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Photosynthesis and aerobic cellular respiration are two key metabolic pathways conducted by plants. The process of photosynthesis transformed life on Earth. By harnessing energy from the sun, photosynthesis evolved to allow living things access to enormous amounts of energy. Because of photosynthesis, living things gained access to sufficient energy that allowed them to build new structures and achieve the biodiversity evident today.
Only certain organisms, called photoautotrophs, can perform photosynthesis; they require the presence of chlorophyll, a specialized pigment that absorbs certain portions of the visible spectrum and can capture energy from sunlight. Photosynthesis uses carbon dioxide and water to assemble carbohydrate molecules and release oxygen as a waste product into the atmosphere. Eukaryotic autotrophs, such as plants and algae, have organelles called chloroplasts in which photosynthesis takes place, and starch accumulates. In prokaryotes, such as cyanobacteria, the process is less localized and occurs within folded membranes, extensions of the plasma membrane, and in the cytoplasm.
The pigments of the first part of photosynthesis, the light-dependent reactions, absorb energy from sunlight. A photon strikes the antenna pigments of photosystem II to initiate photosynthesis. The energy travels to the reaction center that contains chlorophyll a to the electron transport chain, which pumps protons into the thylakoid interior. This action builds up a high concentration of ions. The protons flow through ATP synthase via chemiosmosis to form molecules of ATP, which are used for the formation of sugar molecules in the second stage of photosynthesis. Photosystem I absorbs a second photon, which results in the formation of an NADPH molecule, another energy and reducing power carrier for the light-independent reactions.
Using the electron carriers formed in the first steps of photosynthesis, the light-independent reactions, or the Calvin cycle, take in CO2 from the environment. An enzyme, RuBisCO, catalyzes a reaction with CO2 and another molecule, RuBP. After three cycles, a three-carbon molecule of G3P leaves the cycle to become part of a carbohydrate molecule. The remaining G3P molecules stay in the cycle to be regenerated into RuBP, which is then ready to react with more CO2.
Photorespiration is the process by which RuBisCO binds to O2 instead of CO2 and breaks down rather than builds sugars. C3, C4, and CAM plants have different strategies for reducing photorespiration.
Just like animals, plants must also break down sugars to produce usable energy in the form of ATP through the process of aerobic cellular respiration. The process begins with glycolysis in the cytoplasm, continues with pyruvate oxidation and the citric acid cycle in the mitochondrial matrix, and concludes with oxidative phosphorylation in the cristae of the mitochondria. Overall, it consumes glucose and gaseous oxygen and releases carbon dioxide and water.
After completing this chapter, you should be able to...
• Describe the different types of energy.
• Describe the structure and function of ATP.
• Explain the relevance of photosynthesis to other living things.
• Identify the substrates and products of photosynthesis.
• Describe the main structures involved in photosynthesis.
• Relate the light-dependent and light-independent reactions.
• Summarize the experimental results that revealed details about the process of photosynthesis.
• Relate wavelength, energy, and the type of electromagnetic radiation (and the color of visible light).
• Explain how plants absorb energy from sunlight.
• Detail the steps of the light-dependent interactions.
• Detail the three steps of the light-independent reactions.
• Define carbon fixation.
• Define photorespiration.
• Explain how C3, C4, and CAM plants reduce photorespiration.
• Outline the C4 pathway and compare its use by C4 plants and CAM plants.
• Identify the reactants and products of aerobic cellular respiration.
• Explain each step of aerobic cellular respiration and where in the cell it occurs.
Attribution
Curated and authored by Melissa Ha using 8 Photosynthesis from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org. | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/04%3A_Plant_Physiology_and_Regulation/4.01%3A_Photosynthesis_and_Respiration/4.1.08%3A_Chapter_Summary.txt |
Animals can respond to environmental factors by moving to a new location. Plants, however, are rooted in place and must respond to the surrounding environmental factors. Plants have sophisticated systems to detect and respond to light, gravity, temperature, and physical touch (Figure \(1\)). Plants may grow towards or away from environmental stimuli, and these growth responses are called tropisms. Environmental factors, particularly temperature and the hours of light and dark each day, also control flowering in many species. Plants detect the latter through photoperiodism. In the absence of light, plants respond physiologically to increase the chance of accessing light. Finally, plants rely on environmental cues to break dormancy in seeds and buds on winter twigs.
Attributions
Curated and authored by Melissa Ha using 30.6: Plant Sensory Systems and Responses from General Biology by OpenStax (licensed CC-BY). Access for free at openstax.org.
• 4.2.1: Tropisms
A tropism is a growth movement whose direction is determined by the direction from which the stimulus strikes the plant. There are two forms: Positive = the plant, or a part of it, grows in the direction from which the stimulus originates. and Negative = growth away from the stimulus.
• 4.2.2: Flowering
The flowering plants (angiosperms) go through a phase of vegetative growth producing more stems and leaves and a flowering phase where they produce the organs for sexual reproduction. In "annuals", like the snapdragon, the vegetative phase begins with germination of the seed. Flowering follows and ends with the senescence and death of the plant. In biennials, the vegetative phase takes up the first year; flowering followed by death occurs the second year.
• 4.2.3: Photoperiodism
Many angiosperms flower at about the same time every year. This occurs even though they may have started growing at different times. Their flowering is a response to the changing length of day and night as the season progresses. The phenomenon is called photoperiodism. It helps promote cross pollination.
• 4.2.4: Etiolation and Shade Avoidance
The stems of plants raised in the dark elongate much more rapidly than normal, a phenomenon called etiolation. It is a mechanism that increases the probability of the plant reaching the light.
• 4.2.5: Dormancy
Dormancy refers to periods when a plant is not actively growing, and it is advantageous when environmental conditions are not favorable. Seeds and winter buds undergo dormancy.
• 4.2.6: Chapter Summary
Thumbnail image: Sensitive plant (Mimosa pudica) exhibits a thigmonastic movement. The leaflets and leaves retract when it is touched. Image by piqsels (public domain)
4.02: Environmental Responses
Learning Objectives
• Distinguish among phototropism, gravitropism, hydrotropism, and thigmotropism.
• Discuss the adaptive value of tropisms.
• Describe the mechanism of phototropism in shoots.
• Describe the mechanism of gravitropism in shoots and roots.
• Distinguish among thigmotropism, thigmonastic movements, and thigmomorphogenesis.
A tropism is directional growth in response to a stimulus. A positive tropism occurs when a plant (or a part of the plant) grows towards the stimulus, and a negative tropism is growth away from the stimulus.
Phototropism is directional growth in response to light (Figure \(1\)). (More generally, photomorphogenesis is the growth and development of plants in response to light.) Stems are positively phototropic, and roots are typically negatively phototropic. Gravitropism is directional growth in response to gravity. Stems are negatively gravitropic, and roots are positively gravitropic. The adaptive value of these tropisms is clear. Stems growing upward and/or toward the light will be able to expose their leaves so that photosynthesis can occur. Roots growing downward and/or away from light are more likely to find the soil, water, and minerals they need. Plants can also grow directionally in response to water (hydrotropism) and touch (thigmotropism).
Phototropism in Shoots
Plants can detect different characteristics of light, such as quantity, quality, duration, and direction. They can tell the time of day and time of year by sensing and using various wavelengths of sunlight. Plants are generally capable of detecting and responding to at least three wavelengths of light: blue light, red light, and far-red light. The different wavelengths are detected by different photoreceptors (Figure \(2\)), an example of which are phototropins.
The Shoot Tip Detects Light and Induces Phototropism
In their 1880 treatise The Power of Movements in Plants, Charles Darwin and his son Francis first described phototropism as the bending of seedlings toward light. The Darwins used grass seedlings for some of their experiments. When grass seeds germinate, the primary leaf pierces the seed coverings and the soil while protected by the coleoptile, a hollow, cylindrical sheath that surrounds it (Figure \(3\) ).
Once the seedling has grown above the surface, the coleoptile stops growing and the primary leaf pierces it. If they placed an opaque cover over the tip, phototropism failed to occur even though the rest of the coleoptile was illuminated from one side. However, when they buried the plant in fine black sand so that only its tip was exposed, there was no interference with the tropism — the buried coleoptile bent in the direction of the light (Figure \(4\) ). In conclusion, light was perceived by the tip of the plant (the apical meristem), but that the response (bending) took place in a different part of the plant. The signal had to travel from the apical meristem to the base of the plant.
We now know that phototropism is a response to blue wavelengths of light. The detection of light in the apical meristem occurs via phototropins called phot1 and phot2, which specifically detect blue light (Figure \(5\) ). The aptly-named phototropins are protein-based receptors responsible for mediating the phototropic response. Like all plant photoreceptors, phototropins consist of a protein portion and a light-absorbing portion, called the chromophore. Together, the two are called a chromoprotein. In phototropins, the chromophore is a covalently-bound molecule of flavin; hence, phototropins belong to a class of proteins called flavoproteins.
The Hormone Auxin from the Shoot Tip Stimulates Elongation on the Shaded Side of the Stem
In 1913, the Danish plant physiologist Peter Boysen-Jensen demonstrated that a chemical signal produced in the plant tip was responsible for the bending at the base. He cut off the tip of a seedling, covered the cut section with a layer of gelatin, and then replaced the tip. The seedling bent toward the light when illuminated. However, when impermeable mica flakes were inserted between the tip and the cut base, the seedling did not bend. A refinement of the experiment showed that the signal traveled on the shaded side of the seedling. When the mica plate was inserted on the illuminated side, the plant did bend towards the light. When a horizontal incision was made on the illuminated side and the mica inserted in it, phototropism was normal (Figure \(6\) ). Therefore, the chemical signal was a growth stimulant because the phototropic response involved faster cell elongation on the shaded side than on the illuminated side.
We now know that auxin is the chemical signal that accumulates and stimulates elongation on shaded side of coleoptile. H. W. Went placed a coleoptile that has previously been illuminated from one side on two separated agar blocks. The block on the side that had been shaded accumulated almost twice as much auxin as the block on the previously lighted side (Figure \(7\) ).
Auxin stimulates cell elongation on the shady side of the stem through a process called the acid growth hypothesis: Auxin causes cells to activate proton pumps, which then pump protons out of the cells and into the space between the plasma membrane and the cell wall. The movement of protons into the extracellular space does two things:
• The lower pH activates expansin, which breaks the links between the cellulose fibers in the cell walls, making them more flexible.
• The high concentration of protons causes sugars to move into the cell, which then creates an osmotic gradient where water moves into cell causing the cell to expand.
Phototropism in Roots
Although roots are underground, they can be exposed to light directly as well. Not only can light penetrate up to a few centimeters in the upper layers of some soils, the plant itself can also guide light through the stem to the roots. Additionally, roots can also be exposed to light shortly after germination in the top layer of the soil or because cracks in the soil emerge that trigger a phototropic reaction. Negative phototropism in roots may be evolutionarily advantageous because it increases root efficiency and enhances seedling survival under dry conditions. As in shoots, phototropin 1 is involved in root phototropism. Directional auxin transport and an auxin gradient are associated with root phototropsim, but it is unclear whether they mediate the process.
Gravitropism
Whether or not they germinate in the light or in total darkness, shoots usually sprout up from the ground, and roots grow downward into the ground. A plant laid on its side in the dark will send shoots upward when given enough time. Gravitropism also involves the unequal distribution of auxin.
When an oat coleoptile tip is placed on two separated agar blocks, as shown here, there is no difference in the auxin activity picked up by the two blocks. When the preparation is placed on its side, however, the lower block accumulates twice as much auxin activity as the upper block. Under natural conditions, this would cause greater cell elongation on the underside of the coleoptile and the plant would bend upward (Figure \(8\)).
Specialized amyloplasts called statoliths settle downward in response to gravity. Statoliths are found in the inner part of a stem's cortex (the starch sheath) and in the the central column of the root cap (columella). When a plant is tilted, the statoliths drop to the new bottom cell wall. A few hours later, the shoot or root will show growth in the new vertical direction (Figure \(9\)).
The mechanism that mediates gravitropism is reasonably well understood. When amyloplasts settle to the bottom of the gravity-sensing cells in the root or shoot, they physically contact the endoplasmic reticulum (ER), causing the release of calcium ions from inside the ER. Calcium may be released by ion channels sensitive to mechanical stimulation. This calcium signaling in the cells causes auxin transport proteins (PIN proteins) to redistribute to the underside of the cell leading to the polar transport of auxin to the bottom of the cell. In roots, a high concentration of indoleacetic acid (IAA) inhibits cell elongation. The effect slows growth on the lower side of the root, while cells develop normally on the upper side. IAA has the opposite effect in shoots, where a higher concentration at the lower side of the shoot stimulates cell expansion, causing the shoot to grow up. After the shoot or root begin to grow vertically, the amyloplasts return to their normal position. Another hypotheses—involving the entire cell in the gravitropism effect—have been proposed to explain why some mutants that lack amyloplasts may still exhibit a weak gravitropic response.
Plant Tropisms in Space
Considering the fundamental role of plants in producing fresh food and recycling of air and water, plant tropism research is critical for advancing plant-based life support systems in space. Understanding of the relative strengths of the different root tropisms would be needed to properly guide root growth by technological means, as the gravity vector is absent in space. For example, by exposing roots in microgravity to blue light, they could be induced to develop away from light toward the growth medium. Furthermore, the possibility of performing explorative experiments in the space environment, together with the development of new technologies, is crucial to pave the way toward the goal of deepening our fundamental understanding of plant tropisms and their underlying molecular networks on Earth.
Modified by Melissa Ha from Muthert, L., Izzo, L. G., van Zanten, M., & Aronne, G. (2020). Root Tropisms: Investigations on Earth and in Space to Unravel Plant Growth Direction. Frontiers in plant science, 10, 1807. https://doi.org/10.3389/fpls.2019.01807. CC BY
Hydrotropism
Water acquisition is an important function of plant roots. Because water availability in the soil is often spatially and temporally patchy, roots of many species can exert directional root growth toward water; i.e., positive hydrotropism. The underlying mechanisms of hydrotropism are still being researched. Gravitropism is often dominant over hydrotropic responses, making it difficult to study hydrotropism in isolation. In contrast to phototropism and gravitropism, hydrotropism does not result from an auxin gradient resulting from directional auxin transport. However, auxin still appears to play a role in signaling hydrotropism.
Thigmotropism, Thigmomorphogensis, and Thigmonastic Movements
The shoot of a pea plant winds around a trellis while a tree grows on an angle in response to strong prevailing winds. These are examples of how plants respond to touch or wind.
The movement of a plant subjected to constant directional pressure is called thigmotropism, from the Greek words thigma meaning “touch,” and tropism implying “direction.” Tendrils are one example of plants displaying positive thigmotropism (Video \(1\)). The meristematic region of tendrils is very touch sensitive; light touch will evoke a quick coiling response. Cells in contact with a support surface contract, whereas cells on the opposite side of the support expand. Application of jasmonic acid is sufficient to trigger tendril coiling without a mechanical stimulus.
Video \(1\): This video shows an example of positive thigmotropism in morning glory plants, which require a support structure of some type to grow optimally. The time lapse images were taken at 10 minute intervals. This video has no sound. Here is a description: The morning glory stems (which are modified as tendrils) grow in a counterclockwise pattern, widening their radius with each rotation. When they encounter one of the two wooden stakes on either side of the plant, grow in a tight coil around it.
Plant roots exhibit negative thigmotropism when contacting obstacles in the soil. When plant roots encounter an obstacle in their growth path, the root first continues growing in the same direction until it slips sideways. The root bends away from the stimulus and then bends again in the opposite direction, creating a step-like shape. Note that at this point the root "side stepped" the obstacle and is now continuing to grow in the original direction. If the root contacts another part of the obstacle, the same bending pattern will occur. Thigmotropism in roots may be mediated by calcium, but other signaling mechanisms have also been proposed. Following contact, the calcium concentration in the root cap is greater than that of surrounding regions. (Under regular conditions, calcium concentration in the root cap is lower.) Ultimately, the directional transport of auxin results in an auxin gradient. When touching an obstacle during downward growth, the root bends and auxin accumulates at the higher (concave) side of the root suppressing elongation. Greater elongation on the convex (lower) side of the root causes it to bend away from the obstacle.
Thigmomorphogenesis is a slow developmental change in the shape of a plant subjected to continuous mechanical stress. When trees bend in the wind, for example, growth is usually stunted and the trunk thickens. Strengthening tissue, especially xylem, is produced to add stiffness to resist the wind’s force. Researchers hypothesize that mechanical strain induces growth and differentiation to strengthen the tissues. Ethylene and jasmonate are likely involved in thigmomorphogenesis.
A thigmonastic movement is a touch response independent of the direction of stimulus. In the Venus flytrap, two modified leaves are joined at a hinge and lined with thin fork-like tines along the outer edges (Video \(2\)). Tiny hairs are located inside the trap. When an insect brushes against these trigger hairs, touching two or more of them in succession, the leaves close quickly, trapping the prey. Glands on the leaf surface secrete enzymes that slowly digest the insect. The released nutrients are absorbed by the leaves, which reopen for the next meal.
Video \(2\): This video below shows an example of thigmonastic movements in a Venus flytrap. There is no only ambient sound in this video and no speaking. Here is a description. At 0:08, a fly enters the taco-shaped trap of the Venus flytrap. It escapes before the tines (prongs) interlock, as shown in a slow-motion replay. At 0:42, another fly crawls into the trap. It crawls around a bit before the trap closes, locking it inside.
Another example of thigmonastic movement occurs in the sensitive plant (Mimosa pudica). Its leaflets and leaves retract in response to touch, and this is thought to be an adaptation that deters herbivores (Video \(3\)).
Video \(3\): The leaflets of the sensitive plant retract when touched. This video has no speaking, but there are natural sounds (such as birds) in the background and laughing when the leaflets retract.
Attributions
Curated and authored by Melissa Ha using the following sources: | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/04%3A_Plant_Physiology_and_Regulation/4.02%3A_Environmental_Responses/4.2.01%3A_Tropisms.txt |
Learning Objective
Describe the environmental conditions that induce flowering.
The flowering plants (angiosperms) go through a phase of vegetative growth producing more stems and leaves and a flowering phase where they produce the organs for sexual reproduction. In annuals the vegetative phase begins with germination of the seed. Flowering follows and ends with the senescence and death of the plant. In biennials, the vegetative phase takes up the first year; flowering followed by death occurs the second year. In perennials, flowering typically occurs year after year when conditions are appropriate (Figure \(1\))
Flowering involves the conversion of the apical meristem into a floral meristem, from which all the parts of the flower will be produced. Signals that change the fate of the apical meristem include the following:
• maturity of the plant
• temperature
• the plant hormone gibberellin
• for many plants, photoperiod (daylength; see Photoperiodism section)
Many annual plants (e.g., winter wheat) and biennial plants have their time of flowering delayed unless they have undergone a preceding period of wintertime cold. The change brought about by this prolonged exposure to the cold is called vernalization.
In Arabidopsis, vernalization involves a gene designated Flowering Locus C (FLC), which encodes a transcription factor that blocks the expression of the genes needed for flowering. Recall that one of the two DNA strands in a gene is called the sense (coding) strand and has the same sequence as the RNA transcript. The other DNA strand is called the antisense (non-coding) strand and has a sequence that is complementary to the RNA transcript. The antisense strand acts as a template during transcription. In the fall, the level of FLC mRNA is high. With the onset of cold temperatures, production of an antisense transcript of FLC (called COOLAIR) increases as does, later, a sense transcript of part of the FLC gene. Both of these transcripts are non-coding; that is, they are not translated into protein. Instead, they cooperate in suppressing the production of FLC mRNA and its translation into FLC protein. With the arrival of spring, there is no FLC protein remaining to suppress flowering so flowering can begin.
The buds of many species of woody angiosperms found in temperate climates, such as apples and lilacs, also need a preceding period of cold weather before they can develop into flowers. So these plants cannot be grown successfully at lower latitudes because the winters never get cold enough (a few days at 0–10°C). This bud dormancy is localized. Prior chilling of one bud on a lilac stem enables it to flower while the other, nonchilled, buds on the stem remain dormant (Figure \(2\)).
Attributions
Curated and authored by Melissa Ha using 16.4D: Flowering from Biology by John. W. Kimball (licensed CC-BY)
4.2.03: Photoperiodism
Learning Objectives
• Describe the mechanism of photoperiodism with respect to flowering.
• Distinguish among short-day, long-day, and day-neutral plants.
• Define circadian rhythms and provide examples in plants.
Detection of seasonal changes is crucial to plant survival. Although temperature and light intensity influence plant growth, they are not reliable indicators of season because they may vary from one year to the next. Day length is a better indicator of the time of year. Many angiosperms flower at about the same time every year. This occurs even though they may have started growing at different times. Their flowering is a response to the changing length of day and night as the season progresses. It helps promote cross pollination. The biological response to the timing and duration of day and night is called photoperiodism.
The Phytochrome System and the Red/Far-Red Response
Plants use the phytochrome system to sense the change of season, which can control flowering. The phytochromes are a family of photoreceptors. They are chromoproteins with a linear tetrapyrrole chromophore (a molecule that absorbs light), similar to the ringed tetrapyrrole light-absorbing head group of chlorophyll. A phytochrome is a homodimer: two identical protein molecules, each conjugated to a light-absorbing molecule (compare to rhodopsin). Plants make 5 phytochromes: PhyA, PhyB, as well as C, D, and E. There is some redundancy in function of the different phytochromes, but there also seem to be functions that are unique to one or another. The phytochromes also differ in their absorption spectrum; that is, which wavelengths (e.g., red vs. far-red) they absorb best. Phytochromes have two photo-interconvertible forms: Pr and Pfr. Pr absorbs red light (~667 nm) and is immediately converted to Pfr. Pfr absorbs far-red light (~730 nm) and is quickly converted back to Pr. Absorption of red or far-red light causes a massive change to the shape of the chromophore, altering the conformation and activity of the phytochrome protein to which it is bound. Pfr is the physiologically active form of the protein; therefore, exposure to red light yields physiological activity. Exposure to far-red light inhibits phytochrome activity. Together, the two forms represent the phytochrome system (Figure \(1\)).
The phytochrome system acts as a biological light switch. It monitors the level, intensity, duration, and color of environmental light. The effect of red light is reversible by immediately shining far-red light on the sample, which converts the chromoprotein to the inactive Pr form. Additionally, Pfr can slowly revert to Pr in the dark, or break down over time. In all instances, the physiological response induced by red light is reversed. The active form of phytochrome (Pfr) can directly activate other molecules in the cytoplasm, or it can be transported to the nucleus, where it directly activates or represses specific gene expression.
Unfiltered sunlight is rich in red light but deficient in far-red light. Therefore, at dawn, all the phytochrome molecules in a leaf quickly convert to the active Pfr form, and remain in that form until sunset. In the dark, the Pfr form takes hours to slowly revert back to the Pr form. If the night is long (as in winter), all of the Pfr form reverts. If the night is short (as in summer), a considerable amount of Pfr may remain at sunrise. By sensing the Pr/Pfr ratio at dawn, a plant can determine the length of the day/night cycle. In addition, leaves retain that information for several days, allowing a comparison between the length of the previous night and the preceding several nights. Shorter nights indicate springtime to the plant; when the nights become longer, autumn is approaching. This information, along with sensing temperature and water availability, allows plants to determine the time of the year and adjust their physiology accordingly.
In 1920 two employees of the U. S. Department of Agriculture, W. W. Garner and H. A. Allard, discovered a mutation in tobacco - a variety called Maryland Mammoth - that prevented the plant from flowering in the summer as normal tobacco plants do. Maryland Mammoth would not bloom until late December. Experimenting with artificial lighting in winter and artificial darkening in summer, they found that Maryland Mammoth was affected by photoperiod. Because it would flower only when exposed to short periods of light, they called it a short-day plant. Examples of other short-day plants include chrysanthemums, rice (Oryza sativa), poinsettias, morning glory (Pharbitis nil), and cocklebur (Xanthium).
Experiments with the cocklebur have shown that the term short-day is something of a misnomer; what the cocklebur needs is a sufficiently long night (Figure \(2\)). Short-day (long-night) plants flower in the late summer and early fall, when nights exceed a critical length (often eight or fewer hours). In short-day plants, the active form of phytochrome (Pfr) suppresses flowering. During long periods of darkness (long nights), Pfr is converted to Pr. With Pfr no longer present, flowering is not suppressed, and short-day plants flower. If a flash of light interrupts the dark period, Pr is converted back to Pfr, and flowering is suppressed.
Long-day (short-night) plants flower during the spring, when darkness is less than a critical length (often eight to 15 hours). Examples include spinach, Arabidopsis, sugar beet, and the radish flower.
Flowering in day-neutral plants, such as the tomato, is not regulated by photoperiod.
Photoperiodism also explains why some plant species can be grown only in a certain latitude. Spinach, a long-day plant, cannot flower in the tropics because the days never get long enough (14 hours). Ragweed, a short-day plant, fails to thrive in northern Maine because by the time the days become short enough to initiate flowering, a killing frost in apt to occur before reproduction and the formation of seeds is completed.
Some plants do not neatly fit into the categories of short day, long day, or day neutral. In 1941, Marie Taylor Clark found that flowering in scarlet sage did not flower under daylengths longer than 16 hours, suggesting it was a short-day plant; however, days that were too short (6 hours) slowed flower development. Flower development was optimal with daylengths of 10 hours.
The leaves produce a chemical signal called florigen that is transmitted to the apical meristems to start their conversion into floral meristems. The chemical nature of florigen has been sought for decades. The most recent evidence suggests that at least one component is the protein encoded by the gene FLOWERING LOCUS T (FT). Due to florigen signaling, the entire plant will bloom even if only a part of one leaf is exposed to the correct photoperiod (Figure \(3\)).
Career Connection: Horticulturist
The word “horticulturist” comes from the Latin words for garden (hortus) and culture (cultura). This career has been revolutionized by progress made in the understanding of plant responses to environmental stimuli. Growers of crops, fruit, vegetables, and flowers were previously constrained by having to time their sowing and harvesting according to the season. Now, horticulturists can manipulate plants to increase leaf, flower, or fruit production by understanding how environmental factors affect plant growth and development.
Greenhouse management is an essential component of a horticulturist’s education. To lengthen the night, plants are covered with a blackout shade cloth. Long-day plants are irradiated with red light in winter to promote early flowering. For example, fluorescent (cool white) light high in blue wavelengths encourages leafy growth and is excellent for starting seedlings. Incandescent lamps (standard light bulbs) are rich in red light, and promote flowering in some plants. The timing of fruit ripening can be increased or delayed by applying plant hormones. Recently, considerable progress has been made in the development of plant breeds that are suited to different climates and resistant to pests and transportation damage. Both crop yield and quality have increased as a result of practical applications of the knowledge of plant responses to external stimuli and hormones.
Horticulturists find employment in private and governmental laboratories, greenhouses, botanical gardens, and in the production or research fields (Figure \(4\)). They improve crops by applying their knowledge of genetics and plant physiology. To prepare for a horticulture career, students take classes in botany, plant physiology, plant pathology, landscape design, and plant breeding. To complement these traditional courses, horticulture majors add studies in economics, business, computer science, and communications.
Circadian Rhythms
Circadian rhythms are changes based on a 24-hour cycle. For example, flowers might open every morning and close every evening or vice versa. In Oxalis and silk tree (Albizia julibrissin), leaflets expand during the day and retract at night. Circadian rhythms may also involve physiological processes like photosynthetic rate or the the production of floral scent compounds.
Under constant conditions, circadian rhythms may drift out of phase with the environment (figure \(5\)). However, when exposed to environmental changes (e.g., alternating day and night), the rhythms become entrained; that is, they now cycle synchronized with the cycle of day and night with a period of exactly 24 hours. Internal circadian clocks also adjust to changing photoperiods. Suppose a plant that flowers throughout the spring opens its flowers every morning. The sun rises earlier in the late spring compared to the early spring (photoperiod increases in late spring). As time passes and the plant detects the changing photoperiod (technically, plants measure the length of the night rather than daylength; see above), the circadian clock would adjust such that its flowers opened earlier. In Arabidopsis, the entrainment of circadian rhythms requires that light is detected by phytochromes (absorb red light) and cryptochromes (absorb blue light).
Attributions
Curated and authored by Melissa Ha using the following sources: | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/04%3A_Plant_Physiology_and_Regulation/4.02%3A_Environmental_Responses/4.2.02%3A_Flowering.txt |
Learning Objectives
• Contrast etiolated seedlings with those grown in light.
• Describe the adaptive value of etiolation.
Plants grown in the dark have elongated stems, are white or yellow (due to lack of chlorophyll), and have small leaves (Figures \(1\) and 2). This phenomenon is called etiolation, and it is a mechanism that increases the chance that the plant will access light.
The prevention of etiolation in Arabidopsis is mediated by phytochrome B. When sunlight (660 nm) converts Pr into Pfr, Pfr moves from the cytoplasm into the nucleus. There it stimulates the activity of DELLA proteins, which bind to and inhibit proteins called PIFs (phytochrome-interacting factors). PIFs are transcription factors that bind to and turn on promoters of genes that, among other effects, stimulate cell and thus stem elongation. DELLA proteins prevent PIFs from binding to the promoters of their target genes, resulting in reduced cell elongation (no etiolation) in the presence of light.
Gibberellins are plant hormones that promote stem elongation thus mimicking the etiolation response. They do this by triggering the degradation of DELLA proteins, which frees PIFS to bind to the promoters of genes needed for cell elongation.
The shade-avoidance response also involves phytochrome and and PIFs. This response helps ensure that the plant accesses enough sunlight to be able to conduct photosynthesis. Natural sunlight contains about the same amount of red (660 nm) as far-red (735 nm) light. However, chlorophyll absorbs 660 nm light strongly, so that the light that filters through a canopy of foliage is enriched in far-red light. This is absorbed by Pfr, converting it into inactive Pr such that PIFs are no longer inhibited. The now-active PIFs turn on the genes needed for auxin synthesis, and auxin stimulates stem elongation.
Attributions
Curated and authored by Melissa Ha using 16.4C: Etiolation from Biology by John. W. Kimball (licensed CC-BY)
4.2.05: Dormancy
Learning Objectives
• List the environmental factors that break seed dormancy (induce germination) and define the horticultural techniques based on these factors.
• Explain how phytochrome mediates light-induced germination in lettuce.
During certain time periods or environmental circumstances, active growth is not advantageous for a plant. For example, the embryos in seeds must stay dormant for long enough to disperse from the parent plant or until there are suitable conditions for growth (temperature, light availability, water, etc.). In seeds, the phytochrome system is not used to determine direction and quality of light (shaded versus unshaded). Instead, is it used merely to determine if there is any light at all. This is especially important in species with very small seeds, such as lettuce. Because of their size, lettuce seeds have few food reserves. Their seedlings cannot grow for long before they run out of fuel. If they germinated even a centimeter under the soil surface, the seedling would never make it into the sunlight and would die. In the dark, phytochrome is in the Pr (inactive form) and the seed will not germinate; it will only germinate if exposed to light at the surface of the soil. Upon exposure to light, Pr is converted to Pfr, which signals transcription of the gene that encodes amylase, an enzyme that breaks down starches stored in the seed into simple sugars. Germination then proceeds. Experimentally exposing lettuce seeds to red light induces germination because this converts Pr to Pfr. Exposure to far red light inhibits germination because this causes the active form of phtochrome (Pfr) to be converted to the inactive form (Pr; Figure \(1\)).
Several additional factors are involved in seed germination. Exposure to a cold period can also induce germination in seeds. Cold stratification is a horticultural technique that involves storing seeds in cold conditions to promote germination. Sometimes seeds require mechanical abrasion of the seed coat, facilitated by the soil. Scarification is a horticultural technique that simulates this abrasion by cutting the seed coat with a knife or other means. Seeds may also require exposure to water, which can wash chemicals from the seed coat that inhibit germination. Seeds that are dispersed by animals that eat the fruits are adapted to the acids in the digestive tract. To promote germination in these species, horticulturalists may need to bathe these seeds in acid.
Likewise, trees in certain climates become dormant in the winter (setting of winter buds; Figure \(2\)). Similarly to how it controls flowering, photoperiodism mediated by phytochrome stops vegetative growth and promotes the setting of winter buds. This occurs as the days grow shorter in the autumn. To break dormancy in the spring, the buds may need to first be exposed to cold winter temperatures. In other cases, a certain period of time just needs to pass.
Attributions
Curated and authored by Melissa Ha using the following sources:
4.2.06: Chapter Summary
Plants respond to light, gravity, water, physical contact, temperature, and other environmental factors. Tropisms are a response involve directional growth toward or away from a stimulus. Positive phototropism in stems is mediated by blue-light receptors called photoreceptors and an auxin gradient. Positive gravitropism in roots and negative gravitropism in shoots involves specialized amyloplasts called statoliths as well as an auxin gradient. Plants also change biological activities according to a 24-hour cycle maintained by an internal clock. These circadian rhythms can be entrained to the environment. Flowering is another response to environmental factors, such as temperature and photoperiod. Photoperiodism with respect to flowering involves two interconvertible forms of the red-light phytochrome with which plants can measure the length of the dark period (night). Seedlings respond to a lack of light by etiolated growth, which helps them access light. Light, temperature, rainfall, and other environmental factors can break seed dormancy (induce germination).
After completing this chapter, you should be able to...
• Distinguish among phototropism, gravitropism, hydrotropism, and thigmotropism.
• Discuss the adaptive value of tropisms.
• Describe the mechanism of phototropism in shoots.
• Describe the mechanism of gravitropism in shoots and roots.
• Distinguish among thigmotropism, thigmonastic movements, and thigmomorphogenesis
• Define circadian rhythms and provide examples in plants.
• Describe the environmental conditions that induce flowering
• Describe the mechanism of photoperiodism with respect to flowering.
• Distinguish among short-day, long-day, and day-neutral plants.
• Contrast etiolated seedlings with those grown in light and describe the adaptive value of etiolation.
• List the environmental factors that break seed dormancy (induce germination) and define the horticultural techniques based on these factors.
• Explain how phytochrome mediates light-induced germination in lettuce. | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/04%3A_Plant_Physiology_and_Regulation/4.02%3A_Environmental_Responses/4.2.04%3A_Etiolation_and_Shade_Avoidance.txt |
Cucurbitaceae is a family of plants first cultivated in Mesoamerica, although several species are native to North America (Figure \(1\)). The family includes many edible species, such as squash and pumpkin, as well as inedible gourds. In order to grow and develop into mature, fruit-bearing plants, many requirements must be met and events must be coordinated. Seeds must germinate under the right conditions in the soil; therefore, temperature, moisture, and soil quality are important factors that play a role in germination and seedling development. Soil quality and climate are significant to plant distribution and growth. The young seedling will eventually grow into a mature plant, and the roots will absorb nutrients and water from the soil. At the same time, the aboveground parts of the plant will absorb carbon dioxide from the atmosphere and use energy from sunlight to produce organic compounds through photosynthesis. This chapter will explore the complex dynamics between plants and soils.
• 4.3.1: Essential Elements
Plants require essential elements to grow in large quantities (macronutrients) or small quantities (micronutrients). These form biological macromolelcules, maintain ion balance, aid in enzyme function, and support cell wall structure. Plants display characteristic nutrient deficiencies according to which nutrient they lack.
• 4.3.2: Soils
Soil is the outer loose layer that covers the surface of Earth. Soil quality is a major determinant, along with climate, of plant distribution and growth. Soil quality depends not only on the chemical composition of the soil, but also the topography (regional surface features) and the presence of living organisms. In agriculture, the history of the soil, such as the cultivating practices and previous crops, modify the characteristics and fertility of that soil.
• 4.3.3: Nutrient Cycles
Nutrient cycles describe how elements used by organisms move among the air, water, soil, rocks, and the organisms themselves. The carbon cycle involves photosynthesis and cellular respiration. Most of the nitrogen on Earth is in the form of nitrogen gas in the atmosphere, and plants rely on nitrogen-fixing bacteria to convert usable forms. In the phosphorus cycle, phosphates enter the soil and water through weathering of rocks. Much phosphate is trapped in ocean sediments.
• 4.3.4: Chapter Summary
Attribution
Curated and authored by Melissa Ha using 31.0 Prelude to Soil and Plant Nutrition from Biology 2e by OpenStax (CC-BY). Access for free at openstax.org.
Thumbnail image: A root nodule of broad bean (Vicia faba), which hosts symbiotic nitrogen-fixing bacteria (Rhizobium). Nitrogen fixation converts gaseous nitrogen into ammonium, which is available in the soil to plants. Image by Whitney Cranshaw, Colorado State University, Bugwood.org (CC-BY).
4.03: Nutrition and Soils
Learning Objectives
• List the essential elements required by plants and summarize their functions.
• Describe how plants obtain nutrients, including the mechanism of cation exchange.
• Distinguish between macronutrients and micronutrients.
Plants are unique organisms that can absorb nutrients and water through their root system, as well as carbon dioxide from the atmosphere. Soil quality and climate are the major determinants of plant distribution and growth. The combination of soil nutrients, water, and carbon dioxide, along with sunlight, allows plants to grow.
The chemical composition of plants reflects the essential elements, which are necessary for plant growth and reproduction. For an element to be regarded as essential, three criteria are required: 1) a plant cannot complete its life cycle without the element; 2) no other element can perform the function of the element; and 3) the element is directly involved in plant nutrition. There is some disagreement of the number of essential elements for plants with experts listing as few as 15 or as many as 20. Nineteen essential elements are discussed here. While identifying essential elements may seem straightforward, the nutritional needs of plants somewhat depend on the species and environmental conditions. As a result, some may argue that certain elements, like cobalt (Co), are essential, but it is typically only considered a beneficial element.
Chemical Composition of Plants
The majority of the plant body consists of carbon (C), hydrogen (H), and oxygen (O). Plants obtain carbon from carbon dioxide in the atmosphere and hydrogen from the water absorbed by the roots. Oxygen atoms come from carbon dioxide and gaseous oxygen in the atmosphere, as well as from water. Water typically comprises 80 to 90 percent of the plant’s total weight. However, carbon and oxygen constitute about 45% of dry plant tissue (biomass) each, and hydrogen makes up 6%. The remaining 4% of dry biomass consists of elements that are obtained from the soil. These mineral nutrients are categorized as macronutrients and micronutrients.
Macronutrients are required in relatively large quantities (more than 0.1% of dry biomass). The macronutrients in order of contribution to dry biomass are nitrogen (N), potassium (K), calcium (Ca), magnesium (Mg), phosphorus (P), sulfur (S), and silicon (Si). Nitrogen, potassium, and phosphorus are major components in fertilizer (figure \(1\)). Silicon is only absolutely required by horsetails, but many other plant species contain silicon and benefit from its presence. Some sources consider carbon, oxygen, and hydrogen macronutrients. However, this text will not because they are obtained from the atmosphere and/or water rather than minerals in the soil.
Micronutrients are required in small quantities (less than 0.01% of dry biomass). Plant micronutrients are chlorine (Cl), iron (Fe), boron (B), manganese (Mn), sodium (Na), zinc (Zn), copper (Cu), nickel (Ni), and molybdenum (Mo). Sodium is primarily required by plants using certain photosynthetic pathways (C4 and CAM), but like silicon, it benefits many plant species.
Absorption of Mineral Nutrients
Plants absorb most mineral nutrients from the soil as ions. Some of these essential elements are cations, including potassium (K+), calcium (Ca2+), magnesium (Mg2+), iron (Fe3+ or Fe2+), manganese (Mn2+), sodium (Na+), zinc (Zn2+), copper (Cu+ and Cu2+), and nickel (Ni2+). Other nutrients are found in the form of anions, including dihydrogen phosphate (H2PO4-) or hydrogen phosphate (HPO42-), sulfate (SO42-), chloride (Cl-), and molybdate (MoO42-). Plants obtain nitrogen from the soil as nitrate (NO3-) or ammonium (NH4+). Boron is absorbed as boric acid (H3BO3) or its conjugate base, dihydrogenborate (H2BO3-). Silicon is available as silicic acid (H4SiO4).
Cations in the soil are bound to negatively charged clay particles or the organic acids that form humus (see Soils), and this makes it difficult for plants to absorb them. Plants have a mechanism called cation exchange, which releases cations and frees them for absorption (figure \(2\)). This occurs when the roots pump protons (H+) into the soil. The protons bind to the clay and humus, taking the place of the cation nutrients, such as K+ ,Ca2+, and Mg2+. These nutrients are then freely dissolved in the water in the soil and can enter the roots. Roots can also increase proton concentration (decrease pH) of the soil indirectly by releasing carbon dioxide, which reacts with water to form carbonic acid. Protons released when carbonic acid molecules disassociate can then contribute to cation exchange.
To absorb iron, plants must either release protons creating acidic conditions, which promote the conversion (oxidation) of Fe3+ to Fe2+, or produce special compounds called siderophores. These bind to Fe3+ forming a complex, which can then be transported into the root.
Because anions are not attracted to clay and humus in the soil, it is easier for them to leach from the soil when it is irrigated or when it rains. For this reason, anions like nitrates and phosphates are common causes of eutrophication (see Threats to Biodiversity).
Mycorrhizae, the symbiotic fungi that grow around or inside of root cells, help plants absorb a variety of mineral nutrients, particularly phosphorus.
Functions of Essential Elements
Each essential element has multiple roles. For example, calcium acts a second messenger, transmitting signals within a cell, and as a cofactor, assisting enzyme function. While multiple functions for most of the essential elements are described below, they are organized based on one of their primary functions.
Elements in Biological Macromolecules
Carbon dioxide is a reactant in photosynthesis, and carbon is required to form biological macromolecules (carbohydrates, lipids, proteins, and nucleic acids). For example, the carbohydrate cellulose is the main structural component of the plant cell wall and makes up over thirty percent of plant matter (figure \(3\)). Besides the biological macromolecules, plants contain many other organic molecules, such as the chlorophyll and plastoquinone, and all of these contain carbon. In fact, organic molecules are defined as those that contain carbon and hydrogen, typically have carbon-to-carbon bonds, and are often larger and more complex than inorganic molecules.
Hydrogen and oxygen are components of water and all of the biological macromolecules. Hydrogen is found in all organic compounds, and oxygen is found in many, as well. Gaseous oxygen is also a reactant in aerobic cellular respiration.
Nitrogen is part of proteins, nucleic acids, and chlorophyll. Nitrogen is also used in the synthesis of some vitamins, such as vitamin B6, which serves as a coenzyme in protein synthesis.
Phosphorus is necessary to synthesize nucleic acids and phospholipids, which form the plasma membrane, chloroplasts, and many other plant cell structures. Adenosine triphosphate (ATP), the primary form of ready-to-use energy in the cell, contains three phosphate groups, each with a phosphate in the center (figure \(4\)). Plants generate ATP by binding a phosphate group with adenosine diphosphate (ADP) through oxidative phosphorylation during cellular respiration and during photophosphorylation in photosynthesis.
Sulfur is part of certain amino acids, such as cysteine and methionine, and is present in several coenzymes. For example, it connects coenzyme A to acetyl groups, forming acetyl CoA, the starting molecule for the Krebs cycle (tricarboxylic acid cycle) of cellular respiration. Sulfur also plays a role in photosynthesis as a component of the iron sulfur proteins that transfer electrons from photosystem I to NADP+.
Elements that Maintain Ion Balance
Potassium and chlorine play roles in regulating stomatal opening and closure. As the openings for gas exchange, stomata help maintain a healthy water balance. This process is supported by a potassium ion pump as well as the movement of chloride through symport and anion channels. Potassium is also a cofactor for many enzymes, including those involved in photosynthesis. Chlorine, along with calcium, is essential for water photolysis, which generates oxygen during photosynthesis. Calcium also transmits signals within the cell (including as a second messenger during stomatal closure), acts as a cofactor for many enzymes, and contributes to cell wall structure at the middle lamellae between adjacent cells.
Elements Involved in Enzyme Function
Magnesium, zinc, nickel, copper, and manganese all function as cofactors. Specifically, manganese assists with water photolysis. Zinc aids with the synthesis of chlorophyll. Nickel is involved breaking down urea. Copper can serve as a cofactor or a component of the enzyme itself. For example, the copper in plastocyanin allows it to transport electrons during photophosphorylation. In addition to aiding with enzyme function, magnesium is additionally important to the photosynthetic process because it is part of chlorophyll (figure \(5\)).
Like copper, iron facilitates electron transport in enzymes. It is found in the cytochromes involved in oxidative phosphorylation and photophosphorylation. Iron is also plays a role in synthesizing chlorophyll. Molybdenum is a component of a few enzymes in plants, including one that helps plants use nitrate.
Sodium helps some plants synthesize phosphoenolpyruvate (PEP). Carbon dioxide is added to PEP to form oxaloacetate during C4 and CAM photosynthesis.
Elements in Cell Walls
Of course, polysaccharides such as cellulose, which contain carbon, hydrogen, and nitrogen, are the main components of plant cell walls. However, other elements play more minor roles. Calcium in the middle lamella was already mentioned. Additionally, silicon plays a role in cell wall structure in horsetails. Boron is involved in cell wall structure and elongation.
Nutrient Deficiencies
Deficiencies in any of these nutrients—particularly the macronutrients—can adversely affect plant growth (Figure \(6\)). Depending on the specific nutrient, a lack can cause stunted growth, slow growth, or chlorosis (yellowing of the leaves). Extreme deficiencies may result in leaves showing signs of necrosis (death of the tissues). The location of the symptoms, such as in old leaves versus young leaves, can also be telling. Some nutrients like iron are immobile, so an iron deficiency would cause chlorosis in young leaves. On the other hand, magnesium can be transported from old leaves to developing ones. As a result, a magnesium deficiency would be manifested as chlorsis in the old leaves.
Everyday Connection: Hydroponics
Hydroponics is a method of growing plants in a water-nutrient solution instead of soil (Figure \(7\)). Since its advent, hydroponics has developed into a growing process that researchers often use. Scientists who are interested in studying plant nutrient deficiencies can use hydroponics to study the effects of different nutrient combinations under strictly controlled conditions. Hydroponics has also developed as a way to grow flowers, vegetables, and other crops in greenhouse environments. You might find hydroponically grown produce at your local grocery store. Today, many lettuces and tomatoes in your market have been hydroponically grown.
Attribution
Curated and authored by Melissa Ha using 31.1 Nutritional Requirements of Plants from Biology 2e by OpenStax (CC-BY). Access for free at openstax.org. | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/04%3A_Plant_Physiology_and_Regulation/4.03%3A_Nutrition_and_Soils/4.3.01%3A_Essential_Elements.txt |
Learning Objectives
• Identify and describe each component of soil.
• Distinguish among sand, silt, and clay and explain how particle size influences water holding capacity and soil texture.
• Describe each horizon in a typical soil profile.
• Explain how soils are formed, describing each of the five major factors that affects soil formation and composition.
• Explain what determines soil pH and how soil pH affects nutrient absorption by plants.
Plants obtain mineral nutrients from the soil, which serves as a natural medium for land plants. Soil is the outer loose layer that covers the surface of Earth. Soil quality is a major determinant, along with climate, of plant distribution and growth. Soil quality depends not only on the chemical composition of the soil, but also on climate, topography, and organisms living in the soil. In agriculture, the history of the soil, such as the cultivating practices and previous crops, modifies the characteristics and fertility of that soil.
Soil Composition
Soil consists of organic matter (about 5%), inorganic mineral matter (40-45% of soil volume), and water (about 25%) and air (about 25%). The amount of each of the four major components of soil depends on the amount of vegetation, soil compaction, and water present in the soil.
The organic material consists of dead organisms in various stages of decomposition. It is dark-colored because it contains humus, partially decayed matter containing organic acids. Humus enriches the soil with nutrients, gives the soil a loose texture that holds water, and allows air to diffuse through it. Oxygen is essential to permit cellular respiration in plant roots, decay organisms, and other inhabitants of the soil. The organic component of soil serves as a cementing agent, returns nutrients to the plant, allows soil to store moisture, makes soil tillable for farming, and provides energy for soil microorganisms. Most soil microorganisms—bacteria, algae, or fungi—are dormant in dry soil, but become active once moisture is available.
The inorganic material of soil consists of rock, slowly broken down into smaller particles that vary in size. Soil particles that are 100 μm to 2 mm in diameter are sand. (A micrometer, μm, 10-6 m, or a millionth of a meter.) Soil particles between 2 and 100 μm are called silt, and even smaller particles, less than 2 μm in diameter, are called clay.
Soil should ideally contain 50 percent solid material and 50 percent pore space (Figure \(1\)). Pore space refers to the gaps in between soil particles. The larger the soil particles, the larger the pore spaces. Water can quickly pass through large pore spaces, so soils high in sand drain easily. Smaller soil particles have more surface area relative to volume and produce narrow pore spaces. Water clings to these surfaces, and soils high in clay thus retain water. (Clay is also negatively charged, which attracts water.) About one-half of the pore space should contain water, and the other half should contain air.
Water holding capacity is a measure of of the soils ability to retain water. Technically, it is the field capacity minus the permanent wilting percentage. The field capacity is the percentage of water retained by the soil after excess water is allowed the drain. The permanent wilting percentage (permanent wilting point) is the amount of water left in the soil once the plants have fatally wilted. This measures the amount of water stored in the soil that is actually available to plants.
Soils texture is based on percentages of sand, silt, and clay (Figure \(2\)). Soils that have a high percentage of one particle size are named after that particle (a clay soil has a high percentage of clay). Other soils have a mixture of two particle sizes and very little of the third size. For example, silty clay has roughly 50% clay and 50% silt while sandy clay has 50-60% sand and 35-50% clay. Some soils have no dominant particle size and contain a mixture of sand, silt, and humus. These soils are called loams, and they are optimal for agriculture. A medium loam has roughly 40% sand, 40% silt, and 20% clay. Larger particles (sand) facilitate drainage, and small particles (clay) facilitate water retention, so loam soils both have good drainage and can remain moist. Medium loams have a water holding capacity of approximately 35%. Soils that deviate slightly from a medium loam include loamy sand, sandy loam, sandy clay loam, clay loam, silty clay loam, and silty loam.
Organic Versus Mineral Soils
Soils can be divided into two groups based on how they form. Organic soils are those that are formed from sedimentation and often contain more than 30% organic matter. They form when organic matter, such as leaf litter, is deposited more quickly than it can be decomposed (Figure \(3\)). Mineral soils are formed from the weathering of rocks, typically contain no more than 30% organic matter, and are primarily composed of inorganic material. Weathering occurs when biological, physical, and chemical processes, such as erosion, leaching, or high temperatures, break down rocks.
Soil Horizons
Soil distribution is not homogenous because its formation results in the production of layers; together, the vertical section of a soil is called the soil profile. Within the soil profile, soil scientists define zones called horizons. A horizon is a soil layer with distinct physical and chemical properties that differ from those of other layers.
The typical soil profile has four distinct layers: 1) O horizon; 2) A horizon; 3) B horizon and 4) C horizon (Figure \(4\)). Some soils may have additional layers or lack one of these layers. The thickness of the layers is also variable, and depends on the factors that influence soil formation. In general, immature soils may have O, A, and C horizons, whereas mature soils may display all of these, plus additional layers.
O horizon
The very top of the O horizon (organic layer) consists of partially decayed organic debris like leaves. This horizon is usually dark in color because of humus.
A horizon
The A horizon (topsoil) consists of a mixture of organic material with inorganic products of weathering, and it is therefore the beginning of true mineral soil. In this area, rainwater percolates through the soil and carries materials from the surface. The A horizon may be only 5 cm (2 in.), or it may over a meter. For instance, river deltas like the Mississippi River delta have deep layers of topsoil. Microbial processes occur in the top soil, and this horizon supports plant growth. Many organisms, such as earthworms and insects, live among the plant roots in this horizon.
B horizon
The B horizon (subsoil) consists of small particles that have moved downward, resulting in a dense layer in the soil. In some soils, the B horizon contains nodules or a layer of calcium carbonate. The subsoil is usually lighter in color that topsoil and often contains an accumulation of minerals.
C horizon
The C horizon (soil base), includes the parent material, the organic and inorganic substances from which soils form. Weathering parent material represents the first steps in the chemical breakdown of rock into soil. Often the weathered parent material is underlain by the parent material itself, although in some places it has been carried from another location by wind, water, or glaciers. Beneath the C horizon lies bedrock. The chemical nature of the parent material, whether granite, limestone, or sandstone, for example, has a great influence on the fertility of the soil derived from it.
Factors Affecting Soil Formation and Composition
The fundamental factors that affect soil genesis can be categorized into five elements: climate, organisms, topography (or relief), parent material, and time. One could say that the relief, climate, and organisms dictate the local soil environment and act together to cause weathering and mixing of the soil parent material over time.
Climate
The role of climate in soil development includes aspects of temperature and precipitation. Soils in very cold areas with permafrost conditions (such as the Arctic tundra) tend to be shallow and weakly developed due to the short growing season. In warm, tropical climates, soils tend to be thicker (but lacking in organic matter), with extensive leaching and mineral alteration due to heavy precipitation. In such climates, organic matter decomposition and chemical weathering occur at an accelerated rate. The presence of moisture and nutrients from weathering will also promote biological activity: a key component of a quality soil. The soils of four biomes are described below as examples of the effects of climate on soil composition.
Tropical Rainforests
The lushness of the jungle biome is somewhat illusory. While productivity is high, the soils themselves tend to be of very poor quality (Figure \(5\)). Because of the high rainfall, nutrients are quickly washed out of the topsoil unless they are incorporated in the forest plants. As plant and animal debris falls to the ground, it is quickly decomposed because of the warmth and moisture there. Thus minerals are found mainly in the forest plants, not in the soil. When the plants are removed and cultivation attempted, the soils quickly lose fertility. The situation is made worse by the lack of humus. Additionally, the topsoil may be no thicker than 5 cm (~2 inches), and most of these soils have high iron and aluminum content. Once exposed to the sun, these soils quickly bake into a brick-like material that cannot be cultivated.
Temperate Forests
These regions receive 75–100 cm (~30-39") or more of precipitation each year. Enough water falls on the soil so that much of it passes down to the water table. As it does so, it carries minerals with it. Such soils tend to be acidic and of low and (if unattended) diminishing fertility when used for agriculture. Only by regular fertilization and liming (to restore calcium and raise pH) can productive agriculture be carried out in them. In the U.S., the soils east of the Appalachian Mountains tend to be of this sort (Figure \(6\)).
Temperate Grasslands
In the plains of North America, the annual rainfall is sufficiently low (~50 cm, 20") that little or no rainfall percolates down to the water table. Calcium and other minerals are not carried below the reach of plant roots and so remain available for use. This keeps the pH and general fertility high (Figure \(7\)). Except to the extent that minerals are lost when crops are removed, the minerals simply recycle from subsoil to topsoil and back to the subsoil. The self-restoring fertility of the soils of the plains states accounts for this region being the "breadbasket" of the nation (and other countries as well).
The grasses in undisturbed prairie are perennial. Their extensive root systems help prevent soil erosion, and the return of the season's above-ground growth to the topsoil returns minerals and provides humus to it. These advantages are diminished when annual grasses such as wheat and corn are grown instead and removed in the harvest.
Deserts
The rainfall here is so low, 25 cm (10") per year or less, that any water that does not immediately run off remains near the surface and is largely lost by evaporation. The salts it carries are left near the top of the soil. Their accumulation may make the soil so alkaline and so salty that most crops cannot be grown (Figure \(8\)).
In the U. S., the situation is especially severe in the Great Basin because water flowing down from the mountains—bearing its load of dissolved salts—cannot flow on to the ocean but simply flows out onto the valley floors and evaporates. Large areas of formerly unproductive desert in the United States, Israel, and Egypt have been converted into fertile fields through irrigation. However, even the best irrigation water contains dissolved salts. If just enough water is applied to meet the needs of the crop, the salts are never carried deep in the soil. The high rate of evaporation found in these areas hastens the accumulation of salts in the upper layers of the soil. If uncorrected, the condition may become so severe that only salt-tolerant crops, like sugar beets, can be grown.
Organisms
The presence of living organisms in the soil (soil biota) greatly affects soil formation and structure. A diversity of animals found in the soil such as nematodes, spiders, insects, centipedes, millipedes, pillbugs, slugs, and earthworms (Figure \(9\)). The soil also contains microorganisms like bacteria, archaea, fungi, and "protists". Animals and microorganisms can produce pores and crevices, and plant roots can penetrate into crevices to produce more fragmentation. Plant secretions promote the development of microorganisms around the root, in an area known as the rhizosphere. Through the process of hydraulic redistribution, plants can relocate water in the soil. For example, deep roots can pull water from the lower soil levels and release it at the upper levels. Additionally, leaves and other material that fall from plants decompose and contribute to soil composition. Microorganisms not only decompose organic matter, but contribute to other processes in nutrient cycles, such as nitrogen fixation.
Parent Material
Mineral soils form directly from the weathering of bedrock, the solid rock that lies beneath the soil, and therefore, they have a similar composition to the original rock. Other soils form in materials that came from elsewhere, such as sand and glacial drift. Materials located in the depth of the soil are relatively unchanged compared with the deposited material. Sediments in rivers may have different characteristics, depending on whether the stream moves quickly or slowly. A fast-moving river could have sediments of rocks and sand, whereas a slow-moving river could have fine-textured material, such as clay and silt.
The type of parent material may also affect the rapidity of soil development. Parent materials that are highly weatherable (such as volcanic ash) will transform more quickly into highly developed soils, whereas parent materials that are quartz-rich, for example, will take longer to develop. Parent material also provide nutrients to plants and can affect soil internal drainage.
Topography
Regional surface features (familiarly called “the lay of the land”) can have a major influence on the characteristics and fertility of a soil. Topography affects water runoff, which strips away parent material and affects plant growth. Steeps soils are more prone to erosion and may be thinner than soils that are relatively flat or level. Infiltration, the percolating of water through the soil, is limited in steep soils.
The local topography can have important microclimatic effects. In the northern hemisphere, south-facing slopes are exposed to more direct sunlight angles and are thus warmer and drier than north-facing slopes. The cooler, moister north-facing slopes have a more dynamic plant community due to less evapotranspiration (the combination of liquid water escaping as water vapor and water vapor exiting stomata in plants). They consequently have thicker soils because extensive root systems stabilize the soil and reduce erosion (Figure \(10\)).
Soil drainage affects mineral composition and organic matter accumulation. Well-drained soils, generally on hills or sideslopes, are more brownish or reddish due to conversion of ferrous iron (Fe2+) to minerals with ferric (Fe3+) iron. More poorly drained soils in flat areas tend more be more greyish, greenish-grey (gleyed), or dark colored, due to iron reduction (to Fe2+) and accumulation and preservation of organic matter in areas tending towards anoxic. Areas with poor drainage also tend to be lowlands into which soil material may wash and accumulate from surrounding uplands, often resulting in overthickened A or O horizons. In contrast, steeply sloping areas in highlands may experience erosion and have thinner surface horizons
Time
Time is an important factor in soil formation because soils develop over long periods. Soil formation is a dynamic process. Materials are deposited over time, decompose, and transform into other materials that can be used by living organisms or deposited onto the surface of the soil.
In general, soil profiles tend to become thicker (deeper), more developed, and more altered over time. However, the rate of change is greater for soils in youthful stages of development. The degree of soil alteration and deepening slows with time and at some point, after tens or hundreds of thousands of years, may approach an equilibrium condition where erosion and deepening (removals and additions) become balanced. Young soils (< 10,000 years old) are strongly influenced by parent material and typically develop horizons and character rapidly. Over time, as weathering processes deepen, mix, and alter the soil, the parent material becomes less recognizable as chemical, physical, and biological processes take their effect. Moderate age soils (roughly 10,000 to 500,000 years old) are slowing in profile development and deepening, and may begin to approach equilibrium conditions. Old soils (>500,000 years old) have generally reached their limit as far as soil horizonation and physical structure, but may continue to alter chemically or mineralogically.
Soil development is not always continual. Geologic events such as landslides, glacier advance, or the rising of shorelines can rapidly bury soils. Erosions in rivers and shorelines can cause removal or truncation of soils, and wind or flooding slowly deposited sediment that add to the soil. Animals can mix the soil and sometimes cause soil regression, a reversal or "bump in the road" for the normal path of development, and this increases development over time.
Soil pH and Nutrient Availability
Most plants grow well in soils at a pH of 5.5-6.5. These acidic conditions mean that there is a greater proton concentration. Protons can bind to negative soil particles, freeing essential elements that occur as cations in the soil, similar to the mechanism of cation exchange. Additionally, acidic conditions help rocks break down, which adds mineral nutrients to the soil. Some nutrients also dissolve more easily at this pH, making it easier for roots to absorb them. Finally, some cations will precipitate, becoming unavailable to plants, if the soil is alkaline (basic).
Soil pH also indirectly affects nutrient availability by influencing decomposition rates by microorganisms. When conditions are extremely acidic or basic, decomposition is slowed. Even in the pH range that is suitable for decomposition, the microbial community differs depending on pH with fungi becoming more dominant in slightly acidic conditions.
Several factors determine soil pH. Organic material in soil decreases pH to an extent, but it also acts as a buffer, limiting changes in pH. Climate is also important, with high amounts of rainfall increasing leaching and lowering pH. Some types parent material, such as those high in silicon, decrease pH, while others, such as limestone increase pH.
Soil Taxonomy
Soils are classified into one of 12 soil orders based on soil horizons, how they form, and their chemical compositions. For example, Mollisols (Figure \(7\)), which are found in temperate grasslands, have a thick topsoil rich in organic content. Aridisols, on the other hand, are dry soils that contain calcium carbonate and are found in deserts. Each soil order is further divided into suborders. See USDA's The Twelve Orders of Soil Taxonomy and The Twelve Soil Orders from the University of Idaho for more details.
Career Connections: Soil Scientist
A soil scientist studies the biological components, physical and chemical properties, distribution, formation, and morphology of soils (Figure \(11\)). Soil scientists need to have a strong background in physical and life sciences, plus a foundation in mathematics. They may work for federal or state agencies, academia, or the private sector. Their work may involve collecting data, carrying out research, interpreting results, inspecting soils, conducting soil surveys, and recommending soil management programs.
Many soil scientists work both in an office and in the field. According to the United States Department of Agriculture (USDA), "A soil scientist needs good observation skills to be able to analyze and determine the characteristics of different types of soils. Soil types are complex and the geographical areas a soil scientist may survey are varied. Aerial photos or various satellite images are often used to research the areas. Computer skills and geographic information systems help the scientist to analyze the multiple facets of geomorphology, topography, vegetation, and climate to discover the patterns left on the landscape.” Soil scientists play a key role in understanding the soil’s past, analyzing present conditions, and making recommendations for future soil-related practices.
Attributions
Curated and authored by Melissa Ha using the following sources: | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/04%3A_Plant_Physiology_and_Regulation/4.03%3A_Nutrition_and_Soils/4.3.02%3A_Soils.txt |
Learning Objectives
• Explain the processes of carbon, nitrogen, and phosphorus cycling.
• Explain the process by which rhizobia infect legumes and form root nodules.
Nutrient cycles, also known as biogeochemical cycles, describe the movement of chemical elements through different media, such as the atmosphere, soil, rocks, bodies of water, and organisms. Nutrient cycles keep essential elements available to plants and other organisms. The cycling of three macronutrients are discussed below. The first is the carbon cycle. Plants acquire carbon through photosynthesis. The nitrogen and phosphorus cycles are also discussed, and plant acquire nitrogen and phosphorus as mineral nutrients from the soil.
The Carbon Cycle
The carbon cycle is actually comprised of several interconnected cycles: one dealing with rapid carbon exchange among living organisms and the other dealing with the long-term cycling of carbon through geologic processes (Figure \(1\)). The overall effect is that carbon is constantly recycled in the dynamic processes taking place in the atmosphere, at the surface and in the crust of the earth. The vast majority of carbon resides as inorganic minerals in crustal rocks. Other reservoirs of carbon, places where carbon accumulates, include the oceans and atmosphere. Some of the carbon atoms in your body today may long ago have resided in a dinosaur's body, or perhaps were once buried deep in the Earth's crust as carbonate rock minerals.
Carbon Cycles Slowly between Land and the Ocean
On land, carbon is stored in soil as organic carbon in the form of decomposing organisms or terrestrial rocks. Decomposed plants and algae are sometimes buried and compressed between layers of sediments. After millions of years fossil fuels such as coal, oil, and natural gas are formed. The weathering of terrestrial rock and minerals release carbon into the soil.
Carbon-containing compounds in the soil can be washed into bodies of water through leaching. This water eventually enters the ocean. Atmospheric carbon dioxide also dissolves in the ocean, reacting with water molecules to form carbonate ions (CO32-). Some of these ions combine with calcium ions in the seawater to form calcium carbonate (CaCO3), a major component of the shells of marine organisms. These organisms eventually die and their shells form sediments on the ocean floor. Over geologic time, the calcium carbonate forms limestone, which comprises the largest carbon reservoir on Earth.
Carbonate also precipitates in sediments, forming carbonate rocks, such as limestone. Carbon sediments from the ocean floor are taken deep within Earth by the process of subduction: the movement of one tectonic plate beneath another. The ocean sediments are subducted by the actions of plate tectonics, melted and then returned to the surface during volcanic activity. Plate tectonics can also cause uplifting, returning ocean sediments to land.
Carbon Cycles Quickly between Organisms and the Atmosphere
Carbon dioxide is converted into glucose, an energy-rich organic molecule through photosynthesis by plants, algae, and some bacteria. They can then produce other organic molecules like complex carbohydrates (such as starch), proteins and lipids, which animals can eat. Most terrestrial autotrophs obtain their carbon dioxide directly from the atmosphere, while marine autotrophs acquire it in the dissolved form (bicarbonate, HCO3).
Plants, animals, and other organisms break down these organic molecules during the process of aerobic cellular respiration, which consumes oxygen and releases energy, water and carbon dioxide. Carbon dioxide is returned to the atmosphere during gaseous exchange. Another process by which organic material is recycled is the decomposition of dead organisms. During this process, bacteria and fungi break down the complex organic compounds. Decomposers may do respiration, releasing carbon dioxide, or other processes that release methane (CH4). Photosynthesis and respiration are actually reciprocal to one another with regard to the cycling of carbon: photosynthesis removes carbon dioxide from the atmosphere and respiration returns it. A significant disruption of one process can therefore affect the amount of carbon dioxide in the atmosphere.
Cellular respiration is only one process that releases carbon dioxide. Physical processes, such as the eruption of volcanoes and release from hydrothermal vents (openings in the ocean floor) add carbon dioxide to the atmosphere. Additionally, the combustion of wood and fossil fuels releases carbon dioxide. The level of carbon dioxide in the atmosphere is greatly influenced by the reservoir of carbon in the oceans. The exchange of carbon between the atmosphere and water reservoirs influences how much carbon is found in each.
Nitrogen Cycle
Getting nitrogen into living organisms is difficult. Plants and algae are not equipped to incorporate nitrogen from the atmosphere (where it exists as tightly bonded, triple covalent N2) although this molecule comprises approximately 78 percent of the atmosphere. Because most of the nitrogen is stored in the atmosphere, the atmosphere is considered a reservoir of nitrogen.
Nitrogen Fixation
The nitrogen molecule (N2) is quite inert. To break it apart so that its atoms can combine with other atoms requires the input of substantial amounts of energy. Nitrogen fixation is the process of converting nitrogen gas into ammonia (NH3), which spontaneously becomes ammonium (NH4+). Ammonium is found in bodies of water and in the soil (Figure \(2\)).
Three processes are responsible for most of the nitrogen fixation in the biosphere. The first is atmospheric fixation by lightning. The enormous energy of lightning breaks nitrogen molecules and enables their atoms to combine with oxygen in the air forming nitrogen oxides. These dissolve in rain, forming nitrates, that are carried to the earth. Atmospheric nitrogen fixation probably contributes some 5-8% of the total nitrogen fixed. The second process is industrial fixation. Under great pressure, at a temperature of 600°C, and with the use of a catalyst, atmospheric nitrogen and hydrogen (usually derived from natural gas or petroleum) can be combined to form ammonia (NH3). Ammonia can be used directly as fertilizer, but most of it is further processed to urea and ammonium nitrate (NH4NO3).
The third process is biological fixation. Some nitrogen-fixing bacteria, like Azotobacter, are free-living. Others (Rhizobium and Bradyrhizobium) form a symbiotic relationship with plants in Fabaceae (bean or legume family), which includes beans, peas, soybeans, alfalfa, clovers, and many other species (Figure \(3\)). Bacteria that form root nodules in legumes are informally called rhizobia. Frankia forms root nodules in alders, which are non-legume trees. Nitrogen-fixing cyanobacteria that are symbiotic with the water fern Azolla are essential to maintaining the fertility of semi-aquatic environments like rice paddies. Lichens that contain cyanobacteria can also fix nitrogen. Biological nitrogen fixation requires a complex set of enzymes and a huge expenditure of ATP. Although the first stable product of the process is ammonia, this is quickly incorporated into protein and other organic nitrogen compounds.
Rhizobia live freely in the soil, but they cannot fix atmospheric nitrogen until they have infected the roots of a legume. Legumes provide nitrogen-fixing bacteria with carbohydrates for energy and limit oxygen in the root nodule. Nitrogenase, the enzyme that fixes nitrogen, is inhibited by oxygen, but the bacteroids still need some oxygen to conduct cellular respiration to produce ATP. Legumes produce leghemoglobin, which binds oxygen like the hemoglobin of vertebrates. This reduces oxygen availability in the root nodule. Leghemoglobin contains iron and appears red when bound to oxygen; in fact, a freshly-cut nodule is red or pink (Figure \(4\)).
Root hairs release chemicals called flavonoids, which cause bacteria to synthesize Nod factors. These are chemical signals that bind to root epidermal cell receptors and induce the legume to produce nodules. Nod factors mediate interactions between specific strains of rhizobia and the associated legume species. Different strains of rhizobia produce different Nod factors, and different legumes produce receptors of different specificity. Because of the specificity of the interaction between the Nod factor and the receptor on the legume, some strains of rhizobia will infect only peas, some only clover, some only alfalfa, etc. The treating of legume seeds with the proper strain of rhizobia is a routine agricultural practice.
If the combination is correct, the bacteria enter an epidermal cell of the root and then migrate into the cortex. Their path runs within an intracellular channel that grows through one cortex cell after another. This infection thread is constructed by root hair, not the bacteria, and is formed only in response to the infection. When the infection thread reaches a cell deep in the cortex, it bursts, and the rhizobia are engulfed by endocytosis into membrane-enclosed symbiosomes within the cytoplasm. At this time the cortical (cortex) cell goes through several rounds of mitosis—without cytokinesis —so the cell becomes polyploid. The cortical cells then begin to divide rapidly forming a nodule. This response is driven by the translocation of cytokinins from epidermal cells to the cells of the cortex. The rhizobia also go through a period of rapid multiplication within the nodule cells. Then they begin to change shape and lose their motility. The bacteroids, as they are now called, may almost fill the cell. Only now does nitrogen fixation begin.
Nitrification
Ammonium is converted by bacteria and archaea into nitrites (NO2) and then nitrates (NO3) through the process of nitrification. Like ammonium, nitrites and nitrates are found in water and the soil.
Denitrification
Some nitrates are converted back into nitrogen gas, which is released into the atmosphere. The process, called denitrification, is conducted by bacteria.
Assimilation
Ammonium and nitrates can be used directly by plants and other producers to make organic molecules such as DNA and proteins through the process of assimilation. This nitrogen is now available to consumers. Organic nitrogen is especially important to the study of ecosystem dynamics because many processes, such as primary production, are limited by the available supply of nitrogen.
Ammonification
Consumers excrete organic nitrogen compounds that return to the environment. Additionally dead organisms at each trophic level contain organic nitrogen. Microorganisms, such as bacteria and fungi, decompose these wastes and dead tissues, ultimately producing ammonium through the process of ammonification.
Phosphorus Cycle
Several forms of nitrogen (N2, NH4+, NO3, etc.) were involved in the nitrogen cycle, but phosphorus remains primarily in the form of the phosphate ion (PO43-). Also in contrast to the nitrogen cycle, there is no form of phosphorus in the atmosphere.
Rocks are a reservoir for phosphorus, and these rocks have their origins in the ocean. Phosphate-containing ocean sediments form primarily from the bodies of marine organisms and from their excretions. However, volcanic ash, aerosols, and mineral dust may also be significant phosphate sources. This sediment is then moved to land over geologic time by the uplifting of Earth’s surface (Figure \(5\)). The movement of phosphate from the ocean to the land and through the soil is extremely slow, with the average phosphate ion having an oceanic residence time between 20,000 and 100,000 years.
Weathering of rocks releases phosphates into the soil and bodies of water. Plants can assimilate phosphates in the soil and incorporate it into organic molecules, making phosphorus available to consumers in terrestrial food webs. Waste and dead organisms are decomposed by fungi and bacteria, releasing phosphates back into the soil. Some phosphate is leached from the soil, entering into rivers, lakes, and the ocean. Primary producers in aquatic food webs, such as algae and photosynthetic bacteria, assimilate phosphate, and organic phosphate is thus available to consumers in aquatic food webs. Similar to terrestrial food webs, phosphorus is reciprocally exchanged between phosphate dissolved in the ocean and organic phosphorus in marine organisms.
Attributions
Curated and authored by Melissa Ha using the following sources: | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/04%3A_Plant_Physiology_and_Regulation/4.03%3A_Nutrition_and_Soils/4.3.03%3A_Nutrient_Cycles.txt |
Essential elements are indispensable elements for plant growth. Plants can absorb mineral nutrients and water through their root system, and carbon dioxide from the atmosphere. Mineral nutrients are divided into macronutrients and micronutrients. The macronutrients plants require are nitrogen, potassium, calcium, magnesium, phosphorus, sulfur, and silicon. Important micronutrients include chlorine, iron, boron, manganese, sodium, zinc, copper, nickel, and molybdenum.
Soil consists of organic and inorganic material as well as water and air. The organic material of soil is made of humus, which improves soil structure and provides nutrients. Soil inorganic material consists of rock slowly broken down into smaller particles that vary in size, such as sand, silt, and loam. Soils form slowly as a result of biological, physical, and chemical processes. Soil is not homogenous because its formation results in the production of layers called a soil profile. Most soils have four distinct horizons, or layers: O, A, B, and C. Their composition is influenced by the climate, presence of living organisms, topography, parent material, and time.
The chemical elements that organisms need continuously cycle through ecosystems. Cycles of matter are called biogeochemical cycles, or nutrient cycles, because they include both biotic and abiotic components and processes.
In the carbon cycle, carbon enters the soil through weathering of rocks and leaches into bodies of water. Aquatic organisms produce calcium carbonate, which ultimately forms ocean sediments. Through the process of uplifting, ocean sediments return to land. On shorter time scales, photosynthesis removes carbon dioxide from the atmosphere and converts it to organic carbon. Through aerobic cellular respiration by plants, animals, decomposers, and other organisms, organic carbon breaks down, releasing carbon dioxide back into the atmosphere.
Nitrogen gas in the atmosphere cannot be used by plants. Through nitrogen fixation, bacteria convert nitrogen into ammonium, which can then be converted to nitrites and nitrates through nitrification. Both free-living and symbiotic bacteria, some of which form root nodules, fix nitrogen. Plants can asssimilate ammonium and nitrates, making organic nitrogen available to consumers. Decomposition of organic matter releases ammonium back into the soil through the process of ammonification. Denitrifying bacteria release nitrogen gas from unused nitrates back into the atmosphere.
The phosphorus cycle is a simpler process than the nitrogen cycle. Phosphates enter the soil and water through weathering of rocks, where they can be assimilated. Assimilation and decomposition allow phosphates to cycle in terrestrial and marine ecosystems. Some phosphates are leached from the soil ultimately into the ocean, where the sediment, making ocean sediments a reservoir for phosphorus. Uplifting can move phosphorus-containing rocks to land.
After completing this chapter, you should be able to...
• List the essential elements required by plants and summarize their functions.
• Describe how plants obtain nutrients, including the mechanism of cation exchange.
• Distinguish between macronutrients and micronutrients.
• Identify and describe each component of soil.
• Distinguish among sand, silt, and clay and explain how particle size influences water holding capacity and soil texture.
• Describe each horizon in a typical soil profile.
• Explain how soils are formed, describing each of the five major factors that affects soil formation and composition.
• Explain what determines soil pH and how soil pH affects nutrient absorption by plants.
• Explain the processes of carbon, nitrogen, and phosphorus cycling.
• Explain the process by which rhizobia infect legumes and form root nodules.
Attributions
Curated and authored by Melissa Ha using the following sources: | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/04%3A_Plant_Physiology_and_Regulation/4.03%3A_Nutrition_and_Soils/4.3.04%3A_Chapter_Summary.txt |
Learning Objectives
• Explain the defining characteristics of a hormone.
• Name the five major plant hormones.
A plant’s sensory response to external stimuli relies on chemical messengers (hormones). Plant hormones affect all aspects of plant life, from flowering to fruit setting and maturation (Figure \(\PageIndex{a}\), and from phototropism to leaf fall. Just as in animals, hormones are signaling molecules which are present in very small amounts, transported throughout the plant body, and only elicit in responses in cells which have the appropriate hormone receptors. In plants, hormones travel large throughout the body such as through the vascular tissue (xylem and phloem) or cell-to-cell via plasmodesmata. Potentially every cell in a plant can produce plant hormones. In contrast, many animal hormones are produced only in specific glands. Plants do not have specialized hormone-producing glands.
Plant hormones are a group of unrelated chemical substances that affect plant growth, development, and other physiological processes. Five major plant hormones are traditionally described: auxins (particularly IAA), cytokinins, gibberellins (GAs), abscisic acid (ABA), and ethylene. Auxins, cytokinins, and gibberellins are groups of plant hormones whereas abscisic acid and ethylene are single compounds. Additional signaling molecules, such as jasmonates and salicylic acid are key in defense against pathogens and herbivores.
Attributions
Curated and authored by Melissa Ha from the following sources:
Thumbnail image: A normal barley seedling (right) and a mutant deficient in the hormone gibberellic acid. The mutant produces less of the active hormone, and does not grow as rapidly. Image and description (modified) by CSIRO (CC-BY).
4.04: Hormones
Learning Objectives
• Explain the mechanism of polar auxin transport.
• Identify locations of synthesis and actions of auxin.
• Define apical dominance and explain the role of auxin in maintaining it.
• Describe the commercial applications of auxin.
• Interpret and predict outcomes of experiments that demonstrate the action of auxin.
The term auxin is derived from the Greek word auxein, which means "to grow." While many synthetic auxins are used as herbicides, indole-3-acetic acid (IAA) is the only naturally occurring auxin that shows physiological activity (Figure \(1\)). Auxin is synthesized in apical meristems, young leaves, and developing seeds.
The Discovery of Auxin
Recall from the Tropisms section that the Boysen-Jensen experiment showed a chemical signal must be downward from the tip of the coleoptile tip along the shaded side, resulting in phototropism. Went extracted the chemical signal involved in the Boysen-Jensen experiment. He removed the tips of several coleoptiles of oat, Avena sativa, seedlings. He placed these on a block of agar for several hours. At the end of this time, the agar block itself was able to initiate resumption of growth of the decapitated coleoptile. The growth was vertical because the agar block was placed completely across the stump of the coleoptile and no light reached the plant from the side (Figure \(2\)). The unknown substance that had diffused from the agar block was named auxin. The amount of auxin in coleoptile tips was far too small to be purified and analyzed chemically. Therefore, a search was made for other sources of auxin activity.
This search was aided by a technique called the Avena test developed by Went for determining the relative amount of auxin activity in a preparation. The material to be assayed is incorporated into an agar block, and the block is placed on one edge of a decapitated Avena coleoptile. As the auxin diffuses into that side of the coleoptile, it stimulates cell elongation and the coleoptile bends away from the block (Figure \(3\)). The degree of curvature, measured after 1.5 hours in the dark, is proportional to the amount of auxin activity (e.g., number of coleoptile tips used). The use of living tissue to determine the amount of a substance, such as in the Avena test, is called a bioassay.
The Avena test soon revealed that substances with auxin activity occur widely in nature. One of the most potent was first isolated from human urine. It was indole-3-acetic acid (IAA) and turned out to be the auxin actually used by plants.
Auxin Transport
Auxin moves through the plant by two mechanisms, called polar and nonpolar transport.
Polar Transport
In contrast to the other major plant hormones, auxins can be transported in a specific direction (polar transport) through parenchyma cells. The cytoplasms of parenchyma cells are neutral (pH = 7), but the region outside the plasma membranes of adjacent cells (the apoplast) is acidic (pH = 5). When auxin is in the cytoplasm, it releases a proton and becomes an anion (IAA-). It cannot pass through hydrophobic portion of the plasma membrane as an anion, but it does pass through special auxin efflux transporters called PIN proteins. Eight different types of these transmembrane proteins have been identified so far. When IAA- enters the acidic environment of the apoplast, it is protonated, becoming IAAH. This uncharged molecule can then pass through the plasma membrane of adjacent cells through diffusion or via influx transporters. Once it enters the cytoplasm, it loses its proton, becoming IAA- again. PIN proteins can be unevenly distributed around the cell (for example, only occurring on the bottom of the cell), which directs the flow of auxin (Figure \(4\)).
Nonpolar Transport
Auxins can also be transported nondirectionally (nonpolar transport) through the phloem. It passes in the assimilate that translocates through the phloem from where it is synthesized (its "source", usually the shoot) to a "sink" (e.g., the root).
Actions of Auxin
Tropisms
Auxins are the main hormones responsible for phototropism and gravitropism. The auxin gradients that are required for these tropisms are facilitated by the movement of PIN proteins and the polar transport of auxin in response to environmental stimuli (light or gravity). Note that higher auxin concentration on one side of the stem typically causes that side of the stem to elongate; however, the effect is opposite in roots with higher auxin concentration inhibiting elongation (Figure \(5\)).
Growth and Development
Embryo Development
Auxins play a role in embryo development. From the very first mitotic division of the zygote, gradients of auxin guide the patterning of the embryo into the parts that will become the organs of the plant, including the shoot apex, primary leaves, cotyledon(s), stem, and root.
Vascular Tissue Differentiation
They also control cell differentiation of vascular tissue.
Leaf Development and Arrangement
The formation of new leaves in the apical meristem is initiated by the accumulation of auxin. Already-developing leaves deplete the surrounding cells of auxin so that the new leaves do not form too close to them. In this way, the characteristic pattern of leaves in the plant is established. Auxin also controls the precise patterning of the epidermal cells of the developing leaf.
Root Initiation and Development
The localized accumulation of auxin in epidermal cells of the root initiates the formation of lateral or secondary roots. Auxin also stimulates the formation of adventitious roots in many species. Adventitious roots grow from stems or leaves rather than from the regular root system of the plant. Once a root is formed, a gradient of auxin concentration develops highest at the tip promoting the production of new cells at the meristem, and lowest in the region of differentiation, thus promoting the elongation and differentiation of root cells. The drop in auxin activity in the regions of elongation and differentiation is mediated by cytokinin — an auxin antagonist.
Shade Avoidance
Auxins stimulate cell elongation parts of the plants that have access to light as part of the shade-avoidance response (see Etiolation and Shade Avoidance).
Interactions with Other Growth-Regulating Hormones
Auxin is required for the function of other growth-regulating hormones such as cytokinins; cytokinins promote cell division, but only in the presence of auxin.
Apical Dominance
Apical dominance—the inhibition of axillary bud (lateral bud) formation—is triggered by downward transport of auxins produced in the apical meristem. Many plants grow primarily at a single apical meristem and have limited axillary branches (Figure \(6\)). Growth of the shoot apical meristem (terminal shoot) usually inhibits the development of the lateral buds on the stem beneath. If the shoot apical meristem of a plant is removed, the inhibition is lifted, and axillary buds begin growth. However, if the apical meristem is removed and IAA applied to the stump, inhibition of the axillary buds is maintained (Figure \(7\)). Gardeners exploit this principle by pruning the terminal shoot of ornamental shrubs, etc. The release of apical dominance enables lateral branches to develop and the plant becomes bushier. The process usually must be repeated because one or two laterals will eventually outstrip the others and reimpose apical dominance.
The common white potato also illustrates the principle of apical dominance. Note that a potato is a tuber, which is an underground stem modified for starch storage. As with an ordinary shoot, the potato has a terminal bud (containing the shoot apical meristem) or "eye" and several axillary (lateral) buds. After a long period of storage, the terminal bud usually sprouts but the other buds do not. However, if the potato is sliced into sections, one bud to a section, the axillary buds develop just as quickly as the terminal bud (Figure \(8\)).
As will be discussed in the Abscisic Acid section, abscisic acid in the lateral buds inhibits production of auxin, and removal of the apical bud will release this inhibition of auxin, allowing the lateral buds to begin growing.
Flowering and Fruit Development
Auxins promote flowering and fruit setting and ripening. Pollination of the flowers of angiosperms initiates the formation of seeds. As the seeds mature, they release auxin to the surrounding flower parts, which develop into the fruit that covers the seeds.
Prevention of Abscission
Some plants drop leaves and fruits in response to changing seasons (based on temperatures, photoperiod, water, or other environmental conditions). This process is called abscission, and is regulated by interactions between auxin and ethylene. During the growing season, the young leaves and fruits produce high levels of auxin, which blocks activity of ethylene; they thus remain attached to the stem. As the seasons change, auxin levels decline and permit ethylene to initiate senescence, or aging (see Ethylene).
Figure \(9\) demonstrates the role of auxin in abscission. If the blade of the leaf is removed, as shown in the figure, the petiole remains attached to the stem for a few more days. The removal of the blade seems to be the trigger as an undamaged leaf at the same node of the stem remains on the plant much longer, in fact, the normal length of time. If, however, auxin is applied to the cut end of the petiole, abscission of the petiole is greatly delayed.
Mechanisms of Auxin Action
Auxin effects are mediated by two different pathways: immediate, direct effects on the cell and turning on of new patterns of gene expression. The arrival of auxin in the cytosol initiates such immediate responses as changes in the concentration of and movement of ions in and out of the cell and reduction in the redistribution of PIN proteins. At the cellular level, auxin generally increases the rate of cell division and longitudinal cell expansion. Some of the direct effects of auxin may be mediated by its binding to a cell-surface receptor designated ABP1 ("Auxin-binding protein 1").
Many auxin effects are mediated by changes in the transcription of genes. Auxin enters the nucleus and binds to its receptor, a protein called TIR1 ("transport inhibitor response protein 1") which now can bind to proteins responsible for attaching ubiquitin to one or another of several Aux/IAA proteins. This triggers the destruction of the Aux/IAA proteins by proteasomes. Aux/IAA proteins normally bind transcription factors called auxin response factors (ARF) preventing them from activating the promoters and other control sequences of genes that are turned on (or off) by auxin. Destruction of the Aux/IAA proteins relieves this inhibition, and gene transcription begins.
This mechanism is another of the many cases in biology where a pathway is turned on by inhibiting the inhibitor of that pathway (a double-negative is a positive). The presence in the cell of many different Aux/IAA proteins (29 in Arabidopsis), many different ARFs (23 in Arabidopsis) and several (~4) TIR1-like proteins provides a logical basis for mediating the different auxin effects that are described here, but how this is done remains to be discovered.
Commercial Applications of Auxins
Commercial use of auxins is widespread in for propagation in nurseries, crop production, and killing weeds. Horticulturists may propagate desirable plants by cutting pieces of stem and placing them base down in moist soil. Eventually adventitious roots grow out at the base of the cutting. The process can often be hastened by treating the cuttings with a solution or powder containing a synthetic auxin.
Applying synthetic auxins to tomato plants in greenhouses promotes normal fruit development. Fruit growers often apply auxin sprays to cut down the loss of fruit from premature dropping. Additionally, outdoor application of auxin promotes synchronization of fruit setting and dropping to coordinate the harvesting season. Fruits such as seedless cucumbers can be induced to set fruit by treating unfertilized plant flowers with auxins.
Synthetic auxins are widely used as herbicides. Examples include 2,4-dichlorophenoxy acetic acid (2,4-D) and 2,4,5-trichlorophenoxy acetic acid (2,4,5-T), shown in Figure \(10\). 2,4-D and its many variants are popular because they are selective herbicides, killing broad-leaved eudicots but not narrow-leaved monocots. (No one knows the basis of this selectivity). Why should a synthetic auxin kill the plant? It turns out that the auxin influx transporter works fine for 2,4-D, but that 2,4-D cannot leave the cell through the efflux transporters. Perhaps it is the resulting accumulation of 2,4-D within the cell that kills it. A mixture of 2,4,-D and 2,4,5-T was the "agent orange" used by the U.S. military to defoliate the forest in parts of South Vietnam. Because of health concerns, 2,4,5-T is no longer used in the U.S.
Attributions
Curated and authored by Melissa Ha from the following sources: | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/04%3A_Plant_Physiology_and_Regulation/4.04%3A_Hormones/4.4.01%3A_Auxin.txt |
Learning Objective
Identify the locations of synthesis, transport, and actions of cytokinins.
Cytokinins are plant hormones that promote cytokinesis (cell division) are derivatives of the purine adenine. (They are not to be confused with cytokines.) The effect of cytokinins was first reported when it was found that adding the liquid endosperm of coconuts to developing plant embryos in culture stimulated their growth. Without the cytokinins from the endosperm, plant cells would not divide by mitosis. Almost 200 naturally occurring or synthetic cytokinins are known to date. Zeatin is an example of naturally occurring cytokinin (Figure \(1\)), and kinetin is an example of a synthetic cytokinin.
Cytokinins are synthesized in roots tips and other young structures where cell division is occurring such as embryos and fruits. They are also produced by wounded tissue. Cytokinins are transported through the xylem.
Actions of Cytokinins
One of the clearest examples of cytokinin stimulating cell division involves seed germination. The endosperm of monocot seeds, such as corn (maize), contains large stores of the precursor to the cytokinin zeatin. When the corn kernel germinates, zeatin moves from the endosperm to the root tip where it stimulates vigorous mitosis (Figure \(2\)).
Plant development is controlled by multiple hormones working together or balancing each other's effects. Cytokinins play an important role in plant development. They are involved in leaf formation, and they delay senescence in leaf tissues. Cytokinins also play a role in chloroplast development.
Cytokinins often counter the effects of auxin when regulating shoot and root development. Cytokinins inhibit apical dominance by stimulating axillary bud development, having the opposite effect as auxin. They inhibit the formation of lateral roots while auxin initiates lateral roots. When cytokinins are applied to a callus (mass of undifferentiated cells), shoots form. If auxin is applied, roots form. If the two hormones are applied in equal amounts, much the rate of cell division increases, but the callus does not produce distinct shoots and roots.
With respect to mediating roots gravitropism, however, the effect of cytokinin is similar to that of auxin. When a root is turned on its side, cytokinins accumulate on the lower side, inhibiting elongation there. As the upper surface of the root elongates, it bends downwards.
Mechanism of Cytokinin Action
Like auxins, cytokinin can cause changes in gene expression. To begin this process, a cytokinin binds to a receptor protein embedded in the plasma membrane of the cell. The internal portion of the receptor then attaches a phosphate group to a protein in the cytosol. This protein moves into the nucleus where it activates one or more nuclear transcription factors, which then bind to the promoters of genes. Transcription of these genes produces mRNAs that move out into the cytosol. Translation of these mRNAs produces the proteins that enable the cell to carry out its cytokine-induced function.
Attributions
Curated and authored by Melissa Ha from the following sources:
4.4.03: Gibberellins
Learning Objectives
• Identify the locations of synthesis, transport, and actions of gibberellins.
• Interpret and predict the outcome of an experiment demonstrating the action of gibberellins.
• Describe the commercial applications of gibberellins.
During the 1930s Japanese scientists isolated a growth-promoting substance from cultures of a fungus that parasitizes rice plants. They called it gibberellin. Gibberellins (GAs) are a group of about 125 closely related plant hormones synthesized in the root and stem apical meristems, young leaves, and seed embryos. They are likely transported through the vascular tissue. One of the most active gibberellins - and one found as a natural hormone in the plants themselves - is gibberellic acid (GA; figure \(1\)).
Actions of Gibberellins
Several aspects of plant growth involve GAs, including stimulating shoot elongation, seed germination, and fruit and flower maturation. Other effects of GAs include gender expression (also see Ethylene) and the delay of senescence in leaves and fruit. Synthesis of gibberellins also helps grapevines climb up toward the light by causing meristems that would have developed into flowers to develop into tendrils instead.
Shoot elongation in this case results from both cell division and cell elongation. When applied in low concentrations to a bush or "dwarf" bean, the stem begins to grow rapidly. The length of the internodes becomes so great that the plant becomes indistinguishable from climbing or "pole" beans. GA seems to overcome the genetic limitations in many dwarf varieties.
Gibberellins break dormancy (a state of inhibited growth and development) in the seeds of plants that require exposure to cold or light to germinate. The seeds of some plant species rely on the imbibition (intake) of water to initiate germination. Intake of water activates gibberellins, which then signals to transcribe the gene encoding amylase, an enzyme that breaks down starches stored in the seed into simple sugars (note these final steps are identical to what occurs in phytochrome-regulated germination). Gibberellins also stimulate cell elongation of young roots during germination. When water is absent, germination in this pathway is blocked by a hormone called abscisic acid, which inhibits the activity of gibberellins. Thus gibberellins and abscisic acid act in opposition in regulating the the germination response.
Many plant species first produce a basal rosette of leaves. When daylength increase or the weather becomes cold, they bolt, producing a long stalk. Eventually, flowers and then fruits develop on this stalk. Gibberellins are responsible for inducing bolting (figure \(2\).
Mechanism of Gibberellin Action
Like auxins, gibberellins generally increase the rate of cell division and longitudinal cell expansion. Gibberellins also exert their effects by altering gene transcription through a mechanism similar to auxin in that a pathway is turned on by inhibiting the inhibitor of that pathway (a double-negative is a positive). First, Gibberellin enters the cell and binds to a soluble receptor protein called GID1 ("gibberellin-insensitive dwarf mutant 1") which now can bind to a complex of proteins (SCF) responsible for attaching ubiquitin to one or another of several DELLA proteins. This triggers the destruction of the DELLA proteins by proteasomes. DELLA proteins normally bind gibberellin-dependent transcription factors, a prominent one is designated PIF3/4, preventing them from binding to the DNA of control sequences of genes that are turned on by gibberellin (also see Shade Avoidance and Etiolation).
The dwarf varieties of rice and wheat carry mutations related to GAs. In the case of rice, the mutation interfere with the synthesis of their gibberellins. The wheat mutation reduces is in the gene coding for a DELLA protein and reduce the plant's ability to respond to its own gibberellins. Dwarf varieties of sorghum and more recently maize (corn) also exist, but in these cases, the mutation interferes with auxin transport, not gibberellin activity.
Commercial Applications of Gibberellins
Gibberellin application assists with seedless grape production. Seedless grapes are obtained through standard breeding methods and contain inconspicuous seeds that fail to develop. Because GAs are produced by the seeds, and because fruit development and stem elongation are under GA control, these varieties of grapes would normally produce small fruit in compact clusters. Maturing grapes are routinely treated with GA to promote larger fruit size, as well as looser bunches (longer stems), which reduces the instance of mildew infection (Figure \(3\)). Like auxins, GAs can be used commercially to induce fruit development in a variety of species.
Gibberellins have a few other commercial applications. They can be applied to artificially induce bolting and flowering, such that plants produce seeds earlier. Addition of gibberellic acid to the winter buds of peach trees helps break dormancy. In urban areas, GA antagonists are sometimes applied to trees under power lines to control growth and reduce the frequency of pruning.
Attributions
Curated and authored by Melissa Ha from the following sources: | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/04%3A_Plant_Physiology_and_Regulation/4.04%3A_Hormones/4.4.02%3A_Cytokinins.txt |
Learning Objectives
• Identify the locations of synthesis, transport, and actions of abscisic acid.
• Describe how ABA interacts with other plant hormones.
The plant hormone abscisic acid (ABA) was was once thought to be responsible for abscission; however, this is now known to be incorrect. Instead, ABA accumulates as a response to stressful environmental conditions, such as dehydration, cold temperatures, or shortened day lengths. Unlike animals, plants cannot flee from potentially harmful conditions like drought, freezing, exposure to salt water or salinated soil, and ABA plays in mediating adaptations of the plant to stress. Abscisic acid (Figure \(1\)) resembles the carotenoid zeaxanthin (Figure \(2\)), from which it is ultimately synthesized. It is produced in mature leaves and roots and transported through the vascular tissue.
Maintaining Dormancy
Seed Maturation and Inhibition of Germination
Seeds are not only important agents of reproduction and dispersal, but they are also essential to the survival of annual and biennial plants. These angiosperms die after flowering and seed formation is complete. Abscisic acid is essential for seed maturation and also enforces a period of seed dormancy, by blocking germination and promoting the synthesis of storage proteins. It is important the seeds not germinate prematurely during unseasonably mild conditions prior to the onset of winter or a dry season. As the hormone gradually breaks down over winter, the seed is released from dormancy and germinates when conditions are favorable in spring. As discussed in the Environmental Responses chapter, other environmental cues such as exposure to a cold period, light, or water are often also needed to for germination to occur.
Interestingly, mangrove species with viviparous germination, meaning that seeds germinate while still attached to the parent plant have reduced levels of ABA during embryo formation, providing further evidence of ABA's role in maintain seed dormancy (Farnsworth and Farrant 1998, Am J. Bot.). These mangroves are adapted to drop germinated seeds into surrounding water to be dispersed (Figure \(3\)).
Bud Dormancy
Another effect of ABA is to promote the development of winter buds; it mediates the conversion of the apical meristem into a dormant bud. The newly developing leaves growing above the meristem become converted into stiff bud scales that wrap the meristem closely and will protect it from mechanical damage and drying out during the winter. Abscisic acid in the bud also acts to enforce dormancy so if an unseasonably warm spell occurs before winter is over, the buds will not sprout prematurely. Only after a prolonged period of cold or the lengthening days of spring (photoperiodism) will bud dormancy be lifted.
Response to Water Stress
Stomatal Closure
Abscisic acid also regulates the short-term drought response. Recall that stomata are pores in the leaf and are surrounded by a pair of guard cells. Much of the water taken up by a plant is lost as water vapor exists stomata. Low soil moisture causes an increase in ABA, which causes stomata to close, reducing water loss. Note that stomatal closure also prevents exchange of oxygen and carbon dioxide, which is necessary for efficient photosynthesis (see Photorespiration and Phytosynthetic Pathways). The response to abscisic acid occurs even if blue light is present; that is, signaling from drought via ABA overrides the signaling from blue light to open stomata. See Transport for more details about stomatal opening and closure.
Cellular Protection from Dehydration
Abscisic acid turns on the expression of genes encoding proteins that protect cells - in seeds as well as in vegetative tissues - from damage when they become dehydrated.
Interactions with Other Hormones
At a cellular level, abscisic acid inhibits both cell division and cell expansion. It often opposes the growth-inducing effects of auxin and gibberellic acid. For example, abscisic acid prevents stem elongation probably by its inhibitory effect on gibberellic acid. In maintaining apical dominance, however, ABA synergizes with auxin. Abscisic acid moves up from the roots to the stem (opposite the flow of auxin) and suppresses the development of axillary buds. The result is inhibition of branching (maintaining apical dominance).
Attributions
Curated and authored by Melissa Ha from the following sources: | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/04%3A_Plant_Physiology_and_Regulation/4.04%3A_Hormones/4.4.04%3A_Abscisic_Acid.txt |
Learning Objectives
• Relate the chemical structure of ethylene to its mode of transport.
• Identify the locations of synthesis and actions of ethylene.
• Describe the commercial applications of ethylene.
Ethylene differs from other plant hormones in that it is a smaller and simpler molecule that is a volatile gas (Figure \(1\)). Hundreds of years ago, when gas street lamps were installed in city streets, trees that grew close to lamp posts developed twisted, thickened trunks and shed their leaves earlier than expected. These effects were caused by ethylene volatilizing from the lamps. Aging tissues, such as those that are wilting or ripening, and nodes of stems produce ethylene.
Actions of Ethylene
Ethylene has many functions, and its major functions are associated with senescence, or aging. This includes fruit ripening, flower wilting, and leaf and fruit abscission. Ethylene also promotes germination in some cereals and sprouting of bulbs and potatoes. It is responsible for drooping of leaves and sprouting of potato buds. In monoecious plants, ethylene promotes the production of female flowers where as gibberellic acid promotes male flower production. Ethylene mediates the triple response, which makes the shoots of seedlings that are buried under debris grow short and wide as well as bend horizontally. This makes it possible for the shoot to push through the debris. Ethylene causes stem elongation in rice and other plants that are submerged in water. It promotes the breakdown of abscisic acid (ABA) and thus relieves ABA's inhibition of gibberellic acid.
Fruit Ripening
As they approach maturity, many fruits (e.g., apples, oranges, avocados) release ethylene. During fruit ripening, ethylene stimulates the conversion of starch and acids to sugars. Some people store unripe fruit, such as avocados, in a sealed paper bag to accelerate ripening; the gas released by the first fruit to mature will speed up the maturation of the remaining fruit.
Abscission
Ethylene induces the abscission of leaves, fruits, and flower petals. When auxin levels decline, ethylene triggers senescence and ultimately programmed cell death at the site of leaf attachment to the stem. A special layer of cells — the abscission layer (abscission zone) — forms at the base of the petiole or fruit stalk (Figure \(2\)). In petioles of some plants, there are two parts of the abscission layer: the more distal separation layer and more proximal protective layer. Before abscission occurs, nutrients are absorbed into the stem so that they are not lost with the leaf. As the separation layer breaks down, the leaf breaks free at this point and leaf falls to the ground in a controlled manner without harming the rest of the plant. The protective layer, which was reinforced with suberin, serves as a seal.
Leaf abscission is particularly important for temperate deciduous trees in the autumn. This is a vital response to the onset of winter when ground water is frozen - and thus cannot support transpiration - and snow load would threaten to break any branches still in leaf.
In drought conditions, the immediate response is closing stomata (see Abscisic Acid). However, because closed stomata prevent gas exchange, plants will die if the stomata remain closed for too long. Thus if a drought persists for too long, the plant will begin sacrificing certain areas by allowing the leaves or stems to die in localized regions. This process may be regulated by ethylene, which can induce localized cell death under certain conditions.
Mechanism of Ethylene Action
At a cellular level, ethylene can inhibit or promote cell division. It sometimes inhibits cell expansion. In other circumstances, it stimulates lateral cell expansion. The presence of ethylene is detected by transmembrane receptors in the endoplasmic reticulum (ER) of cells. Binding of ethylene to these receptors unleashes a signaling cascade that leads to activation of transcription factors and the turning on of gene transcription.
Commercial Applications of Ethylene
Ethylene is widely used in agriculture. Commercial fruit growers can buy equipment to generate ethylene so that their harvest ripens quickly and uniformly. Horticulturalists inhibit leaf dropping in ornamental plants by removing ethylene from greenhouses using fans and ventilation.
Attributions
Curated and authored by Melissa Ha from the following sources:
4.4.06: Summary Table
Table \(1\): Summary of the sources, transport mechanism, and major actions of the five main plant hormones
Hormone Source(s) Transport mechanism Major actions
Auxin Apical meristems, young leaves, and developing seeds Polar transport or nonpolar transport through the phloem Tropisms, embryo and leaf development, root initiation, apical dominance, flowering and fruit development, prevention of abscission
Cytokinins Root tips and other young tissues Xylem Gravitropism, cell division, germination, leaf formation, inhibition of apical dominance, inhibition of root initiation
Gibberellins Young shoot tissues and seeds Likely vascular tissue Germination, shoot elongation, bolting, flowering, production of male flowers, fruit maturation
Ethylene Stressed, wilting, or ripening tissues Moves through the air as a gas Abscission, senescence, fruit ripening, triple response, production of female flowers
Abscisic acid Mature leaves and roots Vascular tissue Seed maturation and inhibition of germination, bud dormancy, stomatal closure
Attribution
Melissa Ha (CC-BY-NC) | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/04%3A_Plant_Physiology_and_Regulation/4.04%3A_Hormones/4.4.05%3A_Ethylene.txt |
Learning Objectives
• Identify several signaling molecules beyond the five major plant hormones and describe their effects.
• Distinguish between the hypersensitive response and systemic acquired response.
• Explain the mechanisms by which signaling compounds aid in plant defense against pathogens and herbivores.
Recent research has discovered a number of compounds that also influence plant development. Their roles are less understood than the effects of the major hormones described so far.
Brassinosteroids
Brassinosteroids (Figure \(1\)) are synthesized primarily in young tissues are important to many developmental and physiological processes. In fact, many sources considere them the sixth major plant hormones. Unlike the hormones discussed previously, brassinosteroids do not travel far from their site of synthesis. Signals between these compounds and other hormones, notably auxin and GAs, amplifies their physiological effect. Apical dominance, seed germination, gravitropism, lateral root formation, differentiation of cells in the vascular tissue, and resistance to freezing are all positively influenced by brassinosteroids. Root growth and fruit dropping are inhibited by steroids.
Systemin
Systemin, named for the fact that it is distributed systemically (everywhere) in the plant body upon production, is a short polypeptide that activates plant responses to wounds from herbivores (animals that feed on plant parts). It causes the plant to produce jasmonic acid (see below).
Jasmonates
Jasmonates play a major role in defense responses to herbivory (Figure \(2\)). Their levels increase when a plant is wounded by an herbivore, resulting in an increase in toxic secondary metabolites. For example, jasmonic acid (Figure \(3\)) also induces transcription of protease inhibitors. Protease inhibitors both taste bad and prevent breakdown of proteins in the herbivore’s gut, thus making the insect sick and deterring further herbivory. Jasmonates also contribute to the production of volatile compounds that attract natural enemies of herbivores. Chewing of tomato plants by caterpillars leads to an increase in jasmonic acid levels, which in turn triggers the release of volatile compounds that attract predators of the pest. Jasmonates also elicit the synthesis of volatile compounds that attract parasitoids, which are insects that spend their developing stages in or on another insect, and eventually kill their host.
Jasmonates also work with systemin to mediate responses to drought, damage by ground-level ozone, and ultraviolet light.
Salicylic Acid
Salicylic acid resembles aspirin (Figure \(4\)) and is important for plant defense. It initiates the a systemic (whole body) response called the systemic acquired response (SAR) as a response to infection by parasites or pathogens. When a parasite or pathogen infects a cell, there is an specific, localized response called the hypersensitive response (HR). Following this very localized response, the plant initiates a systemic (whole body) response called the systemic acquired response (SAR). Salicylic acid is produced and converted to methyl salicylate (Figure \(4\)) inducing the SAR in response to the HR. The SAR activates transcription of general “pathogenesis-resistance” genes, which are not pathogen-specific (unlike in the hypersensitive response), but serve as general defense against pathogenic infection. The SAR is slower than the hypersensitive response, and also differs in that it is systemic instead of localized to the site of the infection.
Similar to jasmonic acid, salicylic acid can mediates defense against insect herbivores. It is directly toxic to some herbivores. Additionally, in response to herbivory, salicylic acid can be converted to methyl salicylate, which is released as a gas. This volatile compound can attract natural predators and parasites of the herbivores.
Some plants, such as skunk cabbage (Figure \(5\)) and elephant yam, are adapted to flower while snow still covers the ground. Salicylic acid mediates their ability to produce heat to melt the snow around them. Such plants are thus called thermogenic ("heat producing").
Oligosaccharins
Oligosaccharins are short chains of simple sugars that play a role in plant defense against bacterial and fungal infections. They act locally at the site of injury, and can also be transported to other tissues.
Strigolactones
Strigolactones (Figure \(6\)) promote seed germination in some species and inhibit lateral apical development in the absence of auxins. Strigolactones also play a role in the establishment of mycorrhizae, a mutualistic association of plant roots and fungi.
Florigen
Florigen is a systemic signal that initiates flowering. It is also involved in the formation of storage organs and contributes to plant architecture. It is synthesized in leaves and transported to the shoot apical meristem (SAM) where it promotes flowering in response to daylength cues. At the molecular level, florigen is represented as a protein product encoded by the FLOWERING LOCUS T (FT) gene, which is highly conserved (occurs/has a similar genetic sequence in) across flowering plants.
Florigen is considered one of the important targets for crop improvement. Regulation of flowering time is an important target for plant breeding because the control of flowering to a favorable time provides successful grain production in a given cropping area. Flowering at unfavorable seasons causes loss of yield due to insufficient growth of photosynthetic organs or poor fertility due to heat or cold stress during reproduction. Thus, understanding florigen function can contribute to novel breeding techniques in crops to produce cultivars that can start their reproductive stage at optimal seasons.
Supplemental Reading
Filgueiras, C. C., Martins, A. D., Pereira, R. V., & Willett, D. S. (2019). The Ecology of Salicylic Acid Signaling: Primary, Secondary and Tertiary Effects with Applications in Agriculture. International journal of molecular sciences, 20 (23), 5851. https://doi.org/10.3390/ijms20235851
Attributions
Curated and authored by Melissa Ha from the following sources:
4.4.08: Chapter Summary
Plant hormones—naturally occurring compounds synthesized in small amounts—can act both in the cells that produce them and in distant tissues and organs. Auxins are responsible for apical dominance, root growth, directional growth toward light, and many other growth responses. Cytokinins stimulate cell division and counter apical dominance in shoots. Gibberellins inhibit dormancy of seeds and promote stem growth. Abscisic acid induces dormancy in seeds and buds, and protects plants from excessive water loss by promoting stomatal closure. Ethylene gas speeds up fruit ripening and dropping of leaves. Other signaling molecules, such as salicylic acid and jasmonates are important to plant defense.
After completing this chapter, you should be able to...
• Explain the defining characteristics of a hormone.
• Name the five major plant hormones and identify their locations of synthesis, transport, and actions.
• Describe interactions among the five major plant hormones.
• Explain the mechanism of polar auxin transport.
• Define apical dominance and explain the role of auxin in maintaining it.
• Interpret and predict the outcome of an experiments demonstrating the actions of auxin and gibberellic acid.
• Describe the commercial applications of auxin, ethylene, and gibberellins.
• Identify several signaling molecules beyond the five major plant hormones and describe their effects.
• Distinguish between the hypersensitive response and systemic acquired response.
• Explain the mechanisms by which signaling compounds aid in plant defense against pathogens and herbivores.
Attribution
Curated and authored by Melissa Ha from 30.6 Plant Sensory Systems and Responses from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org. | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/04%3A_Plant_Physiology_and_Regulation/4.04%3A_Hormones/4.4.07%3A_Other_Signaling_Molecules.txt |
The structure of plant roots, stems, and leaves facilitates the transport of water, nutrients, and photosynthates throughout the plant. The phloem and xylem are the main tissues responsible for this movement. Water and nutrients are absorbed from the soil by roots ands transported through the xylem. Much water is lost through the stomata in the leaves, and plants have a variety of adaptations to reduce water loss (Figure \(1\)). The products of photosynthesis move through the phloem from sources to the tissues and organs that need them. These mechanisms of transport allow plant organs to specialize because they can export excess substances and import what they do not produce or collect locally.
Attribution
Curated and authored by Melissa Ha using 30.5 Transport of Water and Solutes in Plants from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org.
Thumbnail image: A potometer is used to measure transpiration rate, the loss of water through the stomata. Image by Theresa Knott and Rachel Knott (CC-BY-SA).
4.05: Transport
Water potential, transpiration, and stomatal regulation influence how water and nutrients are transported in plants. Water potential refers to the potential energy in water, and water moves towards the areas with the lowest water potential. Water is ultimately pulled to the top of the plant (cohesion-tension theory), and lost through transpiration through stomata. Complex mechanisms control stomatal opening and closure. Both adaptations that increase absorption of water through the roots (Figure \(1\)) and those that limit transpiration ensure that plants collect and retain enough water.
Attribution
Curated and authored by Melissa Ha using 30.5 Transport of Water and Solutes in Plants from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org.
4.5.01: Water Transport
Learning Objectives
• Define water potential and explain how it is influenced by solutes, pressure, gravity, and the matric potential.
• Explain which components of water potential plants can manipulate and describe how.
Plants are phenomenal hydraulic engineers. Using only the basic laws of physics and the simple manipulation of potential energy, plants can move water to the top of a tree approaching 116 m (~381 ft, see The Tallest Trees box). Plants can also use hydraulics to generate enough force to split rocks and buckle sidewalks (Figure $1$). Plants achieve this because of water potential.
Water potential is a measure of the potential energy in water. Plant physiologists are not interested in the energy in any one particular aqueous system, but they are very interested in water movement between two systems. In practical terms, therefore, water potential is the difference in potential energy between a given water sample and pure water (at atmospheric pressure and ambient temperature). Water potential is denoted by the Greek letter ψ (psi) and is expressed in units of pressure (pressure is a form of energy) called megapascals (MPa). The potential of pure water (Ψwpure H2O) is, by convenience of definition, designated a value of zero (although pure water contains plenty of potential energy, that energy is ignored). Water potential values for the water in a plant root, stem, or leaf are therefore expressed relative to Ψwpure H2O.
The water potential in plant solutions is influenced by solute concentration, pressure, gravity, and factors called matrix effects. Water potential can be broken down into its individual components using the following equation:
$\psi_\text{system} = \psi_\text{total} = \psi_s + \psi_p + \psi_g + \psi_m$
where Ψs, Ψp, Ψg, and Ψm refer to the solute, pressure, gravity, and matric potentials, respectively. “System” can refer to the water potential of the soil water (Ψsoil), root water (Ψroot), stem water (Ψstem), leaf water (Ψleaf) or the water in the atmosphere (Ψatmosphere): whichever aqueous system is under consideration. As the individual components change, they raise or lower the total water potential of a system. When this happens, water moves to equilibrate, moving from the system or compartment with a higher water potential to the system or compartment with a lower water potential. This brings the difference in water potential between the two systems (ΔΨ) back to zero (ΔΨ = 0). Therefore, for water to move through the plant from the soil to the air (a process called transpiration), Ψsoil must be > Ψroot > Ψstem > Ψleaf > Ψatmosphere.
Water only moves in response to ΔΨ, not in response to the individual components. However, because the individual components influence the total Ψsystem, by manipulating the individual components (especially Ψs), a plant can control water movement. Solutes, pressure, gravity, and matric potential are all important for the transport of water in plants. Water moves from an area of higher total water potential to an area of lower total water potential.
Solute Potential
Solute potentials), also called osmotic potential, is negative in a plant cell and zero in distilled water. Typical values for cell cytoplasm are –0.5 to –1.0 MPa. Solutes reduce water potential (resulting in a negative Ψw) by consuming some of the potential energy available in the water. Solute molecules can dissolve in water because water molecules can bind to them via hydrogen bonds; a hydrophobic molecule like oil, which cannot bind to water, cannot go into solution. The energy in the hydrogen bonds between solute molecules and water is no longer available to do work in the system because it is tied up in the bond. In other words, the amount of available potential energy is reduced when solutes are added to an aqueous system. Thus, Ψs decreases with increasing solute concentration.
Because Ψs is one of the four components of Ψsystem or Ψtotal, a decrease in Ψs will cause a decrease in Ψtotal. Water moves towards areas of lower Ψs (and thus lower Ψtotal). In Figure $2$, the semipermeable membrane that separates the two sides of the tube allows water but not solutes to pass. In the first tube, solute has been added to the right side. Adding solute to the right side lowers Ψs, causing water to move to the right side of the tube. As a result, the water level is higher on the right side.
The internal water potential of a plant cell is more negative than pure water because of the cytoplasm’s high solute content. Because of this difference in water potential water will move from the soil into a plant’s root cells via the process of osmosis. This is why solute potential is sometimes called osmotic potential. Plant cells can metabolically manipulate Ψs (and by extension, Ψtotal) by adding or removing solute molecules. Therefore, plants have control over Ψtotal via their ability to exert metabolic control over Ψs.
Pressure Potential
Pressure potentialp), also called turgor potential, may be positive or negative (Figure $\PageIndex{c}$). Because pressure is an expression of energy, the higher the pressure, the more potential energy in a system, and vice versa. Therefore, a positive Ψp (compression) increases Ψtotal, and a negative Ψp (tension) decreases Ψtotal. The second tube in Figure $2$) has pure water on both sides of the membrane. Positive pressure is applied to the left side. Applying positive pressure to the left side causes Ψp to increase. As a results, water moves to the right so that the water level is higher on the right than on the left. The third tube also has pure water, but this time negative pressure is applied to the left side. Applying negative pressure lowers Ψp, causing water to move to the left side of the tube. As a result, the water level is higher on the left.
Positive pressure inside cells is contained by the cell wall, producing turgor pressure. Pressure potentials are typically around 0.6–0.8 MPa, but can reach as high as 1.5 MPa in a well-watered plant. A Ψp of 1.5 MPa equates to 210 pounds per square inch (1.5 MPa x 140 lb in-2 MPa-1 = 210 lb/in-2). As a comparison, most automobile tires are kept at a pressure of 30–34 psi. An example of the effect of turgor pressure is the wilting of leaves and their restoration after the plant has been watered (Figure $3$). Water is lost from the leaves via transpiration (approaching Ψp = 0 MPa at the wilting point) and restored by uptake via the roots.
A plant can manipulate Ψp via its ability to manipulate Ψs and by the process of osmosis. If a plant cell increases the cytoplasmic solute concentration, Ψs will decline, Ψtotal will decline, the ΔΨ between the cell and the surrounding tissue will decline, water will move into the cell by osmosis, and Ψp will increase. Ψp is also under indirect plant control via the opening and closing of stomata. Stomatal openings allow water to evaporate from the leaf, reducing Ψp and Ψtotal of the leaf and increasing it between the water in the leaf and the petiole, thereby allowing water to flow from the petiole into the leaf.
Gravitational Potential
Gravitational potentialg) is always negative to zero in a plant with no height. The force of gravity pulls water downwards to the soil, reducing the difference in water potential between the leaves at the top of the plant and the roots. The taller the plant, the taller the water column, and the more influential Ψg becomes. On a cellular scale and in short plants, this effect is negligible and easily ignored. However, over the height of a tall tree like a giant coastal redwood, the gravitational pull of –0.1 MPa m-1 is equivalent to an extra 1 MPa of resistance that must be overcome for water to reach the leaves of the tallest trees. Plants are unable to manipulate Ψg.
Matric Potential
Matric potentialm) is always negative to zero. In a dry system, it can be as low as –2 MPa in a dry seed, and it is zero in a water-saturated system. The binding of water to a matrix always removes or consumes potential energy from the system. Ψm is similar to solute potential because it involves tying up the energy in an aqueous system by forming hydrogen bonds between the water and some other component. However, in solute potential, the other components are soluble, hydrophilic solute molecules, whereas in Ψm, the other components are insoluble, hydrophilic molecules of the plant cell wall. Every plant cell has a cellulosic cell wall and the cellulose in the cell walls is hydrophilic, producing a matrix for adhesion of water: hence the name matric potential. Ψm is very large (negative) in dry tissues such as seeds or drought-affected soils. However, it quickly goes to zero as the seed takes up water or the soil hydrates. Ψm cannot be manipulated by the plant and is typically ignored in well-watered roots, stems, and leaves.
Attribution
Curated and authored by Melissa Ha using 30.5 Transport of Water and Solutes in Plants from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org. | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/04%3A_Plant_Physiology_and_Regulation/4.05%3A_Transport/4.5.01%3A_Water_Transport/4.5.1.01%3A_Water_Potential.txt |
Learning Objective
Define transpiration and identify how environmental factors affect transpiration rate.
Less than 1% of the water reaching the leaves is used in photosynthesis and plant growth. Most of it is lost in transpiration, the loss of water vapor to the atmosphere through stomata. It is a passive process, meaning that metabolic energy in the form of ATP is not required for water movement. The energy driving transpiration is the difference in water potential between the soil and the atmosphere. Transpiration serve two functions- it provides the force for lifting the water up the stems and it cools the leaves. However, the volume of water lost in transpiration can be very high. It has been estimated that over the growing season, one acre of corn (maize) plants may transpire 400,000 gallons (1.5 million liters) of water. As liquid water, this would cover the field with a lake 15 inches (38 cm) deep.
Transpiration involves several cellular structures in the leaf (Figure \(1\)). Water on the surface of mesophyll cells saturates the cellulose microfibrils of the primary cell wall. The leaf contains many large intercellular air spaces for the exchange of oxygen for carbon dioxide, which is required for photosynthesis. The wet cell wall is exposed to this leaf internal air space, and the water on the surface of the cells evaporates into the air spaces, decreasing the thin film on the surface of the mesophyll cells.
Several environmental factors influence transpiration rate, which can be measured with a potometer (Figure \(2\)). Light, high temperatures, and wind increase transpiration rate while humidity reduces it. Light stimulates stomatal opening (see Stomatal Opening and Closure), allowing water vapor to easily leave the leaf. Light also speeds up transpiration by warming the leaf. Plants transpire more rapidly at higher temperatures because water evaporates more rapidly as the temperature rises. At 30°C, a leaf may transpire three times as fast as it does at 20°C. Humidity decreases transpiration rate by reducing the difference in water potential between the air and intercellular air spaces. The rate of diffusion of any substance increases as the difference in concentration of the substances in the two regions increases. When the surrounding air is dry, diffusion of water out of the leaf goes on more rapidly. As transpiration occurs, the air surrounding a leaf becomes increasingly humid, reducing the difference in water potential between the intercellular air spaces and the atmosphere and slowing transpiration. When a breeze is present, however, the humid air is carried away and replaced by drier air. Plants can regulate transpiration through opening and closing stomata, and plants that live in especially hot or dry regions have special adaptations to reduce water loss.
Attributions
Curated and authored by Melissa Ha using the following sources:
4.5.1.02: Transpiration
Learning Objective
Describe the various adaptations that help plants reduce transpiration rate.
Plants have evolved over time to adapt to their local environment and reduce transpiration. Leaves are covered by a waxy cuticle on the outer surface that prevents the loss of water. Plants that grow in dry environments and plants that grow on other plants (epiphytes) have a much thicker waxy cuticle than those growing in more moderate, well-watered environments (Figure \(1\)). Additionally, they often have a thick covering of trichomes or of stomata that are sunken below the leaf’s surface (Figure \(2\)). These adaptations impede air flow across the stomatal pore and reduce transpiration. Multiple epidermal layers are also commonly found in these types of plants .
The size and shape of photosynthetic structures also influences transpiration rate. Succulent plants, common in deserts, have thick, fleshy leaves or stems (Figure \(1\), left). Other plants, such as the evergreen shrubs of the chaparral, have small, thick, tough leaves (Figure \(3\)). Compared to thin, broad leaves, these shapes reduce surface area-to-volume ratio and decreases the opportunity for water loss. Plants with thin, broad leaves that live in climates with hot, dry seasons (such as chaparral or tropical forests that have a wet and dry season) may be deciduous, losing their leaves during these seasons to limit transpiration (Figure \(4\)).
As discussed in 13.7: Photorespiration and Photosynthetic Pathways, CAM plants close their stomata during the day when light and high temperatures would otherwise increase transpiration rate. C4 plants reduce the need to frequently open stomata by creating a high carbon dioxide concentration in the bundle sheath cells, which conduct the Calvin cycle. Regardless of photosynthetic pathway, plants can open and close stomata to regulate transpiration rate based on environmental conditions.
Attribution
Curated and authored by Melissa Ha using 30.5 Transport of Water and Solutes in Plants from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org. | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/04%3A_Plant_Physiology_and_Regulation/4.05%3A_Transport/4.5.01%3A_Water_Transport/4.5.1.02%3A_Transpiration/4.5.1.2.01%3A_Adaptations_to_Reduce_Transpiration.txt |
Learning Objectives
• Relate the pattern of cell wall thickening in guard cells to their function.
• Explain the mechanism by which blue light triggers stomatal opening.
• Explain the mechanism by which water stress, signaled by abscisic acid, triggers stomatal closure.
Regulation of transpiration is achieved primarily through the opening and closing of stomata on the leaf surface. Stomata are surrounded by two specialized cells called guard cells (Figure \(1\)). Stomata must open to allow the gas exchange of carbon dioxide and oxygen for efficient photosynthesis (see Photorespiration), and light thus typically triggers stomatal opening. When stomata are open, however, water vapor is lost to the external environment, increasing the rate of transpiration. Therefore, plants must maintain a balance between gas exchange and water loss. Water stress, high temperatures, and high carbon dioxide concentration causes stomata to close.
Stomatal Opening
Guard cell walls are radially thickened such that the thickenings are concentrated around the stoma (plural: stomata; Figure \(2\)). When turgor pressure increases in guard cells, the cells swell. However, the thickened inner walls near the stoma cannot expand, so they curve to accommodate the expanding outer walls. The curving of the guard cells opens the stoma.
How does light cause stomata to open? Phototropins detect blue light, causing a proton pumps to export protons (H+). ATP, generated by the light reactions of photosynthesis, drives the pump. The cytosol usually more negative than the extracellular solution, and this difference in charge (membrane potential) increases as protons leave the cell. This increase in membrane potential is called hyperpolarization, and it causes potassium (K+) to move down its electrochemical gradient into the cytosol. Protons also move down their electrochemical gradient back into the cytosol, bringing chloride (Cl-) with them through symport channels. Meanwhile, starch is broken down, producing sucrose and malate. Nitrate (NO3-) also enters the cell. The solute potential resulting high concentrations of potassium, chloride, sucrose, malate, and nitrate in the cytosol drives the osmosis of water into the the guard cells. This increases turgor pressure, and the guard cells expand and bend, opening the stoma (Figure \(3\)).
Table \(1\) illustrates how osmotic pressure (which results in turgor pressure) increases with light availability during the day. When the osmotic pressure of the guard cells became greater than that of the surrounding cells, the stomata opened. In the evening, when the osmotic pressure of the guard cells dropped to nearly that of the surrounding cells, the stomata closed.
Table \(1\): Osmotic pressure measured at different times of day in typical guard cells. The osmotic pressure within the other cells of the lower epidermis remained constant at ~1 MPa.
Time Osmotic Pressure (MPa)
7 A.M. 1.46
11 A.M. 3.14
5 P.M. 1.88
12 Midnight 1.32
Stomatal Closure
When water is low, roots synthesize abscisic acid (ABA), which is transported through the xylem to the leaves. There, abscisic acid causes calcium channels to open. Calcium (Ca2+) opens anion channels, and malate, chloride, and nitrate exit the cell. The membrane potential decreases (the difference in charge across the membrane becomes less pronounced) as anions leave the cell. Potassium exits the cell in response to this decrease in membrane potential (called depolarization). The loss of these solutes in the cytosol results in water leaving the cell and a decrease in turgor pressure. The guard cells regain their original shape, and the stoma closes (Figure \(4\)).
Attributions
Curated and authored by Melissa Ha using the following sources: | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/04%3A_Plant_Physiology_and_Regulation/4.05%3A_Transport/4.5.01%3A_Water_Transport/4.5.1.02%3A_Transpiration/4.5.1.2.02%3A_Stomatal_Opening_and_Closure.txt |
Learning Objectives
• Explain how water moves upward through a plant according to the cohesion-tension theory.
• Provide experimental evidence for the cohesion-tension theory.
In 1895, the Irish plant physiologists H. H. Dixon and J. Joly proposed that water is pulled up the plant by tension (negative pressure) from above. As we have seen, water is continually being lost from leaves by transpiration. Dixon and Joly believed that the loss of water in the leaves exerts a pull on the water in the xylem ducts and draws more water into the leaf. But even the best vacuum pump can pull water up to a height of only 10.4 m (34 ft) or so. This is because a column of water that high exerts a pressure of 1.03 MPa just counterbalanced by the pressure of the atmosphere. How can water be drawn to the top of a sequoia, the tallest is 113 m (370 ft) high? Taking all factors into account, a pull of at least ~1.9 MPa is probably needed.
The answer to the dilemma lies the cohesion of water molecules; that is the property of water molecules to cling to each through the hydrogen bonds they form (Figure \(1\)). When ultrapure water is confined to tubes of very small bore, the force of cohesion between water molecules imparts great strength to the column of water. It has been reported that tensions as great as 21 MPa are needed to break the column, about the value needed to break steel wires of the same diameter. In a sense, the cohesion of water molecules gives them the physical properties of solid wires.
According to the cohesion-tension theory, transpiration is the main driver of water movement in the xylem. It creates negative pressure (tension) equivalent to –2 MPa at the leaf surface. Water from the roots is ultimately pulled up by this tension. Negative water potential draws water from the soil into the root hairs, then into the root xylem. Cohesion and adhesion draw water up the xylem. Transpiration draws water from the leaf through the stoma. Water potential becomes increasingly negative from the root cells to the stem to the highest leaves, and finally to the atmosphere (Figure \(2\)).
The water potential at the leaf surface varies greatly depending on the vapor pressure deficit, which can be negligible at high relative humidity (RH) and substantial at low RH. At night, when stomata typically shut and transpiration stops, the water is held in the stem and leaf by the adhesion of water to the cell walls of the xylem vessels and tracheids, and the cohesion of water molecules to each other.
The mechanism of the cohesion-tension theory is based on purely physical forces because the xylem vessels and tracheids are not living at maturity. Evaporation of water into the intercellular air spaces creates a greater tension on the water in the mesophyll cells , thereby increasing the pull on the water in the xylem vessels. The xylem vessels and tracheids are structurally adapted to cope with large changes in pressure. Rings in the vessels maintain their tubular shape, much like the rings on a vacuum cleaner hose keep the hose open while it is under pressure. Small perforations between vessel elements reduce the number and size of gas bubbles that can form via a process called cavitation. The formation of gas bubbles in xylem interrupts the continuous stream of water from the base to the top of the plant, causing a break termed an embolism in the flow of xylem sap. The taller the tree, the greater the tension forces needed to pull water, and the more cavitation events. In larger trees, the resulting embolisms can plug xylem vessels, making them non-functional.
The pulling force due to transpiration is so powerful that it enables some trees and shrubs to live in seawater. Seawater is markedly hypertonic to the cytoplasm in the roots of the red mangrove (Rhizophora mangle), and we might expect water to leave the cells resulting in a loss in turgor and wilting. However, the remarkably high tensions in the xylem (~3 to 5 MPa) can pull water into the plant against this osmotic gradient. Mangroves literally desalt seawater to meet their needs.
Experimental evidence supports the cohesion-tension theory. Over a century ago, a German botanist who sawed down a 21-m (70-ft) oak tree and placed the base of the trunk in a barrel of picric acid solution. The solution was drawn up the trunk, killing nearby tissues as it went. If the roots were the driving force, upward water movement would have stopped as soon as the acid killed the roots. However, the solution reached the top of the tree. When the acid reached the leaves and killed them, the water movement ceased, demonstrating that the transpiration in leaves was causing the water the upward movement of water.
According to the cohesion-tension theory, the water in the xylem is under tension due to transpiration. Consistent with this prediction, the diameter of Monterey pines decreases during the day, when transpiration rates are greatest (Figure \(3\)). Because the water column is under tension, the xylem walls are pulled in due to adhesion.
The Tallest Trees
By spinning branches in a centrifuge, it has been shown that water in the xylem avoids cavitation at negative pressures exceeding ~1.6 MPa. And the fact that giant redwoods (Sequoia sempervirens, Figure \(4\)) can successfully lift water 109 m (358 ft), which would require a tension of ~1.9 MPa, indicating that cavitation is avoided even at that value. However, such heights may be approaching the limit for xylem transport. Measurements close to the top of one of the tallest living giant redwood trees, 112.7 m (~370 ft), show that the high tensions needed to transport water have resulted in smaller stomata, causing lower concentrations of CO2 in the needles, reduced photosynthesis, and reduced growth (smaller cells and much smaller needles; Koch et al. 2004). The limits on water transport thus limit the ultimate height which trees can reach. The tallest living tree is a 115.9-m giant redwood, and the tallest tree ever measured, a Douglas fir, was 125.9 m.
Reference: Koch, G., Sillett, S., Jennings, G. et al. The limits to tree height. Nature 428, 851–854 (2004). https://doi.org/10.1038/nature02417
Supplemental Reading
Woodward, I. Tall storeys. Nature 428, 807–808 (2004). https://doi.org/10.1038/428807a
Attributions
Curated and authored by Melissa Ha using the following sources: | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/04%3A_Plant_Physiology_and_Regulation/4.05%3A_Transport/4.5.01%3A_Water_Transport/4.5.1.03%3A_Cohesion-Tension_Theory.txt |
Learning Objectives
• Explain the function of root hairs.
• Define root pressure and explain its mechanism.
• Contrast the three pathways of water movement through the roots and identify each cell type or tissue involved.
Most plants secure the water and minerals they need from their roots. Water moves from the soil to the roots, stems, and ultimately the leaves, where transpiration occurs. The roots absorb enough water to compensate for water lost to transpiration. Rapid absorption is aided by root hairs, which extend from epidermal cells, increasing surface area (Figure \(1\)). As discussed earlier in this chapter, roots draw water from the soil because they have lower water potential than the soil. Much of this difference in water potential is an indirect result of transpiration, but roots can also water potential by decreasing solute potential.
Root Pressure
When a tomato plant is carefully severed close to the base of the stem, sap oozes from the stump (Figure \(2\)). The fluid comes out because roots are constantly absorbing water, drawing it into the vascular cylinder, and pushing it up the xylem. This is called root pressure, and it is created by the osmotic pressure of solutes trapped in the vascular cylinder by the Casparian strip.
Although root pressure plays a small role in the transport of water in the xylem in some plants and in some seasons, it does not account for most water transport. As evidence, few plants develop root pressures greater than ~0.2 kPa, and some develop no root pressure at all. Additionally, the volume of fluid transported by root pressure is not enough to account for the measured movement of water in the xylem of most trees and vines. Furthermore, the highest root pressures occur in the spring, but water moves through the xylem most rapidly in the summer (when transpiration is high).
As discussed in the Cohesion-Tension Theory section, transpiration, rather than root pressure, is typically the driving force for upward water movement in a plant. However, when transpiration rates are very low, such as in cool and humid weather, root pressure pushes water up the xylem faster than water is lost through the stomata. As a result water droplets are forced out of openings on the leaf margin, a phenomenon called guttation (Figure \(3\)). Droplets resulting from guttation are not to be confused with dew droplets, which result from the condensation of water vapor when the air becomes colder and has less capacity to hold water. In other words, guttation results from water that was inside of the plant, but dew droplets form from water vapor that was in the surrounding air.
Pathways of Water Movement
Water can move through the roots by three separate pathways: apoplast, symplast, and transmembrane (transcellular). In the apoplast pathway (apoplastic route), water moves through the spaces between the cells and in the cells walls themselves. In the symplast pathway (symplastic route), water passes from cytoplasm to cytoplasm through plasmodesmata (Figure \(4\)). In the transmembrane pathway, water crosses plasma membranes, entering and exiting each cell. Water moving through the transmembrane pathway thus moves through both the symplast (interconnected cytoplasms) and apoplast (cell walls and spaces in between cells). Water may also cross the tonoplast, entering the central vacuole as part of the transmembrane pathway.
Water from the soil is absorbed by the root hairs of the epidermis and then moves through the cortex through one of the three pathways. However, the inner boundary of the cortex, the endodermis, is impervious to water due to the Casparian strip. Regardless of how the water moved up to this point (apoplast, symplast, transmembrane), it must enter the cytoplasms of the endodermal cells. From here it can pass via plasmodesmata into the cells of the vascular cylinder (stele). Once inside, water is again free to move through the apolast, the symplast, or both (transmembrane).
Once in the xylem, water with the minerals that have been deposited in it (as well as occasional organic molecules supplied by the root tissue) move up in the vessel elements and tracheids. At any level, the water can leave the xylem and pass laterally to supply the needs of other tissues. At the leaves, the xylem passes into the petiole and then into the veins of the leaf. Water leaves the finest veins and enters the cells of the spongy and palisade layers. Here some of the water may be used in metabolism, but most is lost in transpiration.
Figure \(4\) illustrates minerals moving through the apoplast or symplast, but minerals typically move through the symplast. Minerals enter the root by active transport into the symplast of epidermal cells and move toward and into the vascular cylinder through the plasmodesmata connecting the cells. They enter the conducting cells of the xylem from the pericycle and other parenchyma cells via active transport through specialized transmembrane channels.
Attributions
Curated and authored by Melissa Ha using the following sources: | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/04%3A_Plant_Physiology_and_Regulation/4.05%3A_Transport/4.5.01%3A_Water_Transport/4.5.1.04%3A_Water_Absorption.txt |
Learning Objectives
• Distinguish between sources and sinks and provide examples of each.
• Explain the process of phloem loading, distinguishing between apoplastic and symplastic pathways.
• Explain how assimilate translocations through the phloem according to the pressure-flow hypothesis.
Plants need an energy source to grow. In seeds and bulbs, food is stored in polymers (such as starch) that are converted by metabolic processes into sucrose for newly developing plants. Once green shoots and leaves are growing, plants are able to produce their own food through photosynthesis. The products of photosynthesis (mainly the sugar sucrose) are a major component of the substance found in the phloem, called assimilate. Ions, amino acids, certain hormones, and other molecules are also found in assimilate. The movement of assimilate is called translocation, or assimilate transport.
Sources and Sinks
Structures that produce or release sugars for the growing plant are referred to as sources. Examples include mature leaves, which produce sugar through photosynthesis, and storage organs, such bulbs, tubers, or storage roots. Sources produce or store more sugars than they need themselves and can thus export sugars. The points of sugar delivery, such as most roots, young shoots, and developing fruits and seeds, are called sinks (Figure \(1\)). Because sinks do not produce enough sugars to meet their energy needs, they must import sugars from sources.
The pattern of assimilate flow changes as the plant grows and develops. Sugars are directed primarily to the roots early on, to shoots and leaves during vegetative growth, and to seeds and fruits during reproductive development. They are also directed to storage structures. Thus, the same organ may function as a source or a sink depending on the stage of development. For example, a young leaf may initially be a sink, but it will eventually grow and conduct enough photosynthesis to become a source. Similarly, a developing seed is a sink as the embryo develops. Once the seed germinates, however, starches stored in the seed break down, act as a source for the growing seedling structures.
The products from the source are usually translocated to the nearest sink through the phloem. For example, the highest leaves will send sugars upward to the growing shoot tip, whereas lower leaves will direct sugars downward to the roots. Intermediate leaves will send products in both directions, unlike the flow in the xylem, which is always unidirectional (soil to leaf to atmosphere).
Phloem Loading
Before discussing how sources transport sugars into the phloem (phloem loading), let's first review the conducting cells of the phloem. Stacks of these cylindrical cells called sieve-tube elements form column-like structures. Each cell is separated by a sieve plate (sieve-tube plate). The sieve plate has holes in it, like a slice of Swiss cheese, allowing for translocation. Lateral sieve areas on the side of the column allow different phloem tubes to interact. Sieve-tube elements have reduced cytoplasmic contents and rely on special parenchyma cells called companion cells, which assist with metabolic activities and provide energy (Figure \(2\)). In addition to sieve-tube elements and companion cells, the phloem contains other parenchyma cells and may contain sclerenchyma fibers. In leaves, the phloem is found in vascular bundles (leaf veins), which are surrounded by a bundle sheath. Sugars are produced through photosynthesis in the mesophyll cells that fill most of the leaf (see diagram in Transpiration section).
There are two pathways of phloem loading (Figure \(3\)). Both pathways begin the same way. The cytoplasms of most plant cells are interconnected via plasmodesmata.Augars through plasmodesmata from mesophyll cells to bundle sheath cells to the parenchyma cells of the phloem. From there, the pathways differ. The first pathway called apoplastic phloem loading, occurs when the regular parenchyma cells of phloem are not connected to the companion cells. In this case, sucrose must exist the regular parenchyma cells and enter the apoplast. It enters the companion cell cytoplasm as well as the inside of the sieve tube through secondary active transport. Protons are pumped into the apoplast (outside of the cells), when the protons re-enter the cells (move down their concentration gradient), they bring sucrose with them by flowing through carrier proteins called sucrose-proton symporters. In this way, sucrose is actively transported against its concentration gradient, and high sucrose concentrations accumulate in the assimilate of the phloem. The second pathway is symplastic phloem loading. When the regular parenchyma cells of the phloem are connected to companion cells via plamodesmata, sucrose can flow through the symplast all the way to the sieve-tube elements. There is no need for sucrose to leave the cytoplasm (enter the apoplast) and re-enter cells via sucrose-proton symport.
Pressure-Flow Hypothesis
What drives the movement of assimilate in the phloem? According to the pressure-flow hypothesis, assimilate moves due to osmosis of water to and from the xylem. While this is a passive process, it ultimately results from the active transport of sugars during phloem loading and unloading. In contrast with transpiration, an entirely passive process, translocation as a passive process that is indirectly driven by active processes.
Assimilate contains up to 30 percent sugar, and this high solute concentration decreases Ψs, which decreases the total water potential. The sugar concentration of the assimilate near sinks, where phloem loading occurs, is highest. This causes water to move by osmosis from the adjacent xylem into sieve tubes, thereby increasing pressure. The increase in pressure increases in total water potential causes the bulk flow of phloem from source to sink (Figure \(4\)). Sugar concentration in the sink cells is lower than in the sieve-tube elements because the sink sucrose has been metabolized for growth, or converted to starch for storage or other polymers, such as cellulose, for structural integrity. Unloading at the sink end of the phloem tube occurs by either diffusion or active transport of sucrose molecules from an area of high concentration to one of low concentration. Unloading reduces the sugar concentration of assimilate near sinks, increasing water potential. As a result, water moves from the phloem by osmosis and is then transpired or recycled via the xylem back into the phloem sap.
Attribution
Curated and authored by Melissa Ha using 30.5 Transport of Water and Solutes in Plants from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org.
4.5.03: Chapter Summary
Water and minerals move through the xylem, and sugars move through the phloem. Water potential and transpiration influence how water is transported through the xylem in plants as described by the cohesion-tension theory. Water potential (Ψ) is a measure of the difference in potential energy between a water sample and pure water and is influenced by solute concentration, pressure, gravity, and matric potential. Transpiration is primarily regulated by stomatal opening and closure. Roots must absorb water to replace the water lost through transpiration, and the high solute concentrations in the vascular cylinder results in root pressure. Assimilate containing sucrose move from sources to sinks through the plant’s phloem. Phloem loading increases solute concentration and causes water to move by osmosis from the xylem into the phloem. The positive pressure that is produced pushes water and solutes down the pressure gradient. The sucrose is unloaded into the sink, and the water returns to the xylem vessels.
After completing this chapter, you should be able to...
• Define water potential and explain how it is influenced by solutes, pressure, gravity, and the matric potential.
• Explain which components of water potential plants can manipulate and describe how.
• Define transpiration and identify how environmental factors affect transpiration rate.
• Describe the various adaptations that help plants reduce transpiration rate.
• Relate the pattern of cell wall thickening in guard cells to their function.
• Explain the mechanism by which blue light triggers stomatal opening.
• Explain the mechanism by which water stress, signaled by abscisic acid, triggers stomatal closure.
• Explain how water moves upward through a plant according to the cohesion-tension theory.
• Provide experimental evidence for the cohesion-tension theory.
• Explain the function of root hairs.
• Define root pressure and explain its mechanism.
• Contrast the three pathways of water movement through the roots and identify each cell type or tissue involved.
• Distinguish between sources and sinks and provide examples of each.
• Explain the process of phloem loading, distinguishing between apoplastic and symplastic pathways.
• Explain how assimilate translocations through the phloem according to the pressure-flow hypothesis.
Attribution
Curated and authored by Melissa Ha using 30.5 Transport of Water and Solutes in Plants from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org.
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Development is the process by which an organism changes over its life. In angiosperms, development after sperm from the pollen tube fertilizes the egg in the ovule, forming the diploid zygote. Through cell division and differentiation, the zygote forms a mature embryo, which lies dormant within the seed until germination (Figure \(1\)). A seedling emerges and continues to develop into mature plant, which will ultimately flower and undergo pollination, completing the life cycle.
The variety of tissues and organs in the plant body result from differences in gene expression. While all the diploid cells in a plant have the same genetic information, which genes are active in each structure varies, resulting in a complex, multicellular organism.
Attribution
Melissa Ha (CC-BY-NC)
Thumbnail image: Different stages of germination in Arabidopsis. Image by Alena Kravchenko (CC-BY-SA)
4.06: Development
Learning Objectives
• Explain the steps of embryogensis in eudicots.
• Compare embryogenesis in eudicots versus monocots.
The Seed Plants chapter discussed the fertilization of the egg within the ovule. The zygote ultimately divides to produce the mature embryo, the ovule develops into a seed, and the ovary that contained one or more ovules develops into a fruit. A typical seed contains a seed coat, cotyledons, endosperm, and a single embryo. The development of the embryo occurs through a process called embryogenesis.
Embryogenesis in Eudicots
After fertilization in eudicots, the zygote (Figure \(\PageIndex{1-I}\)) divides to form two cells: the upper apical cell and the lower basal cell . The division of the basal cell gives rise to the suspensor, which attaches the embryo to the micropyle (the pore through which the pollen tube original entered). The suspensor provides a route for nutrition to be transported from the mother plant to the growing embryo. The apical cell also divides, initially producing a proembryo (figure \(\PageIndex{1-II}\)).
As the proembryo continues to divide, it takes a spherical form, called the globular stage (Figures \(\PageIndex{1-III}\) and \(\PageIndex{2a}\)). The globular is the first stage that is considered the embryo proper. Next, cotyledons arise from the embryo proper, forming the heart stage (Figures \(\PageIndex{1-IV}\) and \(\PageIndex{2-4}\)). Cotyledons are embryonic leaf-life structures that function in food storage, food absorption and/or photosynthesis. As the cotyledons elongate, and the base of the embryo thickens, it results a torpedo. (This stage is called the torpedo stage; Figures \(\PageIndex{1-V}\) and \(\PageIndex{2c}\)). Cell division is concentrated at the shoot apical meristem, located at the shoot tip in between the cotyledons, and the root apical meristem at the most basal (bottom) part of the embryo. Most of the suspensor deteriorates during the torpedo stage.
The final stage of embryogenesis results in the mature embryo (Figures \(\PageIndex{1-VI}\) and \(\PageIndex{2d}\)). The mature embryo includes an embryonic root called the radicle. The embryo becomes dormant at this point, halting metabolic activity and cell division. At this point, the seed is ready for dispersal. Growth resumes when the seed germinates and the embryo develops into a seedling.
In some eudicots, the endosperm (triploid tissue that was formed when a sperm fertilized the two polar nuclei) cells divide, and endosperm fills a substantial portion of the mature seed. The endosperm stores nutrients. In other (non-endospermic) eudicots, such as Capsella bursa-pastoris, the endosperm develops initially, but is then digested, and the nutrients are moved into the two cotyledons (Figure \(2\)). After germination, the developing seedling relies on these food reserves stored in the endosperm or cotyledons until the first set of leaves begin photosynthesis.
Embryogenesis in Monocots
The process of embryogenesis in monocots is similar to that of eudicots, but as there is only a single cotyledon, no heart stage occurs. Instead, the embryo proper of the monocot becomes cylindrical at this point in development. The shoot apical meristem, while still present at the shoot tip, is not in between cotyledons in the monocot (because there is only a single cotyledon).
Attribution
Curated and authored by Melissa Ha using 32.2 Pollination and Fertilization from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org.
4.6.02: Meristems
Learning Objective
Define and locate the two apical meristems and the three primary meristems.
Meristems are centers of cell division and growth. In animals, totipotent stem cells, which can differentiate into any tissue type are only found early in development; however, plants contains such embryonic tissues throughout their lives.
Apical meristems are located on the very ends of shoots (shoot apical meristem; SAM; Figure \(1\)) and roots (root apical meristem; RAM; Figure \(2\)). They produce three types primary meristems: the protoderm, ground meristem, and procambium. The protoderm gives rise to epidermis, which surrounds the plant. The ground meristem gives rise to ground tissue, a group of tissues with generalized functions such as photosynthesis, storage, and support. Finally, the procambium gives rise to the vascular tissue, which functions in transport. The three primary meristems first appear in the embryo proper, with the protoderm on the outside, the procambium in the center, and the ground meristem in between them.
Secondary meristems (lateral meristems) result in secondary growth, a woody increase in girth. These include the vascular cambium and cork cambium. The vascular cambium arises from from the procambium and pericycle in roots. In stems, it arises from procambium cells of the vascular bundles (fascicular cambium) and parenchyma cells between vascular bundles (interfascicular cambium). The vascular cambium gives rise to secondary phloem (part of the bark) and secondary xylem (wood; Figures \(\PageIndex{4-5}\)).
The cork cambium arises from the pericycle in roots and the parenchyma cells of the cortex in stems, both of which arise from the ground meristem. The cork cambium produces periderm, secondary dermal tissue that is also a component of bark. (See Roots and Secondary Stem for more details; Figures \(\PageIndex{4-5}\)).)
Other meristems include intercalary meristems which elongate stems from the “middle” (in between nodes) and marginal meristems, which are located along leaf edges and are responsible for leaf development.
Attribution
Curated and authored by Melissa Ha using 5.1 Tissues from Introduction to Botany by Alexey Shipunov (public domain) | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/04%3A_Plant_Physiology_and_Regulation/4.06%3A_Development/4.6.01%3A_Embryogenesis.txt |
Learning Objectives
• Locate the major seed structures and identify the function of each.
• Compare eudicot and monocot seeds.
Eudicot Seeds
The seed is protected by a seed coat that is formed from the integument of the ovule (Figure \(1\)). The seed coat is further divided into an outer coat known as the testa and inner coat known as the tegmen. The hilum is a scar on the outside of the seed where it was attached to the endocarp (inner layer of the fruit wall). The micropyle is a small round structure next to the hilum where the pollen tube entered.
The embryonic axis (root-shoot axis) runs the length of the embryo. On end of the embryonic axis is the plumule, the young shoot apex, which includes the shoot apical meristem and developing leaves (leaf primordia). At the other end of the embryonic axis is the radicle (embryonic root). In some species, the radicle is not apparent in the embryo (in which case the distal end of the root is simply the root tip). The embryonic axis does not include the cotyledons. The portion of the embryo between the cotyledon attachment point and the radicle is known as the hypocotyl (hypocotyl means “below the cotyledons”). The portion of the embryonic axis between the cotyledon attachment and the shoot tip is the epicotyl (epicotyl means "above the cotyledons; Figures \(\PageIndex{2-3}\))). Some embryos lack a visible epicotyl because the cotyledons are attached to the embryonic axis at the shoot tip.
The two cotyledons in the eudicot seed are connected to the rest of the embryo via vascular tissue (xylem and phloem). In endospermic dicots, the food reserves are stored in the endosperm. During germination, the two cotyledons therefore act as absorptive organs to take up the enzymatically released food reserves. Tobacco (Nicotiana tabaccum), tomato (Solanum lycopersicum), and pepper (Capsicum annuum) are examples of endospermic dicots. In non-endospermic dicots, the triploid endosperm develops normally following double fertilization, but the endosperm food reserves are quickly remobilized and moved into the developing cotyledon for storage. The two halves of a peanut seed (Arachis hypogaea; Figure \(4\)) or a bean (Phaseolus; Figures \(\PageIndex{2-3}\)) and the split peas (Pisum sativum; Figure \(5\)) of split pea soup are individual cotyledons loaded with food reserves.
Because seeds have food reserves to fuel germination, they also are a nutritious food source for people. Studying the nutrient content of seed crops such as beans can be used to increase the nutritive value of the plant using biotechnology. Maria Elena Zavala (Figure \(6\)) is working to combat world hunger by manipulating plants to improve their nutritional qualities. For example, her research with beans is looking at how genetic engineering can be used to make the bean proteins more digestible and nutritious.
Monocot Seeds
The seeds of the most complex monocot family, Poaceae (the grass family), which includes corn and wheat, are highly specialized. The testa and tegmen of the seed coat are fused. The fruit is a caryopsis (grain), a one-seeded fruit in which the fruit wall (pericarp) is fused to the seed coat. Thus, not only are the two layers of the seed coat fused, but the seed coat is fused to the pericarp.
The single cotyledon is called a scutellum; the scutellum also has vascular connections to the rest of the embryo. The large inner layer of the endosperm that stores nutrients is called the starchy endosperm. The thin outer layer of the endosperm, which is a single layer of cells, is called the aleurone. Upon germination, enzymes are secreted by the aleurone. The enzymes degrade the stored carbohydrates, proteins and lipids, the products of which are absorbed by the scutellum and transported via a vasculature strand to the developing embryo. Therefore, the scutellum can be seen to be an absorptive organ, not a storage organ.
The root tip is protected by a sheath-like structure called the coleorhiza. Similarly, the coleoptile ensheaths the plumule at the shoot tip (Figure \(\PageIndex{7-10}\)).
Other Variations
Seeds are diverse. Pine (Pinus, a gymnosperm and thus neither a monocot nor eudicot) has multiple (five or more) cotyledons. Some plants like orchids (Orchidaceae, a monocot) do not have developed embryo and even endosperm in seeds; their germination depends on a presence of symbiotic (mycorrhizal) fungus.
Attributions
Curated and authored by Melissa Ha using the following sources: | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/04%3A_Plant_Physiology_and_Regulation/4.06%3A_Development/4.6.03%3A_Mature_Embryos_and_Seed_Structure.txt |
Learning Objectives
• Identify the environmental factors that stimulate germination.
• Distinguish between epigeous and hypogeous germination.
• Compare germination in eudicots versus monocots.
Many mature seeds enter a period of inactivity, or extremely low metabolic activity: a process known as dormancy, which may last for months, years or even centuries. Dormancy helps keep seeds viable during unfavorable conditions. Germination occurs when the embryo, which is dormant within a mature seed, resumes growth upon a return to favorable conditions. The embryo becomes a young seedling that is no longer confined within the seed coat.
In many seeds, the presence of a thick seed coat can inhibit germination through several mechanisms: (1) the embryo may not be able to break through the thick seed coat; (2) the seed coat may contain chemicals inhibitors; and (3) the seed coat prevents the embryo from accessing water and oxygen. Dormancy is also maintained by the relative hormone concentrations in the embryo itself.
Environmental Requirements for Germination
The requirements for germination depend on the species. Common environmental requirements include light, the proper temperature, presence of oxygen, and presence of water. Seeds of small-seeded species usually require light as a germination cue. This ensures the seeds only germinate at or near the soil surface (where the light is greatest). If they were to germinate too far underneath the surface, the developing seedling would not have enough food reserves to reach the sunlight. (Recall from 14.5 Dormancy that red light induces germination by converting the inactive form of phytochrome (Pr) to the active form (Pfr), which leads to the production of amylase. This enzyme breaks down the limited food reserves in the seed, facilitating germination.)
Not only do some species require a specific temperature to germinate, but they may also require a prolonged cold period prior to germination. In this case, cold conditions gradually break down a chemical germination inhibitor. This mechanism prevents seeds from germinating during an unseasonably warm spell in the autumn or winter in temperate climates. Similarly, plants growing in hot climates may have seeds that need a hot period in order to germinate, an adaptation to avoid germination in the hot, dry summers.
Water is always needed to allow vigorous metabolism to begin. Additionally, water can leach away inhibitors in the seed coat. This is especially common among desert annuals. Seeds that are dispersed by animals may need to pass through an animal digestive tract to remove inhibitors prior to germination. Similarly, some species require mechanical abrasion of the seed coat, which could be achieved by water dispersal. Other species are fire adapted, requiring fire to break dormancy (Figure \(1\)).
The Mechanism of Germination
The first step in germination and starts with the uptake of water, also known as imbibition. After imbibition, enzymes are activated that start to break down starch into sugars consumed by embryo. The first indication that germination has begun is a swelling in the radicle.
Depending on seed size, the time taken for a seedling to emerge may vary. Species with large seeds have enough food reserves to germinate deep below ground, and still extend their epicotyl all the way to the soil surface while the seedlings of small-seeded species emerge more quickly (and can only germinate close to the surface of the soil).
During epigeous germination, the hypocotyl elongates, and the cotyledons extend above ground. During hypogeous germination, the epicotyl elongates, and the cotyledon(s) remain belowground (Figure \(2\)). Some species (like beans and onions) have epigeous germination while others (like peas and corn) have hypogeous germination. In many epigeous species, the cotyledons not only transfer their food stores to the developing plant but also turn green and make more food by photosynthesis until they drop off.
Germination in Eudicots
Upon germination in eudicot seeds, the radicle emerges from the seed coat while the seed is still buried in the soil.
For epigeous eudicots (like beans), the hypocotyl is shaped like a hook with the plumule pointing downwards. This shape is called the plumule hook, and it persists as long as germination proceeds in the dark. Therefore, as the hypocotyl pushes through the tough and abrasive soil, the plumule is protected from damage. Additionally, the two cotyledons additionally protect the from mechanical damage. Upon exposure to light, the hypocotyl hook straightens out, the young foliage leaves face the sun and expand, and the epicotyl elongates (Figure \(3\)).
In hypogeous eudicots (like peas), the epicotyl rather than the hypocotyl forms a hook, and the cotyledons and hypocotyl thus remain underground. When the epicotyl emerges from the soil, the young foliage leaves expand. The epicotyl continues to elongate (Figure \(4\)).
The radicle continues to grown downwards and ultimately produces the tap root. Lateral roots then branch off to all sides, producing the typical eudicot tap root system.
Germination in Monocots
As the seed germinates, the radicle emerges and forms the first root. In epigeous monocots (such as onion), the single cotyledon will bend, forming a hook and emerge before the coleoptile (Figure \(5\)). In hypogeous monocots (such as corn), the cotyledon remains belowground, and the coleoptile emerges first. In either case, once the coleoptile has exited the soil and is exposed to light, it stops growing. The first leaf of the plumule then pieces the coleoptile (Figure \(6\)), and additional leaves expand and unfold. At the other end of the embryonic axis, the first root soon dies while adventitious roots (roots that arise directly from the shoot system) emerge from the base of the stem (Figure \(7\)). This gives the monocot a fibrous root system.
Attributions
Curated and authored by Melissa Ha using the following sources: | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/04%3A_Plant_Physiology_and_Regulation/4.06%3A_Development/4.6.04%3A_Germination.txt |
Learning Objective
• Define the roles of the shoot apical meristem, leaf primordia, intercalary meristems, and axillary buds in shoot development.
• Explain the mechanism of the ABCDE model of floral development.
Some plant parts, such as stems and roots, continue to grow throughout a plant’s life: a phenomenon called indeterminate growth. Other plant parts, such as leaves and flowers, exhibit determinate growth, which ceases when a plant part reaches a particular size.
Stem and Leaf Development
In comparison to roots, in which cell division, elongation, and maturation occur in distinct regions (see Internal Root Structure), growth in stems occurs in several regions. Rapid cell division occurs at the shoot apical meristem of the shoot tip. Some of the cells produced from these divisions give rise to leaf primordia, which ultimately develop into leaves. Division, elongation, and cell differentiation (maturation) also occurs at the intercalary meristems of internodes, the stem segments between nodes, the points where leaves attach. As a result, multiple internodes can lengthen simultaneously. Under certain circumstances, the axillary buds at each node can become active and produce axillary shoots, which branch from the main stem.
Flower Development
Changes in gene expression can cause a shoot apical meristem to develop into a floral meristem, which leads to flowering. The ABCDE model (ABC model) of floral development directs the formation of each of the four whorls of a complete flower. From outside to inside, these whorls are the calyx (sepals), corolla (petals), androecium (stamens), and gynoecium (carpels). (See Angiosperms for more details; Figure \(1\).) Each unit of these whorls (sepal, petal, etc.) developmentally and evolutionarily originated from the leaf.
The ABCDE model refers to five groups of homeotic genes, each represented by one letter. Homeotic genes are those that control the development and organization of body parts (body plan). Each of the five groups of genes that encode the transcription factors (proteins required for gene expression), which turn on the genes for development of each whorl. Genetic analysis of mutants especially those found in the Arabidopsis thaliana and in the snapdragon (Antirrhinum) support the ABCDE model of flowering. Each group of genes has a distinct role as follows (Figure \(2\)):
• A genes are needed for sepal and petal development
• B genes are needed for petal and stamen development.
• C gene are needed for stamen and carpel (including ovules) development.
• D genes are need for ovule development (part of the carpels).
• E genes are needed to produce any of the floral whorls.
Thus, each whorl results from the expression of a different gene combination as follows:
• Expression of A and E genes produce sepals.
• Expression of A, B, and E genes produce petals.
• Expression of B, C, and E genes produce stamens.
• Expression of C and E genes produce carpels. To produce ovules, which are considered part of the carpels, D genes must also be expressed (C+D+E = ovules).
In summary, the formation of a flower requires a cascade of sequential gene activity that gradually converts a mass of undifferentiated cells (the shoot apical meristem) into the parts of a flower. The genes encode transcription factors that act as master switches, turning on (or off) downstream genes that ultimately make each part of the flower in its appropriate location.
Attributions
Curated and authored by Melissa Ha using the following sources:
4.6.06: Chapter Summary
During embryogenesis (the development of the embryo), the proembryo develops into the embryo proper and finally into the mature embryo. The mature embryo is surrounded by the seed coat. Depending on the species, nutrient-rich endosperm may remain in the mature seed between the embryo and seed coat. The embryo consists of the cotyledon(s) and embryonic axis, consisting of the epicotyl and hypocotyl. The young root is called the radicle. Monocot seeds differ from eudicot seeds in that they contain a single cotyledon (scutellum) and protective sheaths surrounding the young shoot and root, the coleoptile and coleorrhiza, respectively.
Cell division and growth are concentrated at meristems. The shoot apical meristem and root apical meristem produce three primary meristems: the protoderm, ground meristem, and procambium. The cork cambium and vascular cambium are secondary meristems, which allow woody plants to increase in girth.
The embryo then remains dormant until germination. Various environmental conditions such as proper temperature, light, and water may be required for germination depending on the species. Germination may be epigeous, in which cotyledon(s) rise above the ground, or hypogeous, in which cotyledon(s) remain belowground.
Rapid cell division at the shoot apical meristem initiates shoot development, but growth also occurs in the internodes throughout the shoot. Floral meristems develop from shoot apical meristems, and floral structure is controlled by the ABCDE model.
After completing this chapter, you should be able to...
• Explain the steps of embryogensis in eudicots.
• Compare embryogenesis in eudicots versus monocots.
• Define and locate the two apical meristems and the three primary meristems.
• Locate the major seed structures and identify the function of each.
• Compare eudicot and monocot seeds.
• Identify the environmental factors that stimulate germination.
• Distinguish between epigeous and hypogeous germination.
• Compare germination in eudicots versus monocots.
• Define the roles of the shoot apical meristem, leaf primordia, intercalary meristems, and axillary buds in shoot development.
• Explain the mechanism of the ABCDE model of floral development.
Attribution
Melissa Ha (CC-BY-NC) | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/04%3A_Plant_Physiology_and_Regulation/4.06%3A_Development/4.6.05%3A_Shoot_Development.txt |
Ecology is the study of the interaction of organisms with their biotic (living) and abiotic (non-living) environment. Plant ecologists study plants across many different biomes, both terrestrial and aqautic. They may be interested in studying plants at different levels of organization, such as the population level, the community level, or the ecosystem level. Many plant ecologists are interested in conservation biology, protecting the diversity of plants from human impacts. Understanding the connections between plants and other organisms helps us solve scientific problems and understand our world better.
• 5.1: Population Ecology
Community ecology is the study of the interactions between species in communities on many spatial and temporal scales, including the distribution, structure, abundance, demography, and interactions between coexisting populations. The primary focus of community ecology is on the interactions between populations as determined by specific genotypic and phenotypic characteristics.
• 5.2: Communities and Ecosystems
All populations occupying the same habitat form a community: populations inhabiting a specific area at the same time.
• 5.3: Conservation
Conservation aims to protect biodiversity, the variety of life on Earth. There are three main levels of biodiversity: ecosystem, species, and genetic diversity. Each has value to humans. Threats to biodiversity include habitat loss, pollution, overexploitation, invasive species, and climate change. A variety of approaches, including legislation and ecosystem restoration, address these threats.
• 5.4: Terrestrial Biomes
A biome is a large geographical area characterized by climate and vegetation. Biomes may be terrestrial or aquatic, but this chapter will focus only on terrestrial biomes. Terrestrial biomes are characterized by their plant life. The types of plants depend on the amount of precipitation in an area as well as the temperature. Different adaptations evolve based on the abiotic factors.
05: Ecology and Conservation
Researchers study ecology at various levels. For example, at the organismal level, a scientist might be interested in the adaptations that enable individuals to live in specific habitats. These adaptations could be morphological (pertaining to the study of form or structure), physiological, or behavioral. In this chapter, we will focus on population ecology. A population is a group of interbreeding organisms that are members of the same species living in the same area at the same time. Organisms that are all members of the same species, a population, are called conspecifics (Figure \(1\)). A population is identified, in part, by where it lives; its area of population may have natural or artificial boundaries. Natural boundaries might be rivers, mountains, or deserts, while examples of artificial boundaries include mowed grass or manmade structures such as roads. The study of population ecology focuses on the number of individuals in an area and how and why population size changes over time. Population ecologists are particularly interested in counting the Karner blue butterfly, for example, because it is classified as federally endangered.
Attributions
Modified by Kammy Algiers from the following sources:
• Population and Community Ecology from General Biology by OpenStax (licensed CC-BY)
• Organismal Ecology & Population Ecology from General Biology by Boundless (licensed CC-BY-SA)
• 5.1.1: Population Size and Density
Populations are characterized by their population size and their population density. Various methods can be used to measure the size and density of a population. For example, scientists often use quadrats to do this for plants. Dispersion patterns can give scientists information about a particular population. Three common dispersion patters are uniform, random, and clumped.
• 5.1.2: Life Histories and Natural Selection
All species have evolved a pattern of living, called a life history strategy, in which they partition energy for growth, maintenance, and reproduction. These patterns evolve through natural selection. There are different life strategies that species may exhibit. A species may reproduce early in life to ensure surviving to a reproductive age or reproduce later in life to become larger and healthier. A species may reproduce once (semelparity) or many times (iteroparity) in its life.
• 5.1.3: Environmental Limits to Population Growth
Populations with unlimited resources grow exponentially, with an accelerating growth rate. When resources become limiting, populations follow a logistic growth curve. The population of a species will level off at the carrying capacity of its environment.
• 5.1.4: Population Dynamics and Regulation
Populations are regulated by a variety of density-dependent and density-independent factors. Species are divided into two categories based on a variety of features of their life history patterns: r-selected species, which have large numbers of offspring, and K-selected species, which have few offspring.
• 5.1.5: Chapter Summary
5.01: Population Ecology
Learning Objectives
• Describe how ecologists measure population size and density.
• Describe three different patterns of population distribution.
Population Size and Density
Populations are characterized by their population size (total number of individuals) and their population density (number of individuals per unit area). A population may have a large number of individuals that are distributed densely, or sparsely. There are also populations with small numbers of individuals that may be dense or very sparsely distributed in a local area. Population size can affect potential for adaptation because it affects the amount of genetic variation present in the population. Density can have effects on interactions within a population such as competition for food and the ability of individuals to find a mate.
Population Research Methods
Counting all individuals in a population is the most accurate way to determine its size. However, this approach is not usually feasible, especially for large populations or extensive habitats. Instead, scientists study populations by sampling, which involves counting individuals within a certain area or volume that is part of the population’s habitat. Analysis of sample data enables scientists to infer population size and population density about the entire population.
A variety of methods can be used to sample populations. Scientists usually estimate the populations of sessile or slow-moving organisms with the quadrat method. A quadrat is a square that encloses an area within a habitat. The area may be defined by staking it out with sticks and string, or using a square made of wood, plastic, or metal placed on the ground (Figure \(1\)).
A field study usually includes several quadrat samples at random locations or along a transect in representative habitat. After they place the quadrats, researchers count the number of individuals that lie within the quadrat boundaries. The researcher decides the quadrat size and number of samples from the type of organism, its spatial distribution, and other factors. For sampling daffodils, a 1 m2 quadrat could be appropriate. Giant redwoods are larger and live further apart from each other, so a larger quadrat, such as 100 m2, would be necessary. The correct quadrat size ensures counts of enough individuals to get a sample representative of the entire habitat.
Species Distribution
In addition to measuring simple density, further information about a population can be obtained by looking at the distribution of the individuals. Species dispersion patterns (or distribution patterns) show the spatial relationship between members of a population within a habitat at a particular point in time. In other words, they show whether members of the species live close together or far apart, and what patterns are evident when they are spaced apart.
Individuals of a population can be distributed in one of three basic patterns: they can be more or less equally spaced apart (uniform dispersion), dispersed randomly with no predictable pattern (random dispersion), or clustered in groups (clumped dispersion). (Figure \(2\)). Plants with wind-dispersed seeds disperse randomly, as they germinate wherever they happen to fall in a favorable environment. Uniform dispersion is observed in plants that secrete substances inhibiting the growth of nearby individuals (such as the release of toxic chemicals by the sage plant Salvia leucophylla, a phenomenon called allelopathy). A clumped dispersion may be seen in plants that drop their seeds straight to the ground, such as oak trees.
Contributors and Attributions
Curated and authored by Kammy Algiers from the following sources:
• Population Size and Density from General Biology by Boundless (licensed CC-BY-SA).
• Population Demographics and Dynamics from General Biology by OpenStax (licensed CC-BY) | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/05%3A_Ecology_and_Conservation/5.01%3A_Population_Ecology/5.1.01%3A_Population_Size_and_Density.txt |
Learning Objectives
• Describe how life history patterns are influenced by natural selection.
• Compare and contrast semelparity and iteroparity.
Life History Patterns
Energy is required by all living organisms for their growth, maintenance, and reproduction; at the same time, energy is often a major limiting factor in determining an organism’s survival. Plants, for example, acquire energy from the sun via photosynthesis, but must expend this energy to grow, maintain health, and produce energy-rich seeds to produce the next generation. All species have an energy budget: they must balance energy intake with their use of energy for metabolism, reproduction, and energy storage.
Reproductives strategies
Population size can be influenced by the time reproduction begins, how often the organism reproduces, and how many offspring the organism produces in one reproductive period. The timing of reproduction in a life history also affects species survival. Organisms that reproduce early on have a greater chance of producing offspring, but this is usually at the expense of their growth and the maintenance of their health. Conversely, organisms that begin to reproduce later in life often have greater fecundity (ability to produce many offspring), but they risk that they will not survive to reproductive age.
These different energy strategies and tradeoffs are key to understanding the evolution of each species as it maximizes its fitness and fills its niche. In terms of energy budgeting, some species “blow it all” and use up most of their energy reserves to reproduce early before they die. Other species delay having reproduction to become stronger, more experienced individuals and to make sure that they are strong enough to provide parental care if necessary.
For example, an oak tree grows much slower than an annual grass. Though this is a high investment, oaks are well adapted to expected stressors such as fires or drought. On the other hand, alders and willows grow and colonize open riparian areas very quickly.
Single versus Multiple Reproductive Events
Some life history traits, such as fecundity, timing of reproduction, and parental care, can be grouped together into general strategies that are used by multiple species. Semelparity occurs when a species reproduces only once during its lifetime and then dies, as do annual plants. Such species use most of their resource budget during a single reproductive event, sacrificing their health to the point that they do not survive. Some bamboo exhibit a semelparity life strategy: they flower once and then die. The century plant, or agave, also exhibits this strategy, reproducing once towards the end of its life (Figure \(1\)). Iteroparity describes a reproductive strategy where the species reproduces repeatedly during its life. Many plants reproduce using this strategy.
Contributors and Attributions
Curated and authored by Kammy Algiers using Life Histories and Energy Budgets from General Biology by Boundless (licensed CC-BY-SA)
5.1.03: Environmental Limits to Population Growth
Learning Objectives
• Compare and contrast between exponential and logistic growth patterns.
• Give examples of exponential and logistic growth in natural populations.
• Describe the role of carrying capacity in population growth.
• Describe the influences of intraspecific competition in population size.
Although life histories describe the way many characteristics of a population (such as their age structure) change over time in a general way, population ecologists make use of a variety of methods to model population dynamics mathematically. These more precise models can then be used to accurately describe changes occurring in a population and better predict future changes. Certain models that have been accepted for decades are now being modified or even abandoned due to their lack of predictive ability, and scholars strive to create effective new models.
Exponential Growth
Charles Darwin, in his theory of natural selection, was greatly influenced by the English clergyman Thomas Malthus. Malthus published a book in 1798 stating that populations with unlimited natural resources grow very rapidly, and then population growth decreases as resources become depleted. This accelerating pattern of increasing population size is called exponential growth.
Though bacteria are often the noted examples of species that grow exponentially, we do see this in many photosynthetic species as well. For example, algae and cyanobacteria will often grow exponentially for a period of time with warmer temperatures, both in freshwater (ex: lakes) or salt water (ex: oceans). Exponential growth occurs when the population growth rate—the number of organisms added in each reproductive generation—is accelerating; that is, it is increasing at a greater and greater rate. For example, a population of 1000 can increase by 1000 in one hour, but then increase by 2000 the second hour, and to 4000 the third hour, and 8000 by the fourth hour. The number of organisms increases faster at every reproduction event. After 1 day and 24 of these cycles, a population could have increased from 1000 to more than 16 billion. When the population size is plotted over time, a J-shaped growth curve is produced (Figure \(1\), left graph).
We often see examples of exponential growth for a period of time in nature. For example, after recovery, a plant species may exponentially grow for a period of time while establishing its previous niche. Non-native, invasive species, can often also grow exponentially, as they may not have the same environmental pressures (predators, parasites, competitors) in the introduced area and can increase dramatically once established.
Logistic Growth
In the real world, with its limited resources, exponential growth cannot continue indefinitely. Exponential growth may occur in environments where there are few individuals and plentiful resources, but when the number of individuals gets large enough, resources will be depleted, slowing the growth rate. Eventually, the growth rate will plateau or level off (Figure \(1\), right graph). The pattern formed in this type of growth is called logistic growth, or S-curve. This population size, which represents the maximum population size that a particular environment can sustain, is called the carrying capacity.
There are three different sections to an S-shaped curve. Initially, growth is exponential because there are few individuals and ample resources available. Then, as resources begin to become limited, the growth rate decreases. Finally, growth levels off at the carrying capacity of the environment, with little change in population size over time.
Role of Intraspecific Competition
The logistic model assumes that every individual within a population will have equal access to resources and, thus, an equal chance for survival. For plants, the amount of water, sunlight, nutrients, and the space to grow are the important resources.
In the real world, phenotypic variation among individuals within a population means that some individuals will be better adapted to their environment than others. The resulting competition between population members of the same species for resources is termed intraspecific competition (intra- = “within”; -specific = “species”). Intraspecific competition for resources may not affect populations that are well below their carrying capacity—resources are plentiful and all individuals can obtain what they need. However, as population size increases, this competition intensifies. In addition, the accumulation of waste products can reduce an environment’s carrying capacity.
Contributors and Attributions
Curated and authored by Kammy Algiers from the following sources:
• Exponential Population Growth from General Biology by Boundless (licensed CC-BY-SA)
• Logistic Population Growth from General Biology by Boundless (licensed CC-BY-SA) | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/05%3A_Ecology_and_Conservation/5.01%3A_Population_Ecology/5.1.02%3A_Life_Histories_and_Natural_Selection.txt |
Learning Objectives
• Compare and contrast density-dependent growth regulation and density-independent growth regulation.
• Compare and contrast K-selected and r-selected species using examples of their life history traits.
The logistic model of population growth, while valid in many natural populations and a useful model, is a simplification of real-world population dynamics. Implicit in the model is that the carrying capacity of the environment does not change, which is not the case. The carrying capacity varies annually: for example, some summers are hot and dry whereas others are cold and wet. In many areas, the carrying capacity during the winter is much lower than it is during the summer. Also, natural events such as earthquakes, volcanoes, and fires can alter an environment and hence its carrying capacity. Additionally, populations do not usually exist in isolation. They engage in interspecific competition: that is, they share the environment with other species, competing with them for the same resources. These factors are also important to understanding how a specific population will grow.
Nature regulates population growth in a variety of ways. These are grouped into density-dependent factors, in which the density of the population at a given time affects growth rate and mortality, and density-independent factors, which influence mortality in a population regardless of population density. Note that in the former, the effect of the factor on the population depends on the density of the population at onset. Conservation biologists want to understand both types because this helps them manage populations and prevent extinction or overpopulation.
Density-dependent Regulation
Most density-dependent factors are biological in nature (biotic), and include predation, inter- and intraspecific competition, and diseases such as those caused by parasites. Usually, the denser a population is, the greater its mortality rate. For example, during intra- and interspecific competition, the reproductive rates of the individuals will usually be lower, reducing their population’s rate of growth. In addition, low prey density increases the mortality of its predator because it has more difficulty locating its food source. High population density can also increase the transmission of parasites and prevalence of disease.
Density-independent Regulation
Many factors, typically physical or chemical in nature (abiotic), influence the mortality of a population regardless of its density, including weather, natural disasters, and pollution. A frosty night that emerging in early spring does not kill a higher or lower percentage of seedlings based on the density of the seedling population. Its chances of survival for the seedling are the same whether the population density is high or low.
In real-life situations, population regulation is very complicated and density-dependent and independent factors can interact. A dense population that is reduced in a density-independent manner by some environmental factor(s) will be able to recover differently than a sparse population.
Life Histories of K-selected and r-selected Species
While reproductive strategies play a key role in life histories, they do not account for important factors like limited resources and competition. The regulation of population growth by these factors can be used to introduce a classical concept in population biology, that of K-selected versus r-selected species.
K-selected species are species selected by stable, predictable environments. Populations of K-selected species tend to exist close to their carrying capacity (hence the term K-selected) where intraspecific competition is high. These species have few, large offspring, have more parental care, and have a longer period of maturation development period. In plants, scientists think of parental care more broadly: how long fruit takes to develop or how long it remains on the plant are determining factors in the time to the next reproductive event. Oak trees are great examples of K-selected species (Figure \(1\)). Oak trees grow very slowly and take, on average, 20 years to produce their first seeds, known as acorns. As many as 50,000 acorns can be produced by an individual tree, but the germination rate is low as many of these rot or are eaten by animals such as squirrels. In some years, oaks may produce an exceptionally large number of acorns, and these years may be on a two- or three-year cycle depending on the species of oak (r-selection).
As oak trees grow to a large size and for many years before they begin to produce acorns, they devote a large percentage of their energy budget to growth and maintenance. The tree’s height and size allow it to dominate other plants in the competition for sunlight, the oak’s primary energy resource. Furthermore, when it does reproduce, the oak produces large, energy-rich seeds that use their energy reserve to become quickly established (K-selection).
In contrast, r-selected species have a large number of small offspring (hence their r designation (Table \(1\)). This strategy is often employed in unpredictable or changing environments. Many weedy plants are considered r-selected. Dandelion are great examples of r-selected species, producing many small seeds on one flower head that are seed dispersed long distance (Figure \(3\)). Many seeds are produced simultaneously to ensure that at least some of them reach a hospitable environment. Seeds that land in inhospitable environments have little chance for survival since their seeds are low in energy content. Note that survival is not necessarily a function of energy stored in the seed itself.
Table \(1\): Characteristics of K-selected and r-selected species
Characteristics of K-selected species Characteristics of r-selected species
Mature late Mature early
Greater longevity Lower longevity
Increased parental care Decreased parental care
Increased competition Decreased competition
Fewer offspring More offspring
Larger offspring Smaller offspring
Contributors
Curated and authored by Kammy Algiers from the following sources:
• Density-Dependent and Density-Independent Population Regulation from General Biology by Boundless (licensed CC-BY-SA)
• Theories of Life History from General Biology by Boundless (licensed CC-BY-SA)
5.1.05: Chapter Summary
Populations are characterized by their population size (total number of individuals) and their population density (number of individuals per unit area). Counting all individuals in a population is the most accurate way to determine its size. However, this approach is not usually feasible, especially for large populations or extensive habitats. Instead, scientists study populations by sampling, which involves counting individuals within a certain area or volume that is part of the population’s habitat. Analysis of sample data enable scientists to infer population size and population density about the entire population. One method used the collect population size data in the field is using quadrat placed at random locations or along a transect in representative habitat. The size of the quadrat depends on the species monitoring. In addition to measuring simple density, further information about a population can be obtained by looking at the distribution of the individuals. Species may have uniform, random, or clumped distribution.
All species have evolved a pattern of living, called a life history strategy, in which they partition energy for growth, maintenance, and reproduction. These patterns evolve through natural selection; they allow species to adapt to their environment to obtain the resources they need to successfully reproduce. There are different life strategies that species may exhibit. A species may reproduce early in life to ensure surviving to a reproductive age or reproduce later in life to become larger and healthier. A species may reproduce once (semelparity) or many times (iteroparity) in its life.
Populations with unlimited resources grow exponentially, with an accelerating growth rate. When resources become limiting, populations follow a logistic growth curve. The population of a species will level off at the carrying capacity of its environment. Within populations, phenotypic variation among individuals means that some individuals will be better adapted to their environment than others. The resulting competition between population members of the same species for resources is termed intraspecific competition. Populations are regulated by a variety of density-dependent and density-independent factors. Species are divided into two categories based on a variety of features of their life history patterns: r-selected species, which have large numbers of offspring, and K-selected species, which have few offspring.
After completing this chapter, you should be able to...
• Describe how ecologists measure population size and density.
• Describe three different patterns of population distribution.
• Describe how life history patterns are influenced by natural selection.
• Compare and contrast semelparity and iteroparity.
• Compare and contrast between exponential and logistic growth patterns.
• Give examples of exponential and logistic growth in natural populations.
• Describe the role of carrying capacity in population growth.
• Describe the influences of intraspecific competition in population size.
• Compare and contrast density-dependent growth regulation and density-independent growth regulation.
• Compare and contrast K-selected and r-selected species using examples of their life history traits. | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/05%3A_Ecology_and_Conservation/5.01%3A_Population_Ecology/5.1.04%3A_Population_Dynamics_and_Regulation.txt |
Populations typically do not live in isolation from other species. Populations that interact within a given area form a community. The organisms that form a community are found in habitats, physical environments where organisms live. However, only the biotic (living) components are considered part of a community. Scientists study ecology at the community level to understand how species interact with each other and compete for the same resources.
Contributors and Attributions
Modified by Kammy Algiers from the following sources:
5.02: Communities and Ecosystems
Learning Objective
• Describe the difference between intraspecific and interspecific interactions in reference to competition
Biotic Interactions
Biotic interactions refer to the relationships among organisms. They can be intraspecific (between members of the same species) or interspecific (between members of different species). When at least one of the interacting organisms is harmed, the relationship is called an antagonism. Trophic interactions, in which one species consumes another, are antagonisms. Competition, commensalism, and mutualism are some examples of biotic interaction we find in communities.
Competition
Competition occurs when organisms use the the same resources and one or both organisms is harmed. Plants often compete with one another for light (Figure \(1\)). In the tropical rainforests, where water is not a limiting factor, and much of the carbon is aboveground, plants compete with one another intensely. Often times, this is an example of interspecific competition as various species of plants, with different adaptations, compete to get sunlight. However, many plants of the same species are competing for that limited space and sunlight as well. Thus, we also see intraspecific competition.
Attribution
Modified by Kammy Algiers from Community Ecology from General Biology by OpenStax (CC-BY)
5.2.02: Community Interactions
Learning Objectives
• Define herbivory
• Give examples of defenses against predation and herbivory
Herbivory
Herbivory describes the consumption of plants by animals, and it is another interspecific relationship that affects populations. Unlike animals, plants cannot outrun predators or use mimicry to hide from hungry animals (with a few exceptions). Some plants have developed mechanisms to defend against herbivory. Other species have developed strategies that rely on hungry animals; for example, herbivory provides a mechanism of seed distribution that aids in plant reproduction.
Defense Mechanisms against Predation and Herbivory
The study of communities must consider evolutionary forces that act on the members of the various populations contained within it. Species are not static, but slowly changing and adapting to their environment by natural selection and other evolutionary forces. Species have evolved numerous mechanisms to escape predation and herbivory. These defenses may be mechanical, chemical, physical, or behavioral.
Mechanical defenses, such as the presence of thorns on plants, may discourage herbivory by causing physical pain to the predator (Figure \(1\), top). Chemical defenses are also produced by many plants to discourage herbivory. Milkweed plants (Asclepias spp.) produce chemical defenses that protect them against most herbivores. However, as shown in Figure \(1\) (bottom), monarch caterpillars (Danaus plexippus) have evolved a way around these defenses. Monarch butterflies lay their eggs on milkweed plants, the preferred source of food for their developing larvae. If the caterpillars can survive the initial flood of sticky latex when they first bite into the leaf (a front line defense for the milkweed), they can continue to consume the tissues. These tissues contain toxins, but the caterpillar is able to store, rather than metabolize, these toxins and make its own body toxic to predators.
Plants can also be good sources of camouflage for animals. Often times, animals will blend in with the plants they sit on to avoid being detected by predators. The tropical walking stick is an insect with the coloration and body shape of a twig which makes it very hard to see when stationary against a background of real twigs (Figure \(2\)). In another example, the chameleon can change its color to match its plant surroundings (Figure \(2\)). Both of these are examples of camouflage, or avoiding detection by blending in with the background.
Contributors and Attributions
Modified by Kammy Algiers from the following sources: | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/05%3A_Ecology_and_Conservation/5.02%3A_Communities_and_Ecosystems/5.2.01%3A_Communities.txt |
Learning Objectives
• Describe what is considered a symbiotic relationships between species
• Compare and contrast between commensalism, mutualism, and parasitism
• Describe symbiosis as it relates to nitrogen fixation
• Describe how saprophytes, epiphytes, and carnivorous plants depend on other orgnanisms
Symbiosis
Symbiotic relationships, or symbioses (plural), are close interactions between individuals of different species over an extended period of time which impact the abundance and distribution of the associating populations. Most scientists accept this definition, but some restrict the term to only those species that are mutualistic, where both individuals benefit from the interaction. In this discussion, the broader definition will be used.
Commensalism
A commensal relationship occurs when one species benefits from the close, prolonged interaction, while the other neither benefits nor is harmed. Birds nesting in trees provide an example of a commensal relationship (Figure $1$). The tree is not harmed by the presence of the nest among its branches. The nests are light and produce little strain on the structural integrity of the branch, and most of the leaves, which the tree uses to get energy by photosynthesis, are above the nest so they are unaffected. The bird, on the other hand, benefits greatly. If the bird had to nest in the open, its eggs and young would be vulnerable to predators. Another example of a commensal relationship is the clown fish and the sea anemone. The sea anemone is not harmed by the fish, and the fish benefits with protection from predators who would be stung upon nearing the sea anemone.
Mutualism
A second type of symbiotic relationship is called mutualism, where two species benefit from their interaction. Some scientists believe that these are the only true examples of symbiosis. Many animal pollinators have coevolved with plants, including many insects (bees, flies, butterflies, moths), birds (hummingbirds), and some mammals (bats). The pollinator usually receives a reward in the form of nectar or pollen, while the plant is able to distribute its pollen (which will produce the male gametes) to another plant. Flowers have evolved in response to natural selection to attract pollinators by scent, color, shape, phenology, and availability of the reward. Some plants are generalist and are pollinated by many different kinds of pollinators. Other plants are specialists and pollinated by only a few taxa, or perhaps even a single pollinator species (Figure $2$).
Mycorrhizae: A Plant-Fungal Mutualism
One of the most remarkable associations between fungi and plants is the establishment of mycorrhizae. Mycorrhiza, which is derived from the Greek words myco meaning fungus and rhizo meaning root, refers to the fungal partner of a mutualistic association between vascular plant roots and their symbiotic fungi. Nearly 90 percent of all vascular plant species (and many nonvascular plant species) have mycorrhizal partners. In a mycorrhizal association, the fungal mycelia use their extensive network of hyphae and large surface area in contact with the soil to channel water and minerals from the soil into the plant. In exchange, the plant supplies the products of photosynthesis to fuel the metabolism of the fungus.
There are several basic types of mycorrhizae. Ectomycorrhizae (“outside” mycorrhizae) depend on fungi enveloping the roots in a sheath (called a mantle). Hyphae grow from the mantle into the root and envelope the outer layers of the root cells in a network of hyphae called a Hartig net Figure $3$. The fungal partner can belong to the Ascomycota, Basidiomycota or Zygomycota. Endomycorrhizae ("inside" mycorrhizae), also called arbuscular mycorrhizae, are produced when the fungi grow inside the root in a branched structure called an arbuscule (from the Latin for “little trees”). The fungal partners of endomycorrhizal associates all belong to the Glomeromycota. The fungal arbuscules penetrate root cells between the cell wall and the plasma membrane and are the site of the metabolic exchanges between the fungus and the host plant Figures $\PageIndex{3b}$ and $\PageIndex{4b}$. Orchids rely on a third type of mycorrhiza. Orchids are epiphytes that typically produce very small airborne seeds without much storage to sustain germination and growth. Their seeds will not germinate without a mycorrhizal partner (usually a Basidiomycete). After nutrients in the seed are depleted, fungal symbionts support the growth of the orchid by providing necessary carbohydrates and minerals. Some orchids continue to be mycorrhizal throughout their life cycle.
If symbiotic fungi were absent from the soil, what impact do you think this would have on plant growth?
Other examples of fungus–plant mutualism include the endophytes: fungi that live inside tissue without damaging the host plant. Endophytes release toxins that repel herbivores, or confer resistance to environmental stress factors, such as infection by microorganisms, drought, or heavy metals in soil.
Nitrogen Fixation: Root and Bacteria Interactions
Nitrogen is an important macronutrient because it is part of nucleic acids and proteins. Atmospheric nitrogen, which is the diatomic molecule $\ce{N2}$, or dinitrogen, is the largest pool of nitrogen in terrestrial ecosystems. However, plants cannot take advantage of this nitrogen because they do not have the necessary enzymes to convert it into biologically useful forms. However, nitrogen can be “fixed,” which means that it can be converted to ammonia ($\ce{NH3}$) through biological, physical, or chemical processes. As you have learned, biological nitrogen fixation (BNF) is the conversion of atmospheric nitrogen ($\ce{N2}$) into ammonia ($\ce{NH3}$), exclusively carried out by prokaryotes such as soil bacteria or cyanobacteria. Biological processes contribute 65 percent of the nitrogen used in agriculture. The following equation represents the process:
$\ce { N2 + 16 ATP + 8 e^{-} + 8 H^{+} \rightarrow 2 NH3 + 16 ADP + 16 P_i + H_2}$
The most important source of BNF is the symbiotic interaction between soil bacteria and legume plants, including many crops important to humans (Figure $5$). The NH3 resulting from fixation can be transported into plant tissue and incorporated into amino acids, which are then made into plant proteins. Some legume seeds, such as soybeans and peanuts, contain high levels of protein, and serve among the most important agricultural sources of protein in the world.
Farmers often rotate corn (a cereal crop) and soy beans (a legume), planting a field with each crop in alternate seasons. What advantage might this crop rotation confer?
Soil bacteria, collectively called rhizobia, symbiotically interact with legume roots to form specialized structures called nodules, in which nitrogen fixation takes place. This process entails the reduction of atmospheric nitrogen to ammonia, by means of the enzyme nitrogenase. Therefore, using rhizobia is a natural and environmentally friendly way to fertilize plants, as opposed to chemical fertilization that uses a nonrenewable resource, such as natural gas. Through symbiotic nitrogen fixation, the plant benefits from using an endless source of nitrogen from the atmosphere. The process simultaneously contributes to soil fertility because the plant root system leaves behind some of the biologically available nitrogen. As in any symbiosis, both organisms benefit from the interaction: the plant obtains ammonia, and bacteria obtain carbon compounds generated through photosynthesis, as well as a protected niche in which to grow (Figure $6$).
Parasitism
A parasite is an organism that lives in or on another living organism and derives nutrients from it. In this relationship, the parasite benefits and the organism being fed upon (the host) is harmed. The host is usually weakened by the parasite as it siphons resources the host would normally use to maintain itself. The parasite, however, is unlikely to kill the host, especially not quickly, because this would not provide enough time for the organism to complete its reproductive cycle by spreading to another host. Dodder is an annual vine that grows parasitically on shrubs (Figure $7$. Dodder has a weak, cylindrical stem that coils around the host and forms suckers. From these suckers, cells invade the host stem and grow to connect with the vascular bundles of the host. The parasitic plant obtains water and nutrients through these connections. The plant is a holoparasite because it is completely dependent on its host. Some parasitic plants, like leafy mistletoes, are fully photosynthetic and only use the host for water and minerals. These are considered hemiparasites. There are about 4,100 species of parasitic plants.
Heterotrophic Plants
Heterotrophic plants not have chlorophyll (Figure $8$). Instead, they steal sugars from other plants, sometimes through a mycorrhizal fungus. These latter plants are called mycoheterotrophs.
Epiphytes
An epiphyte is a plant that grows on other plants, but is not dependent upon the other plant for nutrition (Figure $9$). Epiphytes have two types of roots: clinging aerial roots, which absorb nutrients from humus that accumulates in the crevices of trees; and aerial roots, which absorb moisture from the atmosphere.
Insectivorous Plants
An insectivorous plant has specialized leaves to attract and digest insects. The Venus flytrap is popularly known for its insectivorous mode of nutrition, and has leaves that work as traps (Figure $10$). The minerals it obtains from prey compensate for those lacking in the boggy (low pH) soil of its native North Carolina coastal plains. There are three sensitive hairs in the center of each half of each leaf. The edges of each leaf are covered with long spines. Nectar secreted by the plant attracts flies to the leaf. When a fly touches the sensory hairs, the leaf immediately closes. Next, fluids and enzymes break down the prey and minerals are absorbed by the leaf. Since this plant is popular in the horticultural trade, it is threatened in its original habitat.
Contributors and Attributions
Modified by Kammy Algiers and Melissa Ha from the following sources: | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/05%3A_Ecology_and_Conservation/5.02%3A_Communities_and_Ecosystems/5.2.03%3A_Symbiosis.txt |
Learning Objectives
• Describe how species richness and relative abundance play a role on biodiversity
• Describe the role of keystone species in a community
• Descdribe the role of invasive species in a community
Biodiversity, Species Richness, and Relative Species Abundance
Biodiversity describes a community’s biological complexity: it is usually measured by the number of different species, or species richness, in a particular area and their relative abundance (evenness). However, genetic diversity is increasingly considered as a component of biodiversity. The area in question could be a habitat, a biome, or the entire biosphere. Species richness is the term that is used to describe the number of species living in a habitat or biome. Species richness varies across the globe (Figure \(1\)). One factor in determining species richness is latitude, with the greatest species richness occurring in ecosystems near the equator, which often have warmer temperatures, large amounts of rainfall, and low seasonality. The lowest species richness occurs near the poles, which are much colder, drier, and thus less conducive to life in Geologic time (time since glaciations). The predictability of climate or productivity is also an important factor. Other factors influence species richness as well. For example, the study of island biogeography attempts to explain the relatively high species richness found in certain isolated island chains, including the Galápagos Islands that inspired the young Darwin. Relative species abundance is the number of individuals in a species relative to the total number of individuals in all species within a habitat, ecosystem, or biome. Foundation species often have the highest relative abundance of species. For more on species richness, see The Value of Biodiversity.
Keystone Species
A keystone species is one whose presence is key to maintaining biodiversity within an ecosystem and to upholding an ecological community’s structure. A keystone species has a disproportionate impact on the overall ecosystem, meaning that even a small number of individuals can cause large scale changes. The intertidal sea star, Pisaster ochraceus, of the northwestern United States is a keystone species (Figure \(2\)). Studies have shown that when this organism is removed from communities, populations of their natural prey (mussels) increase. The increase in mussel results in a decrease in algae, wihch completely alters species composition and reduces biodiversity. Another keystone species is the banded tetra, a fish in tropical streams, which supplies nearly all of the phosphorus, a necessary inorganic nutrient, to the rest of the community. If these fish were to become extinct, the community would be greatly affected.
Invasive Species
Invasive species are exotic or non-native organisms that, when introduced to an area out of their native range, threaten the ecosystem balance of that habitat. They often increase in numbers and decrease the native species composition. Thus, they have a significant impact on the community. For example, pampas grass (Cortaderia selloana), a beloved ornamental found throughout the United States, is actually an invasive species native to South America. Pampas grass is fast growing and each plume produces up to 100,000 seeds. Seeds are wind dispersed and colonize quickly, particularly on disturbed ground. Leaves on this plant have sharp blades and can be harmful to birds. The native community in areas with invasive species often have less biodiversity because many native species in the area are eradicated by the presence of the invasive species. For more on invasive species, see 21.2: Threats to Biodiversity.
Contributors and Attributions
Modified by Kammy Algiers from the following sources:
19.4: Community Ecology - from Concepts of Biology by OpenStax (licensed CC-BY). | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/05%3A_Ecology_and_Conservation/5.02%3A_Communities_and_Ecosystems/5.2.04%3A_Biodiversity_in_Ecosystems.txt |
Learning Objective
Describe community structure and succession
Community Dynamics
Community dynamics are the changes in community structure and composition over time, often following environmental disturbances such as volcanoes, earthquakes, storms, fires, and climate change. Communities with a relatively constant number of species are said to be at equilibrium. The equilibrium is dynamic with species identities and relationships changing over time, but maintaining relatively constant numbers. Following a disturbance, the community may or may not return to the equilibrium state.
Succession describes the sequential appearance and disappearance of species in a community over time after a severe disturbance. In primary succession, newly exposed or newly formed rock is colonized by living organisms. In secondary succession, a part of an ecosystem is disturbed and remnants of the previous community remain. In both cases, there is a sequential change in species until a more or less permanent community develops.
Primary Succession and Pioneer Species
Primary succession occurs when new land is formed, or when the soil and all life is removed from pre-existing land. An example of the former is the eruption of volcanoes on the Big Island of Hawaii, which results in lava that flows into the ocean and continually forms new land. From this process, approximately 32 acres of land are added to the Big Island each year. An example of pre-existing soil being removed is through the activity of glaciers. The massive weight of the glacier scours the landscape down to the bedrock as the glacier moves. This removes any original soil and leaves exposed rock once the glacier melts and retreats.
In both cases, the ecosystem starts with bare rock that is devoid of life. New soil is slowly formed as weathering and other natural forces break down the rock and lead to the establishment of hearty organisms, such as lichens and some plants, which are collectively known as pioneer species (Figure \(1\) because they are the first to appear. These species help to further break down the mineral-rich rock into soil where other, less hardy but more competitive species, such as grasses, shrubs, and trees, will grow and eventually replace the pioneer species. Over time the area will reach an equilibrium state, with a set of organisms quite different from the pioneer species.
Secondary Succession
A classic example of secondary succession occurs in forests cleared by wildfire, or by clearcut logging (figure 2.2.2.c2.2.2.c). Wildfires will burn most vegetation, and unless the animals can flee the area, they are killed. Their nutrients, however, are returned to the ground in the form of ash. Thus, although the community has been dramatically altered, there is a soil ecosystem present that provides a foundation for rapid recolonization.
Before the fire, the vegetation was dominated by tall trees with access to the major plant energy resource: sunlight. Their height gave them access to sunlight while also shading the ground and other low-lying species. After the fire, though, these trees are no longer dominant. Thus, the first plants to grow back are usually annual plants followed within a few years by quickly growing and spreading grasses and other pioneer species. Due, at least in part, to changes in the environment brought on by the growth of grasses and forbs, over many years, shrubs emerge along with small trees. These organisms are called intermediate species. Eventually, over 150 years or more, the forest will reach its equilibrium point and resemble the community before the fire. This equilibrium state is referred to as the climax community, which will remain until the next disturbance. The climax community is typically characteristic of a given climate and geology. Although the community in equilibrium looks the same once it is attained, the equilibrium is a dynamic one with constant changes in abundance and sometimes species identities.
Attributions
• Modified by Kammy Algiers from the following sources:
• 45.5D: Ecological Succession - from Biology by OpenStax (licensed CC-BY)
5.2.06: Ecosystem
Learning Objective
Describe the difference between a community and an ecosystem
Communities are composed of living organisms and their interactions with one another and include biotic (living) components. Ecosystems are communities that include the abiotic (non-living) components of the environment as well (Video \(1\)). These abiotic factors include temperature, amount of rainfall, sunlight, organic material and mineral nutrients, topography and attitude. Ecosystems can be small, such as the tide pools found near the rocky shores of many oceans, or large, such as the Amazon Rainforest in Brazil (Figure \(1\)). Energy flow and nutrient cycling are some of topics central to the study of ecosystems. | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/05%3A_Ecology_and_Conservation/5.02%3A_Communities_and_Ecosystems/5.2.05%3A_Succession.txt |
Learning Objectives
• Describe how organisms acquire energy in a food web and in associated food chains
• Explain how the efficiency of energy transfers between trophic levels affects ecosystem structure and dynamics
Trophic interactions in a community can be represented by diagrams called food chains and food webs. Before discussing these representations in detail, we must first review the basics of energy. Energy flows through a community as a result of trophic interactions.
Energy
Virtually every task performed by living organisms requires energy. In general, energy is defined as the ability to do work, or to create some kind of change. Energy exists in different forms. Examples include light energy, kinetic energy, heat energy, potential energy, and chemical energy.
When an object is in motion, there is energy associated with that object. Think of a wrecking ball. Even a slow-moving wrecking ball can do a great deal of damage to other objects. Energy associated with objects in motion is called kinetic energy. Heat energy is the energy of motion in matter (anything that takes up space and has mass) and is considered a type of kinetic energy. The warmer the substance, the faster its molecules are moving. The rapid movement of molecules in the air, a speeding bullet, and a walking person all have kinetic energy. Now what if that same motionless wrecking ball is lifted two stories above ground with a crane? If the suspended wrecking ball is not moving, is there energy associated with it? The answer is yes. The energy that was required to lift the wrecking ball did not disappear, but is now stored in the wrecking ball by virtue of its position and the force of gravity acting on it. This type of energy is called potential energy. If the ball were to fall, the potential energy would be transformed into kinetic energy until all of the potential energy was exhausted when the ball rested on the ground. Wrecking balls also swing like a pendulum; through the swing, there is a constant change of potential energy (highest at the top of the swing) to kinetic energy (highest at the bottom of the swing). Other examples of potential energy include the energy of water held behind a dam or a person about to skydive out of an airplane (Figure \(1\).
Potential energy is not only associated with the location of matter, but also with the structure of matter. On a molecular level, there is potential energy stored within the bonds holding together the atoms in the food molecules we eat. The type of potential energy that exists within chemical bonds, and is released when those bonds are broken, is called chemical energy. Chemical energy is responsible for providing living cells with energy from food.
To appreciate the way energy flows into and out of biological systems, it is important to understand two of the physical laws that govern energy. The first law of thermodynamics states that the total amount of energy in the universe is constant and conserved. In other words, there has always been, and always will be, exactly the same amount of energy in the universe. Energy exists in many different forms. According to the first law of thermodynamics, energy may be transferred from place to place or transformed into different forms, but it cannot be created or destroyed. The transfers and transformations of energy take place around us all the time. Light bulbs transform electrical energy into light and heat energy. Gas stoves transform chemical energy from natural gas into heat energy. Plants perform one of the most biologically useful energy transformations on earth: that of converting the energy of sunlight to chemical energy stored within biological molecules, such as sugars (Figure \(2\).
The challenge for all living organisms is to obtain energy from their surroundings in forms that are usable to perform cellular work. A living cell’s primary tasks of obtaining, transforming, and using energy to do work may seem simple. However, the second law of thermodynamics explains why these tasks are harder than they appear. All energy transfers and transformations are never completely efficient. In every energy transfer, some amount of energy is lost in a form that is unusable. In most cases, this form is heat energy. For example, when a light bulb is turned on, some of the energy being converted from electrical energy into light energy is lost as heat energy. Likewise, some energy is lost as heat energy during the metabolic reactions that occur in organisms.
The concept of order and disorder relates to the second law of thermodynamics. The more energy that is lost by a system to its surroundings, the less ordered and more random the system is. Scientists refer to the measure of randomness or disorder within a system as entropy. High entropy means high disorder and low energy. Living things are highly ordered, requiring constant energy input to be maintained in a state of low entropy.
Energy Flow
Cells run on the chemical energy found mainly in carbohydrate molecules, and the majority of these molecules are produced by one process: photosynthesis. Through photosynthesis, certain organisms convert solar energy (sunlight) into chemical energy, which is then used to build carbohydrate molecules. The energy that is harnessed from photosynthesis enters the communities continuously and is transferred from one organism to another. Therefore, directly or indirectly, the process of photosynthesis provides most of the energy required by living things on Earth.
Organisms that conduct photosynthesis (such as plants, algae, and some bacteria), and organisms that synthesize sugars through other means are called producers. Without these organisms, energy would not be available to other living organisms, and life would not be possible. Consumers, like animals, fungi, and various microorganisms depend on producers, either directly or indirectly. For example, a deer obtains energy by eating plants. A wolf eating a deer obtains energy that originally came from the plants eaten by that deer (Figure \(3\). Using this reasoning, all food eaten by humans can be traced back to producers that carry out photosynthesis (Figure \(4\)). Dead producers and consumers are eaten by detritivores (which ingest on dead tissues) and decomposers (which further break down these tissues into simple molecules by secreting digestive enzymes). Invertebrate animals, such as worms and millipedes, are examples of detritivores, whereas fungi and certain bacteria are examples decomposers.
Attributions
Modified by Kammy Algiers from the following sources:
2.2.1.1.4: Food Chains and Food Webs - from Biology by OpenStax (licensed CC-BY) | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/05%3A_Ecology_and_Conservation/5.02%3A_Communities_and_Ecosystems/5.2.07%3A_Energy_and_Energy_Flow.txt |
Learning Objectives
• Compare and contrast betrween a food chain and a food webs
• Describe energy transfer efficiency as it relates to trophic levels
Food Chains
A food chain is a linear sequence of organisms through which nutrients and energy pass as one organism eats another. Each organism in a food chain occupies a specific trophic level (energy level), its position in the food chain. The first trophic level in the food chain is the producers. The primary consumers (the herbivores the eat producers) are the second trophic level. Next are higher-level consumers. Higher-level consumers include secondary consumers (third trophic level), which are usually carnivores that eat the primary consumers, and tertiary consumers (fourth trophic level), which are carnivores that eat other carnivores. Higher-level consumers feed on the next lower tropic levels, and so on, up to the organisms at the top of the food chain: the apex consumers. In the Lake Ontario food chain shown in Figure \(1\), the Chinook salmon is the apex consumer at the top of this food chain.
One major factor that limits the number of steps in a food chain is energy. Much of the energy from one tropic level to the next is lost as heat, due to the second law of thermodynamics. Only about 10% of the energy transfers from one trophic level to the next trophic level. Thus, after several transfers, the amount of energy remaining in the food chain may not be great enough to support viable populations at yet a higher trophic level.
Food Webs
While food chains are simple and easy to analyze, there is a one problem when using food chains to describe most communities. Even when all organisms are grouped into appropriate trophic levels, some of these organisms can feed at more than one trophic level. In addition, species feed on and are eaten by more than one species. In other words, the linear model of trophic interactions, the food chain, is a hypothetical and overly simplistic representation of community structure. A holistic model—which includes all the interactions between different species and their complex interconnected relationships with each other and with the environment—is a more accurate and descriptive model. A food web is a concept that accounts for the multiple trophic interactions between each species (Figure \(2\)).
Community Productivity and Transfer Efficiency
The rate at which photosynthetic producers incorporate energy from the sun is called gross primary productivity. In a cattail marsh, plants only trap 2.2% of the energy from the sun that reaches them. Three percent of the energy is reflected, and another 94.8% is used to heat and evaporate water within and surrounding the plant. However, not all of the energy incorporated by producers is available to the other organisms in the food web because producers must also grow and reproduce, which consumes energy. At least half of the 2.2% trapped by cattail marsh plants is used to meet the plants own energy needs.
Net primary productivity is the energy that remains in the producers after accounting for the metabolic needs of the producers and heat loss. The net productivity is then available to the primary consumers at the next trophic level. One way to measure net primary productivity is to collect and weigh the plant material produced on a m2 (about 10.7 ft2) of land over a given interval. One gram of plant material (e.g., stems and leaves), which is largely carbohydrate, yields about 4.25 kcal of energy when burned. Net primary productivity can range from 500 kcal/m2/yr in the desert to 15,000 kcal/m2/yr in a tropical rain forest.
In an aquatic community in Silver Springs, Florida, the gross primary productivity (total energy accumulated by the primary producers) was 20,810 kcal/m2/yr (Figure \(3\)). The net primary productivity (energy available to consumers) was only 7,632 kcal/m2/yr after accounting for energy lost as heat and energy require to meet the producer's metabolic needs.
Only a fraction of the energy captured by one trophic level is assimilated into biomass, which makes it available to the next trophic level. Assimilation is the biomass of the present trophic level after accounting for the energy lost due to incomplete ingestion of food, energy used to conduct work by that trophic level, and energy lost as waste. Incomplete ingestion refers to the fact that some consumers eat only a part of their food. For example, when a lion kills an antelope, it will eat everything except the hide and bones. The lion is missing the energy-rich bone marrow inside the bone, so the lion does not make use of all the calories its prey could provide. In Silver Springs, only 1103 kcal/m2/yr from the 7618 kcal/m2/yr of energy available to primary consumers was assimilated into their biomass. (The trophic level transfer efficiency between the first two trophic levels was approximately 14.8 percent.)
Attributions
Modified by Kammy Algiers from the following sources:
2.2.1.1.4: Food Chains and Food Webs - from Biology by OpenStax (licensed CC-BY) | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/05%3A_Ecology_and_Conservation/5.02%3A_Communities_and_Ecosystems/5.2.08%3A_Food_Chains_and_Food_Webs.txt |
Communities include all the different species living in a given area. Biotic interactions refer to the relationships among organisms. Competition occurs when organisms at the same trophic level use the the same resources, and one or both organisms is harmed. Many organisms have developed defenses against predation and herbivory, including mechanical defenses, warning coloration, and mimicry, as a result of evolution and the interaction with other members of the community. Two species cannot exist in the same habitat competing directly for the same resources. Species may form symbiotic relationships such as commensalism or mutualism. Community structure is described by its foundation and keystone species. Communities respond to environmental disturbances by succession (the predictable appearance of different types of plant species) until a stable community structure is established. The variety of these species is called species richness. Relative abundance is the number of individuals in a species relative to the total number of individuals in all species within a habitat, ecosystem, or biome. In communities, some species play a bigger role than others. keystone species impact the biodiversity within an ecosystem by upholding an ecological community’s structure. Invasive species have an overall negative impact on the community. Community dynamics are the changes in community structure and composition over time. Succession describes the sequential appearance and disappearance of species in a community over time after a severe disturbance.
Ecosystems are communities that include the abiotic components of the environment as well. Energy is defined as the ability to do work, or to create some kind of change. Two of the physical laws that govern energy are the first and second law of thermodynamics. Energy flows from producers to consumers and recycled by detritivores and decomposers. Trophic interactions in a community can be represented by diagrams called food chains and food webs. Organisms in an ecosystem acquire energy in a variety of ways, which is transferred between trophic levels as the energy flows from the bottom to the top of the food web, with energy being lost at each transfer. Modeling of ecosystem energy is best done with ecological pyramids of energy, although other ecological pyramids provide other vital information about ecosystem structure.
After completing this chapter, you should be able to...
• Describe the differences between intraspecific and interspecific interactions in reference to competition
• Describe what is herbivory
• Give examples of defenses against predation and herbivory
• Describe what is considered a symbiotic relationships between species
• Compare and contrast between commensalism, mutualism, and parasitism
• Describe symbiosis as it relates to nitrogen fixation
• Describe how saprophytes, epiphytes, and carnivorous plants depend on other orgnanisms
• Describe how species richness and relative abundance play a role on biodiversity
• Describe the role of keystone species in a community
• Describe the role of invasive species in a community
• Describe community structure and succession
• Describe the difference between a community and an ecosystem
• Describe how organisms acquire energy in a food web and in associated food chains
• Explain how the efficiency of energy transfers between trophic levels affects ecosystem structure and dynamics
• Compare and contrast betrween a food chain and a food webs
• Describe energy transfer efficiency as it relates to trophic levels | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/05%3A_Ecology_and_Conservation/5.02%3A_Communities_and_Ecosystems/5.2.09%3A_Chapter_Summary.txt |
Roughly 1.5 million species have been identified, and this is just a fraction of all the species on Earth. These species exist in a variety of ecosystems. Genetic differences among individuals within a species further contributes to the variety of life on Earth. While this biodiversity provides many benefits to humans, such providing food and building materials, recreational activities, and clean air and water, human population group and resource use threatens many species and ecosystems. Conservation biology is concerned with protecting biodiversity, which ultimately supports humans by promoting ecosystem function (Figure \(1\)).
• 5.3.1: The Value of Biodiversity
Ecosystem diversity refers the number and relative abundance of ecosystems and is the largest scale of biodiversity. Species diversity refers to species richness and species evenness. Genetic diversity is variation among individuals within a species.
• 5.3.2: Threats to Biodiversity
There are five major threats to biodiversity: habitat loss, pollution, overexploitation, invasive species, and climate change.
• 5.3.3: Preserving Biodiversity
Many interrelated strategies help preserve biodiversity. Legislation such as the Endangered Species Act directly protect species at risk of extinction. Non-profit organizations provide additional funding and research. Species-level conservation centers on just one species, but protected areas can preserve whole ecosystems. Ecosystem restoration involves returning an area (as much as possible) to its original state to promote ecosystem services and native species.
• 5.3.4: Chapter Summary
Attribution
Melissa Ha (CC-BY-NC)
Image thumbnail: Maikal Hills in Kabirdham District, Chhattisgarh, India. The traditional healers take advantage of this rich biodiversity zone and use medicinal species in treatment of complicated diseases like different types of cancer, sickle cell anemia, etc. Image and caption (edited) by Pankaj Oudhia (CC-BY-SA).
5.03: Conservation
Learning Objectives
• Define biodiversity.
• Distinguish among ecosystem, species, and genetic diversity, explaining the value of each.
• Define and provide examples of ecosystem services.
• Distinguish between species richness and species evenness.
• Explain the importance of biodiversity hotspots and identify the characteristics of endemic species.
Biodiversity is a broad term for the variety of life on Earth. Traditionally, ecologists have measured biodiversity by taking into account both the number of species and the number of individuals of each species. However, biologists now measure biodiversity at a number of organizational levels, including ecosystem, species, and genetic diversity. This focuses efforts to preserve the biologically and technologically important elements of biodiversity. Biodiversity is important to the survival and welfare of human populations because it has impacts on our health and our ability to feed ourselves through agriculture and harvesting populations of wild animals.
Ecosystem Diversity
Measuring biodiversity on a large scale involves measuring ecosystem diversity, the number of different ecosystems on Earth or in a geographical area as well as their relative abundances (Figure \(1\)). The loss of an ecosystem means the loss of the interactions between species and the loss of biological productivity that an ecosystem is able to create. An example of a largely extinct ecosystem in North America is the prairie ecosystem (Figure \(1\)). Prairies once spanned central North America from the boreal forest in northern Canada down into Mexico. They are now all but gone, replaced by crop fields, pasture lands, and suburban sprawl. Many of the species survive, but the hugely productive ecosystem that was responsible for creating our most productive agricultural soils is now gone. As a consequence, their soils are now being depleted unless they are maintained artificially at great expense.
The soil productivity described above is an example of an ecosystem service. These are the products and processes associated with biological systems are directly or indirectly of immense value to the well-being of people. Some ecosystem services are processes such as the regulation of climate, flooding, and disease. Nutrient cycling, pollination, and regulation of crop pests are ecosystem services important to food production. A 2002 study by Claire Kremen and colleagues found that native pollinators in Central California (those that historically occurred there; Figure \(2\)) provided full pollination to watermelon crops (Figure \(3\)). This was only true on organic farms that were located near the natural habitat for these pollinators, highlighting the importance of sustainable farming practices and habitat conservation in preserving ecosystem services. The water cycle provides fresh water, and photosynthesis adds oxygen to our air. Other ecosystem services are human provisions including food, fuel, and fiber (such as cotton for clothing or timber). Medicines are another important provision (see Species Diversity). Furthermore, healthy ecosystems allow for recreational activities, such as hiking, kayaking, and camping, and educational opportunities, such as field trips. Nature is also the basis for a significant part of aesthetic and spiritual values held by many cultures.
In 1997, Robert Costanza and his colleagues estimated to annual value of ecosystem services to be \$33 trillion dollars (\$53 trillion in 2019 dollars), and many consider this to be an underestimation. For comparison the gross domestic product of the United States in 2020 was \$21 trillion.
Species Diversity
Species diversity includes species richness and species evenness. Species richness, the number of species living in a habitat or other unit, is one component of biodiversity. Species richness varies across the globe. Species evenness is a component of species diversity based on relative abundance (the number individuals in a species relative to the total number of individuals in all species within a system). The area in question could be a habitat, a biome, or the entire biosphere. Areas with low species diversity, such as the glaciers of Antarctica, still contain a wide variety of living organisms, whereas the diversity of tropical rainforests is so great that it cannot be accurately assessed.
Species richness is related to latitude: the greatest species richness occurs near the equator and the lowest richness occurs near the poles (Figure \(4\)). Several hypothesis might explain this dyanamic, but the exact reasons for this pattern are still not clearly understood. One hypothesis is that tropical forests have consistently existed at the same location for a long period of time, allowing more time for speciation to occur. Another hypothesis is that speciation rate is simply higher in the tropics than other regions. The tropics also has a long growing season and a wide variety of ecological niches (different roles that species can occupy), partly due to the different vertical layers in a tropical forest (Video \(1\)). Other factors besides latitude influence species richness as well. For example, ecologists studying islands found that biodiversity varies with island size and distance from the mainland.
Video \(1\): This video discusses possible explanations for why species richness is high at the equator.
In 1988, British environmentalist Norman Myers developed a conservation concept to identify geographical areas rich in species and at significant risk for species loss: biodiversity hotspots. The original criteria for a hotspot included the presence of 1500 or more endemic plant species and 70 percent of the area disturbed by human activity. Endemic species are found in only one location. For example, giant lobelia (Lobelia rhynchopetalum, Figure \(5\)) is only found in the alpine habitats in Ethiopia. Endemic species with highly restricted distributions are particularly vulnerable to extinction. If a population of a widespread species declines in one region, individuals from another region may be able to recolonize the first location, but this is not possible for endemic species. Endemic species are particularly common in isolated regions, such as mountaintops or islands. Identifying biodiversity hotspots aids with conservation efforts, a kind of conservation triage. By protecting hotspots, governments are able to protect a larger number of species. There are now 34 biodiversity hotspots (Figure \(6\)) containing large numbers of endemic species, which include half of Earth’s endemic plants.
Regarding species evenness, foundation species often have the highest relative abundance of species. Two locations with the same richness do not necessarily have the same species evenness. For example, both communities in Figure \(7\) have three different trees species and thus a species richness of three. However, there is a dominant species (represented by six individuals) in community #1. In community #2, there are three of individuals of each species. Therefore, community #2 has a greater species evenness and greater species diversity overall.
Healthy ecosystems contain a diversity of species, and each species plays a role in ecosystem function; therefore, species diversity as well as ecosystem diversity are essential to maintaining ecosystem services. For example, many medications are derived from natural chemicals made by a diverse group of organisms. For example, many plants produce compounds meant to protect the plant from insects and other animals that eat them. Some of these compounds also work as human medicines. Examples of significant medicines derived from plant compounds include aspirin, codeine, digoxin, atropine, and vincristine (Figure \(8\)). It is estimated that, at one time, 25 percent of modern drugs contained at least one plant extract. That number has probably decreased to about 10 percent as natural plant ingredients are replaced by synthetic versions of the plant compounds. Antibiotics, which are responsible for extraordinary improvements in health and lifespans in developed countries, are compounds largely derived from fungi and bacteria. It is estimated that about 35 percent of new drugs brought to market between 1981 and 2002 were from natural compounds.
Genetic Diversity
Genetic diversity is a measure of the variability among individuals within a single species. Genetic diversity is represented by the variety of alleles present within a population. Genetic diversity provides the raw material for evolutionary adaptation. Loss of genetic diversity makes a species less able to reproduce successfully and less adaptable to a changing environment or to a new disease. Small populations of species are especially susceptible to loss of genetic diversity. When a species loses too many individuals, it becomes genetically uniform. Some of the causes for the loss in genetic diversity include: inbreeding among closely related individuals and genetic drift, the process by which the genetic composition of a population fluctuates randomly over time. Genetic drift can lead to the loss of alleles from a population, even if those alleles are adaptive. For more on genetic drift, see Openstax 2e 19.1.
Genetic diversity is important to agriculture. Since the beginning of human agriculture more than 10,000 years ago, human groups have been breeding and selecting crop varieties. This crop diversity matched the cultural diversity of highly subdivided populations of humans. For example, potatoes were domesticated beginning around 7,000 years ago in the central Andes of Peru and Bolivia. The people in this region traditionally lived in relatively isolated settlements separated by mountains. The potatoes grown in that region belong to seven species and the number of varieties likely is in the thousands. Each variety has been bred to thrive at particular elevations and soil and climate conditions. The diversity is driven by the diverse demands of the dramatic elevation changes, the limited movement of people, and the demands created by crop rotation for different varieties that will do well in different fields.
The potato demonstrates a well-known example of the risks of low crop diversity: during the tragic Irish potato famine (1845–1852 AD), the single potato variety grown in Ireland became susceptible to a potato blight—wiping out the crop (Figure \(9\)). The loss of the crop led to famine, death, and mass emigration. Resistance to disease is a chief benefit to maintaining crop biodiversity and lack of diversity in contemporary crop species carries similar risks. Seed companies, which are the source of most crop varieties in developed countries, must continually breed new varieties to keep up with evolving pest organisms. These same seed companies, however, have participated in the decline of the number of varieties available as they focus on selling fewer varieties in more areas of the world replacing traditional local varieties.
The ability to create new crop varieties relies on the diversity of varieties available and the availability of wild forms related to the crop plant. These wild forms are often the source of new gene variants that can be bred with existing varieties to create varieties with new attributes. Loss of wild species related to a crop will mean the loss of potential in crop improvement. Maintaining the genetic diversity of wild species related to domesticated species ensures our continued supply of food.
Since the 1920s, government agriculture departments have maintained seed banks of crop varieties as a way to maintain crop diversity. This system has flaws because over time seed varieties are lost through accidents and there is no way to replace them. In 2008, the Svalbard Global seed Vault, located on Spitsbergen island, Norway (Figure \(10\)), began storing seeds from around the world as a backup system to the regional seed banks. If a regional seed bank stores varieties in Svalbard, losses can be replaced from Svalbard should something happen to the regional seeds. The Svalbard seed vault is deep into the rock of the Arctic island. Conditions within the vault are maintained at ideal temperature and humidity for seed survival, but the deep underground location of the vault in the arctic means that failure of the vault’s systems will not compromise the climatic conditions inside the vault.
Attributions
Curated and authored by Melissa Ha using the following sources: | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/05%3A_Ecology_and_Conservation/5.03%3A_Conservation/5.3.01%3A_The_Value_of_Biodiversity.txt |
Learning Objectives
• Name, define, and provide examples of the five major threats to biodiversity.
• Provide examples of the successes and failures of biological control in regulating invasive species.
Biodiversity loss refers to the reduction of biodiversity due to displacement or extinction of species. According to a 2019 United Nations report, 1 million species at risk of extinction. Considering there are estimated to be 8-11 million species total, that means up to 12.5% of species could go extinct, and many of them within our lifetimes. This will have dramatic effects on human welfare through the loss of ecosystem services.
The core threat to biodiversity on the planet is the combination of human population growth and the resources used by that population. The global population size is 7.8 billion as of August 2020. Population size is continuing to increase, although the rate of population growth is decreasing. Some argue that humans have already surpassed our carrying capacity, meaning that the environment cannot sustain our large population size indefinitely.
The human population requires resources to survive and grow, and many of those resources are being removed unsustainably from the environment. The five main threats to biodiversity are habitat loss, pollution, overexploitation, invasive species, and climate change. Increased mobility and trade has resulted invasive species while the other threats are direct results of human population growth and resource use.
Habitat Loss
Habitat loss includes habitat destruction and habitat fragmentation. Habitat destruction occurs when the physical environment required by a species is altered so that the species can no longer live there. Human destruction of habitats accelerated in the latter half of the twentieth century. For example, half of Sumatra's forests, a biodiversity hotspot, is now gone. The neighboring island of Borneo has lost a similar area of forest, and forest loss continues in protected areas of Borneo. The forests are removed for timber and to plant palm oil plantations (Figure \(1\)). Palm oil is used in many products including food products, cosmetics, and biodiesel in Europe. According to Global Forest Watch, 9.7% of tree cover was lost globally from 2002 to 2019, and 9% of that occurred in Indonesia and Malaysia (where Sumatra and Borneo are located). Figure \(2\) shows average annual change in forest area around the world from 1990 to 2015.
Habitat fragmentation occurs an the living space of a species is divided into discontinuous patches. For example, a mountain highway could divide a forest habitat into separate patches. Wildlife corridors mitigate the damage of habitat fragmentation by connecting patches with suitable habitat (Figure \(2\)).
Overexploitation
Overexploitation (overharvesting) involves hunting, fishing, or otherwise collecting organisms at a faster rate than they can be replenished. While overfishing and poaching are common examples of overexploitation, some fungi and slow-growing plant species are also overexploited. For example, stocks of wild ginseng, which is valued for its health benefits, are dwindling. Peyote cactus, which causes hallucinations and is used in sacred ceremonies, is also declining. Yarsagumba, dead moth larvae that were infected by fungal parasites (caterpillar fungus, Ophiocordyceps sinensis), is overexploited because it is highly valued in traditional medicine and used as an aphrodisiac (Figure \(3\)).
Pollution
Pollution occurs when chemicals, particles, or other materials are released into the environment, harming the organisms there. Pollution has contributed to the decline of many threatened species. For example, a 2007 study by Kingsford and colleagues found that pollution was a major pressure on 30% of threatened species in Australia and surrounding regions.
Power plants, factories, and vehicles are common sources of air pollution. In some cases, the pollutants are directly toxic (for example, lead), but in other cases the pollutants indirectly cause ecological harm when they are present in unnaturally large quantities (for example, carbon dioxide emissions leading to climate change). Not only can air pollutants directly harm animals by causing respiratory issues and cancer as well as damage vegetation, but some interact with the atmosphere to form acid deposition (commonly called acid rain). Acid deposition which disrupts aquatic ecosystems as well as soil communities and plant growth.
Heavy metals, plastics, pesticides, herbicides, fertilizers, and sediments are examples of water pollution. Heavy metals (including copper, lead, mercury, and zinc) can leach into soil and water from mines. Nutrients, such as nitrate and phosphates, are healthy in bodies of water to an extent, but when fertilizer pollution adds too many of these nutrients at one time, algal blooms can result. This has cascading effects that can ultimately shade and kill aquatic plants and deplete oxygen needed by fish and other animals (eutrophication, Figure \(4\)). A particularly concerning water pollution problem is micropollutants. For examples, some chemical residues affect growth, cause birth defects, and have other toxic effects on humans and other organisms even at very low concentrations.
Invasive Species
Invasive species are non-native organisms that, when introduced to an area out of its native range, disrupt the community they invade. Non-native (exotic) refers to species occurring outside of their historic distribution. Invasive species are have been intentionally or unintentionally introduced by humans into an ecosystem in which they did not evolve. Human transportation of people and goods, including the intentional transport of organisms for trade, has dramatically increased the introduction of species into new ecosystems. These new introductions are sometimes at distances that are well beyond the capacity of the species to ever travel itself and outside the range of the species’ natural predators. Invasive species can cause ecological and economic damage.
Invasive plants like the purple loosestrife (Lythrum salicaria) and kudzu (Pueraria montana) threaten native plants through competition for resources, and they drastically altered the ecosystems they invaded (Figure \(5\)). They indirectly harms the animals that depend on native plants to be primary producers and to provide habitat. Some invasive plants, like yellow flag iris (Iris pseudacorus) are toxic, directly poisoning the livestock and wildlife that eat them. The awns projecting from cheat grass (Bromus tectorum) during seed dispersal irritate and injure cattle (Figure \(6\)). Invasive insects and plant pathogens harm crops and native species. The emerald ash borer (Agrilus planipennis ) has killed millions of ash trees in the eastern and midwestern United States and Canada. In spreads through movement of firewood and other wood products. Xylella fastidiosa fastidiosa is an invasive bacterium that is native to Central America that causes several diseases including Pierce's disease of grapes in California and the southeastern United States (Figure \(7\)). It is spread through an invasive insect, the glassy-winged sharpshooter (Figure \(7\)).
One reason why invasive species proliferate dramatically outside of their native range is due to release from predators. This means that parasites, predators, or herbivores that usually regulate their populations are not present, allowing them to outcompete or otherwise decimate native species, which are still regulated. Based on this principle, organisms that regulate the invasive species populations have been introduced the newly colonized areas in some cases. The release of organisms (or viruses) to limit population size is called biological control. As described the examples below, biological control of invasive species has had varying success, exacerbating the problem in some cases and solving it in others.
Introduced into Australia, this cactus soon spread over millions of hectares of range land driving out forage plants. In 1924, the cactus moth, Cactoblastis cactorum, was introduced (from Argentina) into Australia. The caterpillars of the moth are voracious feeders on prickly-pear cactus, and within a few years, the caterpillars had reclaimed the range land without harming a single native species. However, its introduction into the Caribbean in 1957 did not produce such happy results. By 1989, the cactus moth had reached Florida, and now threatens five species of native cacti there.
In 1946 two species of Chrysolina beetles were introduced into California to control the Klamath weed (St. Johnswort, Hypericum perforatum) that was ruining millions of acres of range land in California and the Pacific Northwest. Before their release, the beetles were carefully tested to make certain that they would not turn to valuable plants once they had eaten all the Klamath weed they could find. The beetles succeeded beautifully, restoring about 99% of the endangered range land and earning them a commemorative plaque at the Agricultural Center Building in Eureka, California.
To summarize the lessons learned from biological control successes and failures, only candidates that have a very narrow target preference (eat only a sharply-limited range of hosts) should be chosen. Each candidate should be carefully tested to be sure that once it has cleaned up the intended target, it does not turn to desirable species. Biological controls must not be used against native species. Finally, introduction of non-native species into the environment should be avoided because they could themselves be invasive.
Climate Change
Global climate change is also a consequence of human population needs for energy, and the use of fossil fuels to meet those needs. Essentially, burning fossil fuels, including as oil, natural gas, and coal, increases carbon dioxide concentrations in the atmosphere (see Nutrient Cycles for details about the carbon cycle). Carbon dioxide, methane, and other greenhouse gases trap heat energy from the sun, resulting not only in an average increase in global temperature but also in changing precipitation patterns and increased frequency and severity of extreme weather events, such as hurricanes (Figure \(8\)). Scientists overwhelmingly agree the present warming trend is caused by humans.
Climate change is recognized as a major extinction threat, particularly when combined with other threats such as habitat loss. Scientists disagree about the likely magnitude of the effects, with extinction rate estimates ranging from 15 percent to 40 percent of species committed to extinction by 2050. By altering regional climates, it makes habitats less hospitable to the species living in them. While increased carbon dioxide levels can help plants conduct photosynthesis more efficiently, they are threatened by harsh temperatures and extreme weather events. Additionally, with warmer conditions, moisture from snow melt arrives earlier in the season, lengthening the fire season.
The warming trend will shift colder climates toward the north and south poles. Climate gradients will also move up mountains, eventually crowding species higher in altitude and eliminating the habitat for those species adapted to the highest elevations. Some climates will completely disappear. In response to changing conditions, range shifts have also been observed in plants, butterflies, other insects, freshwater fishes, reptiles, amphibians, and mammals. Because individual plants cannot physically move to cooler regions, plant range shifts result from seed dispersal. Seeds are often dispersed in all directions away from a parent plant, but more of the seedlings that establish in northern locations or higher elevations survive, resulting in a gradual shift towards the poles or up mountains (Figure \(9\)). However, species that cannot adapt to new conditions or shift their ranges quickly enough face extinction.
Changing climates also throw off the delicate timing adaptations that species have to seasonal food resources and breeding times. Scientists have already documented many contemporary mismatches to shifts in resource availability and timing. For example, pollinating insects typically emerge in the spring based on temperature cues. In contrast, many plant species flower based on daylength cues. With warmer temperatures occurring earlier in the year, but daylength remaining the same, pollinators ahead of peak flowering. As a result, there is less food (nectar and pollen) available for the insects and less opportunity for plants to have their pollen dispersed.
Ocean levels rise in response to climate change due to meltwater from glaciers and the greater volume occupied by warmer water. Shorelines will be inundated, reducing island size, which will have an effect on some species, and a number of islands will disappear entirely. Additionally, the gradual melting and subsequent refreezing of the poles, glaciers, and higher elevation mountains—a cycle that has provided freshwater to environments for centuries—will be altered. This could result in an overabundance of salt water and a shortage of fresh water.
Finally, increased carbon dioxide levels in the atmosphere react with ocean water to form carbonic acid, a phenomenon called ocean acidification. In combination with warmer temperatures, ocean acidification is responsible for coral bleaching, the process by which coral expel the algae that typically conduct photosynthesis within the corals. Ocean acidification can also dissolve the calcium carbonate skeletons formed by the coral. Overall, climate change plays a major role in the loss of nearly one third of coral reefs.
The impacts of climate change extend to humans as well. Warmer temperatures will affect agricultural yield. In fact, a 2017 study by Zhao et al. found that for every degree Celsius increase in average global temperature, wheat yields are expected to decrease by 6%, rice yields by 3.2%, and maize by 7.4%. Additionally sea level rise and extreme weather events damage property and force people to move inland. Human health is directly impacted by heat-related illnesses and an expanding range of tropical diseases.
Attributions
Curated and authored by Melissa Ha using the following sources: | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/05%3A_Ecology_and_Conservation/5.03%3A_Conservation/5.3.02%3A_Threats_to_Biodiversity.txt |
Learning Objectives
• Describe the legislative framework for conservation, providing and describing examples of national and state laws and international agreements.
• Summarize the role of non-profit organizations in conservation.
• Provide specific examples of species-level conservation and discuss the shortcomings of this approach.
• Explain the importance of protected areas.
• Describe principles preserve design.
• Define ecosystem restoration.
• Provide examples of how citizen science and botanical gardens contribute to conservation efforts.
The field of conservation focuses on preserving biodiversity. Effective conservation depends on ecological knowledge. Today, the main efforts to preserve biodiversity involve legislative approaches to regulate human and corporate behavior, setting aside protected areas, and ecosystem restoration. Additionally, citizen science, botanical gardens, and seed banks are critical to conservation efforts.
Policies
Within many countries there are laws that protect endangered species. For example, the Endangered Species Act (ESA) was enacted in 1973 in the United States. The U.S. Fish and Wildlife Service (FWS), which enforces the ESA, assesses candidates for protected status as threatened or endangered (Figure \(1\)). Consideration of candidate species can be initiated by the FWS itself or at the request of the public. Through the Species Status Assessment Framework, the FWS compiles biological data, such habitat and population information and current threats to the species. This biological data is used to inform decisions.
Once a species is listed, the FWS is required by law to develop a management plan to protect the species and bring it back to sustainable numbers. The ESA, and others like it in other countries, is a useful tool, but it suffers because it is often difficult to get a species listed or to get an effective management plan in place once a species is listed.
State laws can also aid in conservation. Through California Endangered Species Act (CESA), originally passed in 1970 and subsequently amended, the California Fish and Game Commission assesses species to be listed as threatened or endangered by the state. A listed species, or any part or product of the plant or animal, may not be imported into the state, exported out of the state, “taken” (killed), possessed, purchased, or sold without proper authorization.
The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) treaty came into force in 1975. The treaty, and the national legislation that supports it, provides a legal framework for preventing listed species from being transported across nations’ borders, thus protecting them from being caught or killed when the purpose involves international trade.
Species can be listed in one of three CITES appendices. Trade is banned for Appendix I species, which are threatened with extinction. For example the trade of the cycad Cycas beddomei, which has medicinal value, is banned by CITES. Trade is regulated for Appendix II species, such the timber from the big-leaf mahogany (Figure \(2\)). Appendix III species are protected in at least one country, and the local government needs a coordinated response through CITES. For example, several species of red and pink corals have been added to Appendix III at the request of China.
Approximately 35,800 species are protected by the CITES. The treaty is limited in its reach because it only deals with international movement of organisms or their parts. It is also limited by various countries’ ability or willingness to enforce the treaty and supporting legislation.
Non-profit Organizations
The private non-profit sector plays a large role in the conservation effort both in North America and around the world. Some non-profit organizations are directed at specific groups of organisms, like the Orchid Conservation Coalition. Others are broadly focused, such as the International Union for Conservation of Nature (IUCN), which categorizes species based on extinction risk and maintains this information on the Red List. The Nature Conservancy (Figure \(3\)) takes a novel approach. It purchases land and protects it in an attempt to set up preserves for ecosystems.
Species-level Conservation
Some conservation efforts center around a single species. Often this is a charismatic animal that elicits public interest, such tigers, sea otter, or the California Condor. The specific approach depends on specific threats based by the species of focus. A common strategy is to propagate rare plants and reintroduce them to locations where they were extirpated (went locally extinct). Protecting or restoring habitat is another component of the conservation of rare plant species. For example, removal of the invasive iceplant Carpobrotus edulis from California coasts, restores conditions for endangered dune vegetation (Figure \(4\)-6).
Seemingly unimpressive species can still serve vital ecological roles, but they are often overlooked in conservation efforts. In fact, a 2007 study by Colléony and colleagues found that people more often donated to conservation efforts for species that were more similar to humans rather than choosing those that were at greatest risk of extinction. Broad approaches such as establishing protected areas and ecosystem restoration benefit charismatic and non-charismatic species alike. Additionally, broad approaches protect unidentified and species that have not been assessed.
Protected Areas
It is important to protect natural areas (Figure \(7\)) for several reasons. Some people feel a cultural or spiritual connection to the wilderness. Every year, millions of people visit recreational lands such as parks and wilderness areas to experience attractions of the great outdoors: hiking among the giant sequoias in California, traveling on a photo safari in Kenya or just picnicking at a local county park. Besides providing people with obvious health benefits and aesthetic pleasures, recreational lands also generate considerable tourist money for government and local economies. Outdoor recreation activities such as hiking and camping benefit tourist industries and manufacturers of outdoor clothes and equipment.
Establishment of preserves is one of the key tools in conservation efforts. A preserve is an area of land set aside with varying degrees of protection for the organisms that exist within the boundaries of the preserve. Governments or private organizations establish nature preserves. In 2016, the IUCN estimated that 14.7 percent of Earth’s land surface was covered by preserves of various kinds. This area is large, but only 20% of the key biodiversity areas identified by the IUCN were sufficiently protected.
There has been extensive research into optimal preserve designs for maintaining biodiversity. Preserves can be seen as “islands” of habitat within “an ocean” of non-habitat. In general, large preserves are better because they support more species, they have more core area of optimal habitat for individual species, and they have more niches to support more species. One large preserve is better than the same area of several smaller preserves because there is more core habitat unaffected by less hospitable ecosystems outside the preserve boundary. For this same reason, preserves in the shape of a square or circle will be better than a preserve with many thin “arms.” If preserves must be smaller, then providing wildlife corridors between two preserves is important so that species and their genes can move between them. All of these factors are taken into consideration when planning the nature of a preserve before the land is set aside. In addition to the physical specifications of a preserve, there are a variety of regulations related to the use of a preserve. These can include anything from timber extraction, mineral extraction, regulated hunting, human habitation, and nondestructive human recreation.
Public lands differ in their level of protection. For example, national parks and forests allow camping whereas wildlife refuges place more limitations on human activities. Wilderness areas, comprise ecosystems in which human activity has not significantly affected the plant and animal populations or their environment. National parks and forests and wildlife refuges can contain wilderness areas. The National Park System consists of more than 80 million acres nationwide. Their mission is to "preserve unimpaired the natural & cultural resources & values of the national park system for the enjoyment, education, and inspiration of this & future generations". Science, conservation, and outreach are a big part of the National Park (Figure \(8\)).
The California State Park System manages more than one million acres of parklands including: coastal wetlands, estuaries, scenic coastlines, lakes, mountains and desert areas. The National Forest System manages more than 170 forestlands and grasslands, which are available for activities such as camping, fishing, hiking and hunting. The Coronado National Forest in Arizona is famous for "sky islands", or steep mountain ranges surrounded by low-lying areas. The dramatic increase in elevation is associated with changes in the flora and fauna (figure \(9\)). Explore national forests using this interactive map. The U.S. Fish and Wildlife Service manage more than 500 national wildlife refuges, which not only protect animal habitats and breeding areas but also provide recreational facilities.
Ecosystem Restoration
Ecosystem restoration is the process of bringing an area back to its natural state, before it was impacted through destructive human activities. Reintroducing wolves, a top predator, to Yellowstone National Park in 1995 led to dramatic changes in the ecosystem that increased biodiversity. The wolves (figure \(10\)) function to suppress elk and coyote populations and provide more abundant resources to the detritivores. Reducing elk populations has allowed revegetation of riparian areas (those along the banks of a stream or river), which has increased the diversity of species in that ecosystem. In this ecosystem, the wolf is a keystone species, meaning a species that is instrumental in maintaining diversity within an ecosystem.
Citizen Science
Citizen science, research conducted by laypeople (non-scientists), provides the opportunity to be directly involved in biological conservation efforts. In other words, it is scientific research that engages the public. For some opportunities, like the Flora of the California Floristic Province, data can be collected independently and submitted online. Others, like mapping the urban heat island effect (the phenomenon of higher temperatures in cities than surrounding areas), are scheduled events in which experts train a group of volunteers to collect data. The federal government's citizen science database lists many such opportunities.
Botanical Gardens
Botanical gardens maintain live specimens of a variety of plant species, including those facing extinction (Figure \(11\)). Some botanical gardens have programs to research the cultivation, ecology, and disease prevention of rare species. Additionally, they emphasize the value of preserving biodiversity to the public and provide education on conservation efforts.
Supplemental Reading
America's Public Lands Explained. 2016. U.S. Department of the Interior.
Attributions
Curated and authored by Melissa Ha using the following sources:
5.3.04: Chapter Summary
Biodiversity exists at multiple levels of organization, including ecosystem diversity, species diversity, and genetic diversity. Biodiversity is negatively correlated with latitude for most taxa, meaning that biodiversity is higher in the tropics.
The core threats to biodiversity are human population growth and unsustainable resource use. These are habitat loss, overexploitation, pollution, invasive species, and climate change. Deforestation is an example of habitat loss. Water and air pollution introduce toxic substances into the environment that harm plants and animals. The collection of plants and fungi at faster rate than they can be replenished is an example of overexploitation. Invasive plant species can outcompete native plants, poison livestock and wildlife, and cause plant diseases. Climate change is causes range shifts and extinctions and disrupts species interactions. Climate change will also raise sea levels, eliminating some islands and reducing the area of all others.
Conservation involves a variety of approaches, and many factors influence the success of conservation efforts. In the United States, the Endangered Species Act protects listed species but is hampered by procedural difficulties and a focus on individual species. International treaties such as CITES regulate the transportation of endangered species across international borders. The non-profit sector is also very active in funding and organizing conservation efforts. Species-level conservation of rare plants can involve propagating and reintroducing rare plant species and restoring their habitat by removing invasive species. Presently, 14.7 percent of Earth’s land surface is protected in some way. Preserves are a major tool in conservation efforts, and large, interconnected preserves favor biodiversity. Ecosystem restoration promotes biodiversity, improves conditions for native species, and reinstate ecosystem services. Through citizen science and botanical gardens, the public can engage directly in conservation efforts.
After completing this chapter, you should be able to...
• Define biodiversity.
• Distinguish among ecosystem, species, and genetic diversity, explaining the value of each.
• Define and provide examples of ecosystem services.
• Distinguish between species richness and species evenness.
• Explain the importance of biodiversity hotspots and identify the characteristics of endemic species.
• Name, define, and provide examples of the five major threats to biodiversity.
• Provide examples of the successes and failures of biological control in regulating invasive species.
• Describe the legislative framework for conservation, providing and describing examples of national and state laws and international agreements.
• Summarize the role of non-profit organizations in conservation.
• Provide specific examples of species-level conservation and discuss the shortcomings of this approach.
• Explain the importance of protected areas.
• Describe principles preserve design.
• Define ecosystem restoration.
• Provide examples of how citizen science and botanical gardens contribute to conservation efforts.
Attributions
Curated and authored by Melissa Ha using the following sources: | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/05%3A_Ecology_and_Conservation/5.03%3A_Conservation/5.3.03%3A_Preserving_Biodiversity.txt |
A biome is a large geographical area characterized by climate and vegetation. Biomes may be terrestrial or aquatic, but this chapter will focus only on terrestrial biomes. Terrestrial biomes are characterized by their plant life. The types of plants depend on the amount of precipitation in an area as well as the temperature. Different adaptations evolve based on the abiotic factors. For example, desert environments with less rainfall will have a different type of vegetation and plant life will be more scarce than tropical rainforests which have plenty of rainfall throughout the year (Figure \(1\)).
• 5.4.1: Introduction
Plants are main components of terrestrial ecosystems as they are primary producers. Almost all terrestrial life if based on the types of plants found in the area.
• 5.4.2: Climate and Biomes
Biomes are large-scale environments that are distinguished by characteristic vegetation and climate. Since a biome is defined by climate, the same biome can occur in geographically distinct areas with similar climates.
• 5.4.3: Tropical Rainforest
Tropical rainforests are the most diverse terrestrial biome. Temperatures and precipitation are both high, allows plants to flourish. Tropical rainforests are characterized by vertical layering of vegetation and the formation of distinct habitats for animals within each layer.
• 5.4.4: Tropical Deciduous Forest
Tropical deciduous forests contain two seasons: the dry season (with no to little rain) and the wet season (with heavy rain). Deciduous trees lose their leaves during the dry season and re-grow them during the rainy season. This allows the plants to save energy when liquid water is not as available. This biome contains a constant warm temperature throughout the year.
• 5.4.5: Savannas
Savannas are characterized rolling grasslands scattered with shrubs and isolated trees. Savannas are also known as tropical grasslands.
• 5.4.6: Desert
Deserts can be found between 15-30 degrees north and south latitude as well as on the downwind or lee side of mountain ranges, which create a rain shadow. This biome has low species diversity because of its low and unpredictable precipitation. Many annual plants can be found in desert biomes, as can some perennials.
• 5.4.7: Mediterranean (Chaparral)
The Mediterranean ecosystem occurs only in five relatively small areas around the planet. Mediterranean ecosystems are characterized by mild, rainy winters and warm, dry summers and are moderated by cold ocean currents offshore. The chaparral vegetation is dominated by shrubs and is adapted to periodic fires, with some plants producing seeds that germinate only after a hot fire.
• 5.4.8: Temperate Grasslands (Prairie)
Temperate grasslands have pronounced annual fluctuations in temperature with hot summers and cold winters. They are areas of open grass with few trees, most of which are found growing along rivers or streams.
• 5.4.9: Temperate Deciduous Forests
Temperate forests are dominated by deciduous trees. Deciduous trees in this biome begin to lose their leaves in autumn and grew them back in the spring.
• 5.4.10: 22.10 Temperate Rainforest
Temperate rainforests, sometimes called mixed evergreen forests, are common on the western coast of the United States. The conditions of this biome are similar to temperate deciduous forests but winters are colder and last longer. Both deciduous and evergreen (coniferous) trees are common in this biome.
• 5.4.11: Boreal (Coniferous) Forests
The boreal forest, also known as coniferous forest has cold, dry winters and short, cool, wet summers. In the winters, little evaporation occurs because of the cold temperatures. Though evergreen, the net primary productivity of boreal forests is relatively low, as is species richness. These are evergreen coniferous trees like pines, spruce, and fir, which retain their needle-shaped leaves year-round.
• 5.4.12: Arctic Tundra
The Arctic tundra lies north of the subarctic boreal forests and is located throughout the Arctic regions of the Northern Hemisphere. Plants in the Arctic tundra are generally low to the ground and include low shrubs, grasses, lichens, and small flowering plants with small leaves. There is little species diversity, low net primary productivity, and low aboveground biomass. Permafrost makes it impossible for roots to penetrate far into the soil.
• 5.4.13: Chapter Summary
5.04: Terrestrial Biomes
Learning Objective
Identify the two major abiotic factors that determine terrestrial biomes.
Plants are main components of terrestrial ecosystems as they are primary producers. Almost all terrestrial life if based on the types of plants found in the area. These types of plants are able to survive in the given environment based on the temperature and precipitation range of the biome. Annual totals and fluctuations of precipitation affect the kinds of vegetation and animal life that can exist in broad geographical regions. Temperature variation on a daily and seasonal basis is also important for predicting the geographic distribution of a biome. Since a biome is defined by climate, the same biome can occur in geographically distinct areas with similar climates (Figure \(1\))
Given the amount of rainfall, for example, tall evergreen trees can dominate one area, whereas small shrubs may dominate another. The plants, thus, determine the types of vegetation (i.e., visually different plant communities). We will explore some of the most important types (also called biomes).
Attributions
Curated and authored by Kammy Algiers from the following sources: | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/05%3A_Ecology_and_Conservation/5.04%3A_Terrestrial_Biomes/5.4.01%3A_Introduction.txt |
Learning Objective
The Identify the role of temperature and percipitation play on distribution of biomes.
Biomes are large-scale environments that are distinguished by characteristic vegetation and climate (Figure \(1\)). Biomes are also characterized by the animals and other organisms there, which are influenced by vegetation and climate patterns. The Earth’s biomes are categorized into two major groups: terrestrial and aquatic. Terrestrial biomes are based on land, while aquatic biomes include both ocean and freshwater biomes. Altitude and latitude, which affect temperature and precipitation determine the distribution of biomes.
Low latitudes (near the equator) have high temperatures and low latitudes (near the poles) have low temperatures. This is because the sun hits the equator more directly. Sunlight hits the poles at an angle, reducing the intensity of light (and heat energy) per unit of area. Temperature also decreases with altitude. At high altitudes, the atmosphere is thinner and traps less heat energy from the sun. Because temperatures decline with altitude as well as latitude, similar biomes exist on mountains even when they are at low latitudes. As a rule of thumb, a climb of 1000 feet (about 300 m) is equivalent in changed flora and fauna to a trip northward of some 600 miles (966 km).
Where precipitation is moderately abundant — 40 inches (about 1 m) or more per year — and distributed fairly evenly throughout the year, the major determinant is temperature. It is not simply a matter of average temperature, but includes such limiting factors as whether it ever freezes or length of the growing season. Biomes are thus characterized not only by average temperature and precipitation but also their seasonality.
Not only does latitude influence temperature, but it also affects precipitation. For example, deserts tend to occur at latitudes of around 30° and at the poles, both north and south, driven by circulation and prevailing wind patterns in the atmosphere. The engine that drives circulation in the atmosphere and oceans is solar energy, which is determined by the average position of the sun over the Earth’s surface. Direct light provides uneven heating depending on latitude and angle of incidence, with high solar energy in the tropics, and little or no energy at the poles. Atmospheric circulation and geographic location are the primary causal agents of deserts. At approximately 30° north and south of the equator, sinking air produces trade wind deserts like the Sahara and the Outback of Australia (Figure \(2\) and Video \(1\)).
Video \(1\): This MinuteEarth video discusses the global climate patterns which lead to deserts.
Attributions
Curated and authored by Melissa Ha from the following sources:
5.4.03: Tropical Rainforest
Learning Objective
• Recognize distinguishing characteristics of tropical rainforests & plant adaptations of the biome.
Also referred to as tropical wet forest, this biome is found in equatorial regions. Tropical rainforests are the most diverse terrestrial biome. This biodiversity is still largely unknown to science and is under extraordinary threat primarily through logging and deforestation for agriculture. Tropical rainforests have also been described as nature’s pharmacy because of the potential for new drugs that is largely hidden in the chemicals produced by the huge diversity of plants, animals, and other organisms. The vegetation is characterized by plants with spreading roots and broad leaves that fall off throughout the year, unlike the trees of deciduous forests that lose their leaves in one season. These forests are “evergreen,” year-round, meaning they retain they leaves throughout the year.
The temperature and sunlight profiles of tropical rainforests are stable in comparison to that of other terrestrial biomes, with average temperatures ranging from 20oC to 34oC (68oF to 93oF). Month-to-month temperatures are relatively constant in tropical rainforests, in contrast to forests further from the equator. This lack of temperature seasonality leads to year-round plant growth, rather than the seasonal growth seen in other biomes. In contrast to other ecosystems, a more constant daily amount of sunlight (11–12 hours per day) provides more solar radiation, thereby a longer period of time for plant growth.
The annual rainfall in tropical rainforests ranges from 250 cm to more than 450 cm (8.2–14.8 ft) with considerable seasonal variation. Tropical rainforests have wet months in which there can be more than 30 cm (11–12 in) of precipitation, as well as dry months in which there are fewer than 10 cm (3.5 in) of rainfall. However, the driest month of a tropical rainforest can still exceed the annual rainfall of some other biomes, such as deserts.
Tropical rainforests have high net primary productivity because the annual temperatures and precipitation values support rapid plant growth (Figure \(1\)) . However, the high rainfall quickly leaches nutrients from the soils of these forests, which are typically low in nutrients. Any nutrients that reach the soil (fallen leaves, tree branches, or dead animals) quickly decompose and are used by plants as raw material. Thus, the nutrients are always above ground, and not stored in the soil.
Tropical rainforests are characterized by vertical layering of vegetation and the formation of distinct habitats for animals within each layer. On the forest floor is a sparse layer of plants and decaying plant matter. Above that is an understory of short, shrubby foliage. A layer of trees rises above this understory and is topped by a closed upper canopy—the uppermost overhead layer of branches and leaves. Some additional trees emerge through this closed upper canopy. These layers provide diverse and complex habitats for the variety of plants, animals, and other organisms within the tropical wet forests. Many species of animals use the variety of plants and the complex structure of the tropical wet forests for food and shelter. Some organisms live several meters above ground rarely ever descending to the forest floor.
Rainforests are not the only forest biome in the tropics; there are also tropical dry forests, which are characterized by a dry season of varying lengths. These forests commonly experience leaf loss during the dry season to one degree or another. The loss of leaves from taller trees during the dry season opens up the canopy and allows sunlight to the forest floor that allows the growth of thick ground-level brush, which is absent in tropical rainforests. Extensive tropical dry forests occur in Africa (including Madagascar), India, southern Mexico, and South America.
Adaptations
Plants living in tropical rainforests have many unique adaptations. For example, due to the poor nutrient soil, they cannot have deep roots. They withstand many rain events and compete with other plants for sunlight, causing them to sometimes grow at an angle. Due to all these restrictions, trees often have buttresses, which are large aerial extensions of the lateral surface roots, to help stabilize the tree. Another common adaptation are epiphytes. These are plants that live on the surface of other plants, using moisture and nutrients from the air or rain. They grow on plants instead of the shady forest floor, where they cannot obtain enough sunlight. Epiphytes do not have any attachment to the ground and are not parasitic on the plant. Orchids, bromeliads, and mosses are common epiphytes. Some plants have leaves with drip tips, pointy tips that help remove water off the leaves quickly to reduce the cumulating of fungi and bacteria. It also helps protect the leaves from breakage during heavy rains.
Attributions
Curated and authored by by Kammy Algiers using Terrestrial Biomes from Biology 2e by OpenStax (CC-BY). Access for free at openstax.org. | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/05%3A_Ecology_and_Conservation/5.04%3A_Terrestrial_Biomes/5.4.02%3A_Climate_and_Biomes.txt |
Learning Objective
Recognize distinguishing characteristics of tropical deciduous rainforests & plant adaptations of the biome.
This biome is distinguished by seasonal rainfall and a constant warm temperature throughout the year. There are two seasons in this biome, are often referred to as the dry season (with no to little rain) and the wet season (with heavy rain). This biome receives up to 80 inches of rain per year and temperatures range between 68°F to 77°F.
One would find this biome on the north and south ends of tropical rainforests, generally between 10° and 20°. Parts of South America, including parts of amazon and southern Mexico, as well as parts of south east Asia and India contain tropical deciduous forests. This biome is also sometimes called Monsoon forest or mixed deciduous forest, depending on its location and slight variations.
Adaptations
Though there is less biodiversity in this biome than tropical rainforests, there are still a large number of species, including many endemic species that reside here. Deciduous trees lose their leaves during the dry season and re-grow them during the rainy season. This allows the plants to save energy when liquid water is not as available. During the rainy season, the forest is lush and full of foliage, whereas the dry season creates an open canopy, allowing plants under the canopy to access sunlight. You can still see the various layers of vegetation in this biome, including lianas (woody vines) and epiphytes such as bromeliads and orchids. The canopy contains tall trees above and smaller trees and shrubs underneath. Though most of the tall trees are often deciduous, there are still plenty of evergreen plants that retain their leaves throughout the year. Many birds and mammals live in this biome and use the vegetation as habitat and food.
5.4.05: Savannas
Learning Objective
Recognize distinguishing characteristics of savannas & plant adaptations of the biome.
Savannas are rolling grasslands scattered with shrubs and isolated trees. Not enough rain falls on a savanna to support forests. Savannas are also known as tropical grasslands. They are found in a wide band on either side of the equator on the edges of tropical rainforests. One can find savannas in Africa, South America, and northern Australia. Savannas are hot, tropical areas with temperatures averaging from 24oC –29oC (75oF –84oF) and an annual rainfall of 51–127 cm (20–50 in). Savannas have both a wet season and an extensive dry season with frequent fires. As a result, scattered in the grasses and forbs (herbaceous flowering plants) that dominate the savanna, there are relatively few trees (Figure \(1\)) . Since fire is an important source of disturbance in this biome, plants have evolved well-developed root systems that allow them to quickly re-sprout after a fire.
Adaptations
In the dry season, plants have adapted to the lack of water in Savannas. For example, trees and larger shrubs have deep roots so they can access underground water during the direst time of the year. Some trees, like the Baobob tree in Madagascar, store water in their trunk (up to 120,000 liters or 32,000 US gallons) to endure harsh drought conditions. Some trees may lose their leaves (deciduous) and grow them back during the rainy season, when water is available. In general, many savanna plants have small leaves to reduce water loss. Grasses are dormant during the drought, with their growth season mainly during the wet season.
Attributions
Curated and authored by Kammy Algiers using Terrestrial Biomes from Biology 2e by OpenStax (CC-BY). Access for free at openstax.org
5.4.06: Desert
Learning Objective
Recognize distinguishing characteristics of Deserts & plant adaptations of the biome.
Subtropical deserts exist between 15o and 30o north and south latitude and are centered on the Tropic of Cancer and the Tropic of Capricorn. Deserts are frequently located on the downwind or lee side of mountain ranges, which create a rain shadow after prevailing winds drop their water content on the mountains Figure dd This is typical of the North American deserts, such as the Mohave and Sonoran deserts. Deserts in other regions, such as the Sahara Desert in northern Africa or the Namib Desert in southwestern Africa are dry because of the high-pressure, dry air descending at those latitudes. Subtropical deserts are very dry; evaporation typically exceeds precipitation. Subtropical hot deserts can have daytime soil surface temperatures above 60oC (140oF) and nighttime temperatures approaching 0oC (32oF). The temperature drops so far because there is little water vapor in the air to prevent radiative cooling of the land surface. Subtropical deserts are characterized by low annual precipitation of fewer than 30 cm (12 in) with little monthly variation and lack of predictability in rainfall. Some years may receive tiny amounts of rainfall, while others receive more. In some cases, the annual rainfall can be as low as 2 cm (0.8 in) in subtropical deserts located in central Australia (“the Outback”) and northern Africa.
Adaptations
The low species diversity of this biome is closely related to its low and unpredictable precipitation. Despite the relatively low diversity, desert species exhibit fascinating adaptations to the harshness of their environment. Very dry deserts lack perennial vegetation that lives from one year to the next; instead, many plants are annuals that grow quickly and reproduce when rainfall does occur, then they die within the year. Perennial plants in deserts are characterized by adaptations that conserve water: deep roots to tap groundwater., reduced foliage to reduce water loss, and large, fleshy, water-storing stems (Figure \(2\)). Seed plants in the desert produce seeds that can lie dormant for extended periods between rains. The Namib Desert is the oldest on the planet, and has probably been dry for more than 55 million years. It supports a number of endemic species (species found only there) because of this great age. For example, the unusual gymnosperm Welwitschia mirabilis is the only extant species of an entire order of plants.
In addition to subtropical deserts there are cold deserts that experience freezing temperatures during the winter and any precipitation is in the form of snowfall. The largest of these deserts are the Gobi Desert in northern China and southern Mongolia, the Taklimakan Desert in western China, the Turkestan Desert, and the Great Basin Desert of the United States.
Attributions
Curated and authored by Kammy Algiers using Terrestrial Biomes from Biology 2e by OpenStax (CC-BY). Access for free at openstax.org. | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/05%3A_Ecology_and_Conservation/5.04%3A_Terrestrial_Biomes/5.4.04%3A_Tropical_Deciduous_Forest.txt |
Learning Objective
Recognize distinguishing characteristics of Mediterranean (Chaparral) & plant adaptations of the biome.
The Mediterranean ecosystem occurs only in five relatively small areas around the planet: the area bordering the Mediterranean Sea, central Chile, the Cape region of South Africa, southwestern and southern Australia, and the California Floristic Province (CFP) located on the Pacific Coast of North America. These areas occur along the western edges of continents and are distributed between roughly 30 and 40 degrees latitude in the northern and southern hemispheres. Mediterranean ecosystems are characterized by mild, rainy winters and warm, dry summers and are moderated by cold ocean currents offshore. The annual rainfall in this biome ranges from 65 cm to 75 cm (25.6–29.5 in) and the majority of the rain falls in the cool, wet winter. Summers are very dry and many chaparral plants are dormant during the summertime.
Mediterranean regions make up only 2.5% of the earth’s land but contain more than 16% of the world’s plant species, an impressive level of diversity concentrated in a relatively small area. In the CFP, the majority of the primary vegetation has been damaged or completely removed. The threat of habitat destruction continues today along the coast of California due to the high number of attractive living locations (e.g. close proximity to the ocean) and desirable climate. In addition to the high level of biodiversity, more than a third of the plant species found in the CFP are endemic (not found anywhere else in the world). Because of all these qualities, the CFP is considered to be one of the earth’s biodiversity hotspots, an area of high biodiversity and endemism at risk of destruction. There are a number of widely recognized plant communities in this biome, including Coastal Sage Scrub, Chaparral, Grassland, Wetland, Riparian, Coastal Salt Marsh, and Oak Woodland (Figure \(1\)).
Adaptations
The chaparral vegetation is dominated by shrubs and is adapted to periodic fires, with some plants producing seeds that germinate only after a hot fire. The ashes left behind after a fire are rich in nutrients like nitrogen that fertilize the soil and promote seeds to germinate. Other plants store nutrients in their roots and will recover after a fire by re-sprouting from the root crown. Many have fire resistant bark that protects the plant from harm. Periodic fire is a natural part of the maintenance of this biome. Though these communities are well adapted to infrequent, early season fires that are naturally ignited by lightening (and follow rain), they are not adapted to the frequent, late season fires caused by humans. These late season fires often occur during the dryer part of the year, when high Santa Ana winds can produce devastating results. Fires that occur during Santa Ana wind conditions burn longer and hotter, and since they are more frequent, native plant recovery is more difficult (Figure \(2\)).
Plants in the Mediterranean biome are also well adapted for conserving water. Small leaves, waxy cuticles, trichomes, and succulence are some adaptations you find on such plants. Others may be deciduous, or have large, deep taproots.
Attributions
Curated and authored by Kammy Algiers using Terrestrial Biomes from Biology 2e by OpenStax (CC-BY). Access for free at openstax.org | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/05%3A_Ecology_and_Conservation/5.04%3A_Terrestrial_Biomes/5.4.07%3A_Mediterranean_%28Chaparral%29.txt |
Learning Objective
Recognize distinguishing characteristics of Temperate Grassland (Prairie) & plant adaptations of the biome.
Temperate grasslands are found throughout central North America, where they are also known as prairies, and in Eurasia, where they are known as steppes. Temperate grasslands have pronounced annual fluctuations in temperature with hot summers and cold winters. The annual temperature variation produces specific growing seasons for plants. Plant growth is possible when temperatures are warm enough to sustain plant growth, which occurs in the spring, summer, and fall. Annual precipitation ranges from 25.4 cm to 88.9 cm (10–35 in).
Adaptations
Temperate grasslands are areas of open grass with few trees, most of which are found growing along rivers or streams. The treeless condition is maintained by low precipitation, frequent fires, and grazing (Figure \(1\)). The vegetation is very dense and the soils are fertile because the subsurface of the soil is packed with the roots and rhizomes (underground stems) of these grasses. The roots and rhizomes act to anchor plants into the ground and replenish the organic material (humus) in the soil when they die and decay. Thus, temperate grasslands contain some of the most fertile soils in the world. Agriculture and development is often associated with this biome due to its rich soil.
Fires, which are a natural disturbance in temperate grasslands, can be ignited by lightning strikes. It also appears that the lightning-caused fire regime in North American grasslands was enhanced by intentional burning by humans. When fire is suppressed in temperate grasslands, the vegetation eventually converts to scrub and dense forests. Often, the restoration or management of temperate grasslands requires the use of controlled burns to suppress the growth of trees and maintain the grasses.
Attributions
Curated and authored by Kammy Algiers using Terrestrial Biomes from Biology 2e by OpenStax (CC-BY). Access for free at openstax.org
5.4.09: Temperate Deciduous Forests
Learning Objective
Recognize distinguishing characteristics of temperate forests & plant adaptations of the biome.
Temperate deciduous forests are the most common biome in eastern North America, Western Europe, Eastern Asia, Chile, and New Zealand. This biome is found throughout mid-latitude regions and is the second most common biome in the world at 25% forest cover. Temperatures range between –30oC and 30oC (–22oF to 86oF) and drop to below freezing on an annual basis. These temperatures mean that temperate forests have defined growing seasons during the spring, summer, and early fall. Precipitation is relatively constant throughout the year and ranges between 75 cm and 150 cm (29.5–59 in). However, in the winter, the precipitation falls in the form of snow.
Deciduous trees are the dominant plant in this biome with fewer evergreen conifers. Deciduous trees lose their leaves each fall and remain leafless in the winter. Thus, little photosynthesis occurs during the dormant winter period. Each spring, new leaves appear as temperature increases. Due to the dormant period, the net primary productivity and diversity of tree species is less than that of tropical rainforests. Yet, since deciduous trees lose their leaves during the drier part of the year, the sunlight that reaches the ground provides energy for the growth of wildflowers, ferns, mosses, and lichens. The thick layer of leaf litter on forest floors allows provides cover for invertebrates. Decay of leaf litter and the reduced leaching of nutrients by rainfall allows detritivores and decomposers to thrive, returning nutrients to the soil. The leaf litter also protects soil from erosion & insulates the ground (Figure \(1\)).
Adaptations
Deciduous trees in this biome begin to lose their leaves in autumn. As light levels are reduced, leaves stop receiving nutrients from the tree due to changes in hormone level and nutrients are instead stored in the root for future use. Chlorophyll molecules begin to break down within the leaves. This allows the carotenoid pigments, which are orange, red, and yellow, to become visible. By winter, the trees have shed their leaves completely. The thick bark of the tree will be protected from the cold as the tree stays dormant for the winter. However, as spring arrives, an increase in sunlight and liquid water from melted snow triggers new growth of leaves and the stored nutrients within the soil are used to begin leaf growth.
Attributions
Curated and authored by Kammy Algiers using Terrestrial Biomes from Biology 2e by OpenStax (CC-BY). Access for free at openstax.org | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/05%3A_Ecology_and_Conservation/5.04%3A_Terrestrial_Biomes/5.4.08%3A_Temperate_Grasslands_%28Prairie%29.txt |
Learning Objective
Recognize distinguishing characteristics of temperate rainforests & plant adaptations of the biome.
Temperate rainforests, sometimes called mixed evergreen forests, are common on the western coast of the United States, from Alaska to California. However, smaller patches of this biome can be found on other contents as well. The conditions of this biome are similar to temperate deciduous forests but winters are colder and last longer. The soil in this biome is generally poor in nutrient levels. Being close to the coast, the temperature rarely goes below freezing, averaging between 39°F and 54°F. Precipitation averages about 55 inches but can reach up to 100 inches a year.
Adaptations
The term mixed evergreen forest comes from the fact that both deciduous and evergreen (coniferous) trees are common in this biome. These evergreen trees are often tall and conspicuous. The tallest trees in the world, the coast redwoods, can be found in this biome. Often, moss and epiphytes can be found growing on the tall trees as they reach out for the sun (Figure \(1\)) . In the understory, ferns are common, as well as other shade tolerant plants. Showy wildflowers and fungi can also be seen in the understory.
5.4.11: Boreal (Coniferous) Forests
Learning Objective
Recognize distinguishing characteristics of boreal (coniferous) forests & plant adaptations of the biome.
The boreal forest, also known as coniferous forest, is found roughly between 50o and 60o north latitude across most of Canada, Alaska, Russia, and northern Europe. Boreal forests in North America are sometimes referred to as Taiga. Boreal forests are also found above a certain elevation (and below high elevations where trees cannot grow) in mountain ranges throughout the Northern Hemisphere. This biome has cold, dry winters and short, cool, wet summers. The annual precipitation is from 40 cm to 100 cm (15.7–39 in) and usually takes the form of snow; little evaporation occurs because of the cold temperatures.
Though evergreen, the net primary productivity of boreal forests is relatively low, as is species richness. The aboveground biomass of boreal forests is high because these slow-growing tree species are long-lived and accumulate standing biomass over time. Boreal forests lack the layered forest structure seen in tropical rainforests or, to a lesser degree, temperate forests. The structure of a boreal forest is often only a tree layer and a ground layer. When conifer needles are dropped, they decompose more slowly than broad leaves; therefore, fewer nutrients are returned to the soil to fuel plant growth (Figure \(1\)).
Adaptations
The long and cold winters in the boreal forest have led to the predominance of cold-tolerant cone-bearing plants. These are evergreen coniferous trees like pines, spruce, and fir, which retain their needle-shaped leaves year-round. These small, waxy leaves are adapted to little water loss during the winter where liquid water is not available.
Evergreen trees can photosynthesize earlier in the spring than deciduous trees because less energy from the Sun is required to warm a needle-like leaf than a broad leaf. Evergreen trees grow faster than deciduous trees in the boreal forest. In addition, soils in boreal forest regions tend to be acidic with little available nitrogen. Leaves are a nitrogen-rich structure and deciduous trees must produce a new set of these nitrogen-rich structures each year. Therefore, coniferous trees that retain nitrogen-rich needles in a nitrogen limiting environment may have had a competitive advantage over the broad-leafed deciduous trees.
Attributions
Curated and authored by Kammy Algiers using Terrestrial Biomes from Biology 2e by OpenStax (CC-BY). Access for free at openstax.org | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/05%3A_Ecology_and_Conservation/5.04%3A_Terrestrial_Biomes/5.4.10%3A_22.10_Temperate_Rainforest.txt |
Learning Objective
Recognize distinguishing characteristics of arctic tundra & plant adaptations of the biome.
The Arctic tundra lies north of the subarctic boreal forests and is located throughout the Arctic regions of the Northern Hemisphere. Tundra also exists at elevations above the tree line on mountains. The average winter temperature is –34°C (–29.2°F) and the average summer temperature is 3°C–12°C (37°F –52°F). Plants in the Arctic tundra have a short growing season of approximately 50–60 days. Though the sun is low to the ground and the days are limited, there are almost 24 hours of daylight and plant growth is considerably rapid. The annual precipitation of the Arctic tundra is low (15–25 cm or 6–10 in) with little annual variation in precipitation. And, as in the boreal forests, there is little evaporation because of the cold temperatures.
Adaptations
Plants in the Arctic tundra are generally low to the ground and include low shrubs, grasses, lichens, and small flowering plants with small leaves (Figure \(1\)). There is little species diversity, low net primary productivity, and low aboveground biomass. The soils of the Arctic tundra may remain in a perennially frozen state referred to as permafrost. The permafrost makes it impossible for roots to penetrate far into the soil and slows the decay of organic matter, which inhibits the release of nutrients from organic matter. The melting of the permafrost in the brief summer provides water for a burst of productivity while temperatures and long days permit it. During the growing season, the ground of the Arctic tundra can be completely covered with plants or lichens.
Attributions
Curated and authored by Kammy Algiers using Terrestrial Biomes from Biology 2e by OpenStax (CC-BY). Access for free at openstax.org
5.4.13: Chapter Summary
Plants are main components of terrestrial ecosystems as they are primary producers. Almost all terrestrial life if based on the types of plants found in the area. Biomes are large-scale environments that are distinguished by characteristic vegetation and climate. Since a biome is defined by climate, the same biome can occur in geographically distinct areas with similar climates. In terrestrial biomes, temperature and precipitation determine the type of biome. There are ten major terrestrial biomes: tropical rainforests, deciduous rainforests, savannas, deserts, Mediterranean (or chaparral), temperate grasslands (or prairie), temperate deciduous forests, temperate rainforests, boreal (or coniferous) forests, and Arctic tundra. Some biomes, such as temperate grasslands and temperate forests, have distinct seasons, with cold weather and hot weather alternating throughout the year. In warm, moist biomes, such as the tropical rainforest, net primary productivity is high, as warm temperatures, abundant water, and a year-round growing season fuel plant growth and supply energy for high diversity throughout the food web. Other biomes, such as deserts and tundras, have low primary productivity due to extreme temperatures and a shortage of available water.
After completing this chapter, you should be able to
• Identify the two major abiotic factors that determine terrestrial biomes.
• The Identify the role of temperature and precipitation play on distribution of biomes.
• Recognize distinguishing characteristics of tropical rainforests & plant adaptations of the biome.
• Recognize distinguishing characteristics of tropical deciduous rainforests & plant adaptations of the biome.
• Recognize distinguishing characteristics of savannas & plant adaptations of the biome.
• Recognize distinguishing characteristics of deserts & plant adaptations of the biome.
• Recognize distinguishing characteristics of Mediterranean (chaparral) & plant adaptations of the biome.
• Recognize distinguishing characteristics of temperate grasslands (prairie) & plant adaptations of the biome.
• Recognize distinguishing characteristics of temperate deciduous forests & plant adaptations of the biome.
• Recognize distinguishing characteristics of temperate rainforests & plant adaptations of the biome.
• Recognize distinguishing characteristics of boreal (coniferous) forests & plant adaptations of the biome.
• Recognize distinguishing characteristics of Arctic tundra & plant adaptations of the biome.
Contributors and Attributions
Curated and authored by Kammy Algiers and Melissa Ha using 43.3 Terrestrial Biomes - from Biology2e by OpenStax (licensed CC-BY) | textbooks/bio/Botany/Botany_(Ha_Morrow_and_Algiers)/05%3A_Ecology_and_Conservation/5.04%3A_Terrestrial_Biomes/5.4.12%3A_Arctic_Tundra.txt |
Learning Objectives
Content Objectives
• Research the roles of macro- and micronutrients in plant life cycles
• Connect characteristic symptoms to specific nutrient deficiencies
Skill Objectives
• Use the process of science to investigate questions about nutrient deficiency
• Develop a testable, falsifiable hypothesis
• Collect data on a variety of plant characteristics and organize this data in a meaningful way
• Summarize and communicate your findings in a report
Thumbnail: This pic shows a Brassica rapa. (CC BY-SA 3.0 Unported; TeunSpaans via Wikipedia)
01: Long term Experiment - Nutrient Deficiency in Wisconsin Fast Plants (Brassica rapa)
Throughout this course, you will learn about the requirements for plant survival. One of these requirements is nutrients--elements without which a plant could not complete its life cycle. There are currently 17 known essential nutrients for plants. These include macronutrients, needed in large quantities, and micronutrients, needed only in trace amounts. The dry weight of a plant is 96% carbon, oxygen, and hydrogen, macronutrients that plants can get from air and water. Nitrogen, phosphorus, and potassium are three that you will often see reported on bags of soil (NPK ratios). Calcium, magnesium, and sulfur are the final three macronutrients.
The purpose of this experiment is to familiarize you with the process of science and observe plant nutrition in action! This process often requires a few key components: detailed observation, asking a question, doing some background research, developing a hypothesis/prediction, experimental design, collection of data, and analysis of results. To do this, we will use plants that have been bred to go through their entire life cycle in a single month: Wisconsin Fast Plants. These plants require 24 hours of light each day and do best around \(70^\circ F\) (see the seed packet for details).
1.02: Preparing for the Experiment
In this experiment, you will be investigating the effects of different nutrient deficiencies. To do this, you will first need to do some background research on the role of different essential nutrients within the plant. Use your background research to develop a question about plant nutrition.
Examples of questions
What is the effect of nutrient deficiency on vertical growth?
What effect did nutrient deficiency have on the biomass?
How does nutrient deficiency differentially affect roots vs. shoots?
Feel free to be creative with your questions!
Next, you’ll need to form a hypothesis about how plants grown in the absence of a particular nutrient will respond. A hypothesis should be a statement that predicts some influence of the independent variable on the dependent variable. Because it includes the dependent variable, your hypothesis should include measurable terms. For example, “plant health” is not measurable, but plant height and biomass are.
Hypothesis
Your statement or prediction concerning nutrient deficiency in Brassica rapa is based on what you know about plants, on your observations, from literature we have in lab, or from information you can glean from the internet, books, nursery fertilizer boxes, etc. If the data you collect does not support your hypothesis, it is okay. In fact, that is often the case in science. We are using science to test your hypothesis, not to prove it right. Here is a brief checklist I like to use when making a hypothesis:
Hypothesis checklist:
• It is a statement, not a question
• It does NOT use the phrase "I think..."
• It makes a prediction
• It is falsifiable (it is possible to collect data that proves it incorrect)
• It includes terms that I can measure
• It is specific enough that someone else could read it and know the parameters of the experiment
Next, you will need to decide what data (evidence) you will need to collect to test your hypothesis. Review the description of the experimental design on the following page to get some ideas.
Data collection
During each lab meeting your group will measure/observe at least 5 plant characteristics and record them. These data will then be included in your final written report. Be sure that your data collecting is consistent. Terms like small or large mean very little. However, smaller or larger compared with the control (complete fertilizer) may tell us something about the treatments you are using. Below are some examples of variables you might choose to measure, but do not feel restricted to this list.
Some characteristics that can be observed in comparison with the control and without the use of measuring instruments are basically qualitative:
• Leaf size – use terms like smaller, larger, same size, thicker, thinner
• Leaf appearance – terms might include darker, lighter, rougher, smoother, more hairy
• Leaf number – more, fewer, similar
• Height – taller, shorter
• Form – bushier, more elongated, more squat
• Speed of maturity – faster, slower, similar
• Disease Symptoms/Abnormalities – chlorate (yellowing), pale, spotted, blotched, wilted, curling, rotting, crinkled
Some characteristics that require measuring are basically quantitative:
• Leaf size
• Plant Height
• Leaf length
• Amount of fertilizer added/used
• Number of leaves
• Number of flowers
• Final biomass
You can take pictures to aid in cataloguing your results.
Analysis of results
We will discuss in class a few ways to analyze your results. Often obtaining averages for collected data and comparing to the control is an effective method.
If you have any questions, bring them up in class—you are probably not the only one! We will spend some time during each of the following labs making observations and discussing what we see. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/01%3A_Long_term_Experiment_-_Nutrient_Deficiency_in_Wisconsin_Fast_Plants_(Brassica_rapa)/1.01%3A_Introduction.txt |
Before you begin the experiment, record the following information:
Hypothesis (H1):
Independent variable:
Dependent variable:
Standardized variables:
Positive control(s):
Negative control(s):
1.04: The Experiment
Materials (per group)
• 6 bottles, label with group name, date, time, solution type: -P, -N, -K, -Fe, -Mg, or C for control, and a number (1-6)
• 24 seeds (Wisconsin fast plants, Brassica rapa)
• Vermiculite
• Planting medium (recommend coco coir or other nutrient-free medium)
Procedure
Planting seeds. Make 6 of the following bottle growing systems (BGS)
1. Put your wick (a strip of felt) through the hole in the cap. The wick should be 1/2 inch from the bottom of the bottle.
2. Layer approximately 1/4 cup of vermiculite into the funnel portion of the BGS.
3. Layer approximately 1/2 cup of your planting medium on top of the vermiculite so that it fills the funnel to the rim. Tap the funnel to help the soil settle loosely, then level off the excess. Do not press or compact the planting medium.
4. Gently soak the soil and vermiculite with tap water, letting it percolate through the soil until it drips from the wick at the bottom of the growing funnel. It is important to go slowly, or planting medium can splash out. This is called the run-off. The layer of planting medium should shrink down in the funnel about 0.5 - 1 cm from the rim.
5. Uniformly distribute 2*Fast Plants seeds on the surface of the moist planting medium around the perimeter of the funnel about 5 mm from the clear wall. *Seed arrangement may change, depending on the number of seeds available.
6. Cover the seeds and planting medium with a layer of vermiculite (1 cm) so that the vermiculite is level with the rim of the growing funnel.
7. Gently moisten the vermiculite with tap water until water again drips from the wick at the base of the funnel.
8. Pour off the water remaining in the reservoir, replace it with the liquid fertilizer (-P, -N, -K, -Fe, -Mg, or C). about 2 inches from the bottom – be sure that the wick is at least 1/2 inch into the fertilizer.
9. Clearly label each bottle of the six with the name of your group and which fertilizer solution you are using.
10. Place all BGS in the light rack in the back of the botany lab. Keep light on 24 hours a day.
Procedure: Weekly Data Collection
Each week replenish your fertilizer solutions in the appropriate BGS. You may also need to stake your plants to help support them and adjust the height of the lights. Try to keep your plant tops approximately 10 cm from the light, if possible.
Keep a log of your activities with the plants:
• Did all of your seeds germinate?
• Dates plants are lowered from light source or any other changes are made
• Date, type, and amount of fertilizer added
• Any observed differences in the plants within each BGS. Here are some examples
• Color changes (which colors and where on the plant)
• Growth patterns
• Flower production
• Leaf production (#, color, shape, etc...)
• Patterns of chlorosis or necrosis
1.05: Suggested Rubric
Introduction (11 pts)
( ) A brief description of the experimental design 2 pts
( ) Your question 2 pts
( ) Your hypothesis 2 pts
( ) Which variables you chose to measure 2 pts
( ) Background information to justify your hypothesis and variable selection 3 pts (no need to cite)
Data (5 pts)
( ) A presentation of your collected data (tables, graphs, etc...) 3 pts, clearly labeled! 2 pts
Analysis (9 pts)
( ) A written interpretation of your data/findings. Connect your results to plant nutrition.*(see below) 4 pts
( ) A discussion of the results in relation to your hypothesis: How do these findings relate to your hypothesis? Do they support your hypothesis or not? 2 pts
( ) A brief analysis on improvement: How you could make this experiment better? Was there anything that you think impacted your results? Are there data that you wish you had collected? etc...3 pts
This should be 1-2 pages, not including tables and graphs. Your grade will be based on inclusion of necessary components. As you write your report, use this guide as a checklist! It is also posted to Canvas.
*In this section reiterate the importance of these nutrients to the plant, their function, if they are components of any molecules (micro or macro) that we discussed in this class. This is very important – it shows me that you can apply your knowledge. Relate lecture material regarding nutrients/molecules to the appearance of your plants – how/why the lack of these nutrients/molecules might have affected the plant structure, color, flowering ability, growth, etc . of your plants.
For example: Don’t just state “phosphorus provides energy to the plant” –Tell me that phosphorus is a component of nucleic acids, of which ATP is one, and ATP is the energy molecule of the cell – driving chemical reactions.
Don’t just list the nutrients and a general description of their function – relate them to the health of your plants. Explain why the deficiency in these nutrients might explain the appearance of your plants. What role do these nutrients play in the processes like photosynthesis, mitosis, cell communication, etc. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/01%3A_Long_term_Experiment_-_Nutrient_Deficiency_in_Wisconsin_Fast_Plants_(Brassica_rapa)/1.03%3A_Before_the_experiment.txt |
Learning Objectives
Content Objectives
• Become familiar with different ecological roles organisms can have, including feeding strategies, nutrient cycling, and symbioses
• Get an introduction to some of the diversity of life covered under the umbrella of botany
Skill Objectives
• Describe the flow of energy through a community of organisms
• Ascribe ecological roles to organisms in an ecosystem
Thumbnail: Bumblebee pollinating Aquilegia vulgaris. Image used with permission (CC BY-SA 3.0 Unported; Roo72).
Contributors and Attributions
• Thuymbnail: Wasp Pollination
02: Introduction to Ecology
Complete before lab.
1. How are plants different from other organisms? What traits would you use to classify an organism as a “plant”?
2. The first law of thermodynamics states that energy cannot be created or destroyed. Instead, it is transferred and transformed into other types of energy. In a food web, how is energy transferred between organisms? How is the energy transformed in this process?
2.02: Introduction
Read the following sections before lab.
This lab manual begins by looking at the microscopic, working its way up the hierarchical organization of organisms through cells, tissues, organs, and organisms, until it arrives at large-scale evolutionary groups. For many, this series of content will only make sense if it is first situated into a larger context. This can be accomplished in a variety of ways -- perhaps a field trip looking at interacting pieces of an ecosystem, designing and building a terrarium, or a survey of samples from the different organismal groups covered in botany. How you experience this introduction will be dependent upon the resources (and weather) available to you! The content in this first lab is designed to supplement multiple methods of approach by explaining the foundational principles of ecology and allowing you the flexibility to apply them to any context.
Ecosystems
An ecosystem includes both the biotic (living) and abiotic (nonliving) components in a given environment. Though every ecosystem will be composed of a unique assemblage of interacting factors, we can classify the organisms in them by the roles that they fulfill. For example, all ecosystems require a primary producer. A primary producer is an organism that uses some external, abiotic energy source (e.g. electromagnetic or chemical energy) to build organic molecules.
In many ecosystems, plants act as the primary producers, using energy from sunlight to build sugars from the carbon atoms in carbon dioxide through a process called photosynthesis (photo- meaning light, synthesis- meaning to form or put together). However, we often overlook the other organisms who share this role, algae and photosynthetic bacteria.
Energy Flow Through Ecosystems
Collectively, we can refer to plants, algae, photosynthetic bacteria, and chemosynthetic bacteria as autotrophs (auto- meaning self, troph- meaning feeding), because they all use an abiotic energy source to form organic molecules: they make their own food. Organisms who cannot make their own food must consume other organisms to survive, obtaining energy by breaking down the complex molecules built within those other organisms. These are referred to interchangeably as heterotrophs (hetero- meaning other) and consumers.
In any given ecosystem, there may be a hierarchy of who eats whom. Primary consumers eat primary producers and can also be called herbivores. Secondary consumers eat primary consumers, while tertiary consumers eat secondary consumers. Rarely is ecology quite so clear cut, and any of these levels might also consume “lower” levels of the food chain, which is why it might be more accurate to refer to this transfer of energy as a food web.
There is another important category of organisms that aids in the completion of this network--the decomposers. Decomposers break down dead organic matter, transforming the molecules that made up those organisms into forms that are released into the environment and reabsorbed by primary producers. These organisms have a multitude of names that they may be referred to as, including detritivores, saprotrophs, saprophytes, and saprobes (sapro- meaning death or decay). Some important decomposers studied under the umbrella of botany are the true fungi, Oomycota (water molds), and the Myxogastria (slime molds).
Note
This is the end of the material that you should have completed reading prior to lab. In future labs, this won’t be explicitly stated, it will be your responsibility to keep up with the pre-lab information. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/02%3A_Introduction_to_Ecology/2.01%3A_Formative_Questions.txt |
Draw a food web. If you are on a field trip, include organisms that you see in the same area, interacting together. If in the lab, try to include organisms that you have seen or learned about. Make sure to include a primary producer, at least two levels of consumers, and a decomposer. Label which organisms are autotrophs and which are heterotrophs. Use arrows to show how energy is transferred and lost throughout this web.
What is the ultimate source of energy for your food web?
What happens to this energy as it moves through the food web? Where does it end up?
2.04: Nutrient Cycling in Ecosystems
In drawing your food web, you depicted a combination of the flow of energy and the cycling of nutrients through an ecosystem. Where as energy has an input and an output, nutrients are continually recycled. These elements often cycle between the biotic and abiotic components in an ecosystem.
For example, water is composed of hydrogen and oxygen atoms. Water molecules precipitate from the atmosphere and fall as rain, saturating the soil. A plant absorbs those molecules through its roots, transporting them up to its leaves. Some of these water molecules will be broken apart during the process of photosynthesis, the oxygen exiting the plant as \(\ce{O2}\) gas that you will breathe in while the hydrogen might be used to form molecules of glucose that you will eat. The rest of the water exits the plant through its stomata, evaporating back into the atmosphere in a process called transpiration, the evaporation of water from plant tissues. The oxygen atoms that you breathed in will be used for cellular respiration and be joined back to hydrogen molecules to once again form water. This water might exit your body as vapor on your breath or as perspiration and return directly to the atmosphere. It might also soak into your clothes as sweat or exit as urine, be processed in a wastewater facility, then sent out to the ocean where it will evaporate back into the atmosphere.
The atoms of hydrogen and oxygen are broken apart and reassembled into other molecules multiple times in this process, but the overall outcome is a cycle, with atoms traveling between the atmosphere, hydrosphere, lithosphere, and biosphere, but rarely ever exiting our Earth system.
In the space below, diagram the flow of water through the ecosystem around you.
What is the role of plants in the global cycling of water?
2.05: Mycorrhizal Networks
A symbiosis (sym- meaning shared, bio- meaning life) is when two or more organisms of different species live in close proximity to one another, sharing some aspect of their life cycle. A mutualism is a type of symbiosis in which both partners get a net benefit from the interaction. In other types of symbiosis, only one partner benefits and in a parasitic symbiosis, it is to the detriment of the other partner.
Most plants form a mutualistic symbiosis with fungi called a mycorrhizal relationship (myco- meaning fungus, rhiz- meaning root). In this particular partnership, the fungus enters the plant through the roots and takes sugars from the plant. In exchange, the plant takes water and dissolved nutrients (particularly nitrogen and phosphorus) from the fungus. Though both of these organisms lose something in the exchange, it is something that they tend to have a surplus of, while the thing they get in return is one that they would not have access to alone.
The fungus obtains its sugars from the living plant tissue. What terms could you apply to this organism?
The plant synthesizes its own sugars during photosynthesis. What terms could you apply to this organism?
Somewhat recently, a researcher named Suzanne Simard began tracking the exchange of nutrients through mycorrhizae. Not just from tree to fungus, but from tree to tree via connections to the same mycorrhizal partner. By providing certain trees with carbon dioxide containing a heavier isotope of carbon (C14), Dr. Simard could track the sugars formed from those heavy carbon atoms as they moved from tree to tree. What she found was that the connections extended beyond just one tree to another, but that almost all of the trees within her plot were connected through a network of different mycorrhizal fungi. Trees of different species and fungi of different species could exchange nutrients with each other. Further study revealed that the exchange was not limited to nutrients and water, but also included transfer of plant defense compounds that worked like raising an alarm, cuing the other plants to start producing defensive chemicals.
How might this information change the way we study ecosystems?
Sketch a few of the above ground plants that you can see (or imagine) and connect them below ground via a mycorrhizal network. Use arrows to indicate the flow of sugar through this network. Add in some disturbance that would set off a plant’s immune system (attack of the caterpillars, fungal infection, etc…) and depict the movement of the signal throughout the plant community.
Some plants have evolved to be parasitic by taking advantage of the mycorrhizal network. These plants no longer photosynthesize. Instead, they form relationships with mycorrhizal fungi that are attached to other plants, siphoning sugars from those plants through the fungal connection. For example, the parasitic plant Allotropa parasitizes the Matsutake mushroom, which is mycorrhizal with tanoaks and a few other trees.
Add a parasitic plant into your ecosystem above and use arrows to depict the movement of sugars.
Are these parasitic plants autotrophic or heterotrophic? How would you classify them, using ecology terminology? Explain your reasoning. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/02%3A_Introduction_to_Ecology/2.03%3A_Modeling_a_Food_Web.txt |
Each species in an ecosystem is considered a population. This represents a group of actually or potentially interbreeding organisms and is an important area of study for both ecologists and geneticists. When multiple species in an ecosystem are considered together, this is called a community. For example, a forest could be composed of a single population of redwood trees, but it is more likely to be a community, including other tree species like Sitka spruce and Douglas-fir, and animals like the Northern spotted owl or Coho salmon. All living organisms are considered to be a part of the community in a given ecosystem.
What is the difference between a community and an ecosystem?
Pollination
One important community interaction we study in botany (in addition to mycorrhizae) is pollination. This is the transfer of pollen from one plant to another and can be mediated by animals, water, wind, or another mechanism. Since we are talking about community interactions, we will focus on animal pollination. Many flowering plants have evolved over time to attract a specific animal pollinator. Because the plant often provides food in the form of nectar or pollen, the pollinator often relies on this relationship as well and coevolves with the plant. One interesting example of this coevolution is the pollination of figs by parasitic wasps.
The Story of the Fig and the Wasp
Figs are made of many small flowers in an inside-out inflorescence called a syconium. How, then, are these flowers pollinated to develop into fruits? Figs are pollinated by tiny chalcid wasps known as fig wasps. In the spring, female fig wasps laden with pollen and fertilized eggs enter into the fig syconium through a small opening at the top. As she enters, her wings are ripped off, making her unable to leave again. She has reached her destination.
As she walks along the tops of the flowers, which are in their female stage, she lays her eggs inside the ovaries by inserting a long tube called an ovipositor down through the style.
Pollen falls off of her body onto the flowers and travels down the styles as well to fertilize the ovaries. Each flower she deposits an egg into will develop into a wasp instead of a fruit. However, some of the styles are longer than others, and in these she will not be able to lay her eggs. These flowers will develop into the fig fruit, while the others will serve as nurseries for the fig's strange pollinators.
The male wasps hatch first. Blind and wingless, they roam around the enclosed inflorescence, impregnating their still-sleeping sisters. As carbon dioxide builds up within the chamber, they begin to suffocate and burrow out through the sides of the figs, allowing oxygen to flow back in.
After their brothers have died, the females hatch, climbing over the flowers that are now in their male stage, collecting pollen and stuffing it into pouches on their bodies. The young female wasps, laden with pollen and fertilized eggs, leave the fig through the tunnels their brothers made and fly off to find a developing fig to lay their eggs and pollinate the enclosed florets. The timeline of a fig wasp's life is intricately interwoven into the phenology of the fig tree.
View the diagram on the following page and identify the stages in the connected life cycles of these organisms. If available in lab or on your field trip, cut open a fig to view the florets developing inside the syconium.
Is this relationship a parasitism or mutualism? Explain your reasoning. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/02%3A_Introduction_to_Ecology/2.06%3A_Community_Interactions.txt |
In the pollination story above, you saw the product of a long history of coevolution. Coevolution is driven by the slow accumulation of changes in organisms (mutations) that are then acted on by a process called natural selection. For example, a purple daisy could have a mutation that caused it to produce a sugary substance. All sorts of animals would be attracted to this -- ants, bees, birds, flies, beetles, and mammals. This might help the flower get its pollen spread around, passing the sugar-production gene on to its offspring, but a lot of pollen would be wasted in the process.
Generations down the line, its offspring could have another mutation that caused it to produce a red pigment instead of purple. Birds are drawn to the color red and might visit the flower to investigate it, discovering and drinking the sugar, and picking up pollen during their visit. However, other animals would still be drawn to the sugar. If some of this flower’s offspring were to (completely by chance) develop a deeper place within the flower to store the nectar, then maybe only birds could reach it. If only birds were visiting the flowers, it is more likely that the pollen is being transferred directly to another flower of the same species.
If so, the deeper flowers with sugar and red pigment would be more likely to reproduce and pass on their genes, perhaps resulting in even deeper flowers in future generations. Birds would need to keep up with these changes if they were to continue to access the sugary food source. Offspring with longer beaks would be able to access the sugar more easily, spending less work finding food and perhaps more energy could be devoted to mating, resulting in the longer beak genes passing on to future generations. In this way, flowers and their pollinators can coevolve through a mutual codependence and the accumulation of random mutations. Individuals with mutations that result in more successful pollination for the flower and (usually) more reliable food for the pollinator are more likely to reproduce and pass these genes on to future generations.
How does the diagram on the right illustrate the process of natural selection? Circle the moths that will survive to pass on their genes.
2.08: Summative Questions
1. What roles do plants fill in an ecosystem?
2. How are plants involved in nutrient cycling?
3. What organisms play complementary roles to plants with regard to nutrient cycling? Explain your answer.
4. Explain how Allotropa, Matsutake, and tanoaks are connected within an ecosystem. Which of these organisms are producers and which are consumers?
5. Symbioses often result in coevolution of the organisms involved. In the lab, you saw coevolution from mutualistic symbioses. Describe how a parasitic symbiosis could result in coevolution of both organisms.
6. Make a concept map using the following terms. The terms below should go into bubbles. Arrange these bubbles in a way that helps communicate relationships between the terms, then connect the bubbles with lines that have a verb or action phrase attached to them. It might help to start by organizing the words into related groups. You can use words more than once. For example, I could connect the bubbles “Autotroph” and “Photosynthesis” with a line that said “makes food via”. The concept would be “an autotroph makes food via photosynthesis.”
• Autotroph
• Photosynthesis
• Chemosynthesis
• Heterotroph
• Primary producer
• Primary consumer
• Secondary consumer
• Tertiary consumer
• Decomposer | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/02%3A_Introduction_to_Ecology/2.07%3A_Coevolution_and_Natural_Selection.txt |
Learning Objectives
Content Objectives
• Get an idea of what the earliest life forms on Earth looked like.
• Know some of the roles of cyanobacteria in ecosystems, including primary production, nitrogen fixation, and mutualisms.
• Differentiate between prokaryotic and eukaryotic cells.
• Understand how cells can increase in complexity through endosymbiosis.
• Familiarize yourself with plant cell structure and function.
Skill Objectives
• Successfully use the dissecting and compound microscopes to view specimens.
• Prepare a wet mount of a specimen.
• Differentiate between prokaryotic and eukaryotic cells based on size and cellular composition.
• Select the appropriate stain to view specific aspects of a cell.
Contributors and Attributions
• Thumbnail: Anabaena colony
03: From Prokaryotes to Eukaryotes
1. What do you think the first organisms on Earth looked like?
2. What sort of ecological roles must they have been able to fulfill?
3.02: Introduction
The Earth formed around 4.54 billion years ago. As of 2019, our first evidence of life appears in fossils somewhere around 3.77-4.22 billion years old. The particular date is still much debated, as the Earth’s crust had not cooled enough to form continents until about 2.5 billion years ago and there was a rapid turnover (melting and reformation) of rocks up to this point. Additionally, these early life forms were so small and simple that some argue the fossils are likely complex mineral structures, not organisms at all.
The earliest fossils are interpreted to be unicellular organisms similar to modern day cyanobacteria, sometimes referred to as blue green algae. The most widely accepted of these fossils dates back to 3.4 billion years ago from the Strelley Pool Formation in Western Australia. These particular fossils are called stromatolites and are composed of alternating layers of fossilized cells and calcium carbonate. We can use evidence from modern day stromatolite formation in Western Australia to infer that these fossilized cells were doing a process called photosynthesis, using dissolved CO2 in the water to form sugar molecules. This causes calcium to precipitate out of the seawater, forming hardened layers of calcium carbonate on top of the colony of organisms. Because they need access to light to continue photosynthesizing, living cells begin forming a new layer on top of the calcium carbonate. This process continues, making a ringed pattern as the formation grows, much like we see in trees and corals.
3.03: Using the Dissecting Microscope
A dissecting microscope is a useful tool for viewing small features or fine details. Usually the range of magnification is around 10x to 50x. This type of microscope uses what is called incident lighting, where the light is shone onto the specimen (rather than through it, as you will see later).
View a specimen from today’s lab under the dissecting microscope. Position your lighting so that it creates as few shadows as possible, then look through the ocular lenses. You will have two knobs on the side of the microscope. One of these is a coarse focus that determines your overall magnification. Use this knob to decide on how closely you’d like to view features of your specimen. You’ll notice that it gets quite blurry very quickly. Once you have the magnification you want, use the fine focus to resolve the image (de-blur it).
Draw an interesting feature from your specimen below.
3.04: Prokaryotic Cells
Photosynthesis is one of the major ecological roles cyanobacteria currently have in modern day ecosystems. Another is nitrogen fixation. Nitrogen is an essential nutrient for plants, yet it is difficult to obtain in many ecosystems. Though it is abundant in our atmosphere, this form a nitrogen (\(\ce{N2}\)) is triple-bonded to itself, a bond which most organisms cannot break. However, certain bacteria have an enzyme called nitrogenase that can break the triple bond and convert nitrogen into usable forms for plants, such as ammonia (\(\ce{NH3}\)). These bacteria can be found free-living in the environment or in mutualistic relationships with certain plants. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/03%3A_From_Prokaryotes_to_Eukaryotes/3.01%3A_Formative_Questions.txt |
Prepare a Wet Mount
Place a drop of water onto a glass slide. Obtain a tiny piece of the water fern, Azolla, and place it on the water droplet. You are looking for a cyanobacterium, Anabaena, that lives inside the leaves of the water fern. Using a razor blade, chop the sample into small pieces, much like you would mincing garlic, to release the bacteria. Hold a glass coverslip at an angle, touching the base of the coverslip to the water droplet on your slide. Slowly lower the coverslip over the drop of water until it is flat against the slide.
Observe your Specimen
Place the slide you have prepared onto the microscope stage, holding it in place with the arm, and use the stage controls to place your specimen directly above the light source. Rotate the revolving nosepiece to the scanning objective (4x), adjust the light intensity (it should not hurt your eyes), and look through the ocular lenses (10x).
Checkpoint
Are there black crescents obstructing your field of view? If so, you need to adjust the ocular lenses to match your interpupillary distance (the distance between your eyes). As you look through the ocular lenses, adjust the distance between them until the image resolves into a single white circle.
Anatomy of a Compound Microscope
As you look through the ocular lenses, rotate the coarse focus knob away from you until your specimen comes into focus. You are viewing your specimen at 40x its actual size. What you are looking for are small strands of cells that look similar to a chain of tiny green beads. These are colonies of Anabaena that live inside the leaves of the water fern, fixing nitrogen in exchange for a safe place to live. The colonies will appear a slightly darker green than the pieces of the water fern cells.
Scan your specimen, using the stage controls to move the slide around, until you find a cluster of these dark green chains. When you find some, rotate the nosepiece to the low power objective (10x) and use the fine focus knob to adjust the focus on your specimen.
Note
If you lose your specimen when moving to a higher magnification, go back to the lower magnification and find it again.
Anabaena
When looking at the colonies of cyanobacteria, you should be able to see three different types of cells: vegetative cells, heterocysts, and akinetes.
Note
The image below shows only vegetative cells and heterocysts.
Vegetative cells are the “normal” smaller, darker green cells. These cells are doing photosynthesis and making sugars for the colony.
Heterocysts are larger, round, thick-walled cells that can appear yellow and have two polar bodies, one on each end where they attach to other cells in the colony.
Individual cyanobacteria that have converted into heterocysts are not photosynthesizing because photosynthesis produces oxygen. Instead, these cyanobacteria are fixing nitrogen -- the enzyme nitrogenase will not function in the presence of oxygen. This individual relies on the colony to make it food and, in return, it supplies the colony (and the landlord, Azolla) with nitrogen.
Akinetes are larger, football to nearly rectangular-shaped, thick-walled cells that tend to be more granular in appearance. These individuals still perform photosynthesis, but also function as a sort of failsafe. Akinetes store large amounts of lipids and carbohydrates so that they have enough energy to begin a new colony if conditions become too cold or too dry for survival. Their formation is triggered by these conditions (dry or cold), so you may not see them from a fresh water fern leaf, as this is a relatively stable, comfortable environment. If you left this slide out overnight and allowed the water to evaporate, you might see akinetes forming when you rehydrated it the following day.
Draw a colony of Anabaena, including all three cell types, labelling each with its name and function. Draw a cell of the water fern next to the colony for a size comparison.
Cyanobacteria belong to Domain Bacteria, one of two large groups of organisms called prokaryotes. Prokaryotes are unicellular organisms that have relatively simple cellular makeup. The cell is surrounded by a structure called the cell wall that protects the organism from desiccation (drying out) and other external stressors. In bacteria, this cell wall contains a compound called peptidoglycan. Inside the cell wall is a semi-permeable membrane called the plasma membrane (also called the cell membrane), which controls what enters and exits the cytoplasm of the cell. Prokaryotes have no nucleus or organelles, instead bundling their circular DNA into a compact structure called a nucleoid.
In your drawing of the Anabaena above, label the cell wall, plasma membrane, and cytoplasm of one of the vegetative cells.
The diagram below is a generalized prokaryote. There is a large circle of DNA, the circular chromosome, and a small circle of DNA, a plasmid. On the exterior, there is a long projection called a flagellum which is used for movement. The smaller projections are called pili (pilus, singular) and are used for interacting with other cells. Label these structures in the diagram below, as well as the cell wall, cytoplasm, and ribosomes.
3.06: Eukaryotic Cells
Label the following features in the diagram above:
1. Nucleus
2. Nucleolus
3. Rough endoplasmic reticulum
4. Smooth endoplasmic reticulum
5. Tonoplast
6. DNA
7. Nuclear envelope
8. Central vacuole
9. Golgi apparatus
10. Chloroplast
11. Mitochondrion
12. Ribosome
13. Cytosol
14. Cell wall
15. Middle lamella
16. Plasmodesma
*Add the cell membrane!
3.07: Primary Endosymbiosis
The water fern, Azolla, belongs to a different domain of life, Domain Eukarya. Eukaryotes have a more complex cellular makeup and can be either unicellular or multicellular. Eukaryotic cells get their name from the nucleus (eu- meaning true and karyo- meaning seed). In addition to the nucleus, eukaryotic cells contain other membrane-bound organelles. The DNA stored inside the nucleus is linear, as opposed to circular.
Around 1.56 billion years ago, a heterotrophic eukaryote engulfed a cyanobacterial ancestor in an attempt to eat it, a process called phagocytosis (phago- meaning to eat, cyto- meaning cell). Instead of being digested, this photosynthetic prokaryote remained inside the cell and continued to photosynthesize. The eukaryotic host must have benefited from the sugars produced by the prokaryotic captive and this relationship developed into a symbiosis (sym- meaning shared, bio- meaning life), much like the Anabaena and the Azolla.
Overtime, the captive prokaryote evolved into the chloroplasts and other plastids that we see in plant cells today. This process of converting a free-living organism into an organelle is called endosymbiosis (endo- meaning inner) and is also the way eukaryotes evolved mitochondria. All plants, such as the water fern you are looking at, descended from this primary endosymbiosis event that led to the first chloroplasts.
In the diagram of endosymbiosis above, label the cyanobacterium, the eukaryote, nucleoid, nucleus, and chloroplast.
What evidence could we look for in a chloroplast that would indicate evolution from a bacterium? | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/03%3A_From_Prokaryotes_to_Eukaryotes/3.05%3A_Using_the_Compound_Microscope.txt |
Because Azolla is a plant, its cells are surrounded by a rigid cell wall containing cellulose. Like the bacteria, the plasma membrane is located just inside the cell wall and contains the cytoplasm. The cell wall tends to give plant cells a boxy, rigid structure.
The most obvious of the membrane-bound organelles you will see are the chloroplasts. These numerous, green, disc-like structures are responsible for doing photosynthesis, making food for the plant. Their color is derived from the presence of chlorophylls, green-pigmented molecules involved in harvesting light for photosynthesis. You might also see that these chloroplasts are pushed toward the edges of the cell. This is because the majority of the cell is taken up by a large, liquid-filled structure called the central vacuole, surrounded by a membrane called the tonoplast. The central vacuole contributes to the plant’s overall structure by drawing in water, inflating the cell to a turgid state (more on this in lab 5).
Above is a cell from the aquatic plant Elodea. The cell wall, nucleus, and chloroplasts are visible.
Draw a cell from the Azolla in the space below. Label the cell wall, plasma membrane, cytoplasm, chloroplasts, nucleus (if you see it), central vacuole, and tonoplast.
The layers in an onion bulb are fleshy leaves that have been modified to store starches for the plant. At the base of the bulb, you will see a short stem with roots emerging. The part of the onion that you will be observing is the epidermis (outermost layer) of the leaf.
Cut a small section of an onion leaf. Carefully peel off a section of the epidermis--not the pigmented side, but the paler side that faces the interior of the onion. It should come off as a translucent sheet. Prepare a wet mount of onion epidermis using a drop or two of IKI solution.
This iodine solution increases the refractivity of the light and makes the nucleus visible. You should also be able to see at least one dark dot in the nucleus. This is the nucleolus (plural, nucleoli), a region of the nucleus where ribosomes are assembled.
View your specimen with the compound microscope. Draw an onion cell and label the cell wall, plasma membrane, cytoplasm, nucleus, nucleolus, central vacuole, and tonoplast.
Why are there no chloroplasts in this plant cell? Consider where an onion bulb would be located in a garden setting.
Plant cells are static within the plant (they do not move from place to place) and are surrounded by a cell wall. Adjacent cells are “glued” together by a pectin-rich layer called the middle lamella. As you saw above, the onion cells did not contain chloroplasts, meaning they are not producing their own food, yet they are used to store large amounts of sugars in the form of starch. How then do these cells communicate with each other and exchange nutrients? When plant cells divide, the division is incomplete, leaving small channels through the cell wall and middle lamella. These channels are called plasmodesmata (plasmodesma, singular). You can look for these in the epidermal cells of a red pepper (or similar fruit).
Peppers are fruits whose function is to attract animals to disperse the seeds of the pepper plant. To do this, the pepper goes through a ripening process. Pectins (a form of starch) in the middle lamella begin to break down into simple sugars, making the pepper both sweeter and softer. The color of the pepper also begins to change as chloroplasts are converted to chromoplasts. These organelles contain pigments called carotenoids that produce yellows, oranges, and reds, the last of which is highly attractive to animals like us.
The image above shows epidermal cells in a red pepper. They are filled with small, red, disc-like plastids. These are chromoplasts filled with carotenoids.
In the cell walls, you can see tiny gaps that look more like constrictions. These are the plasmodesmata.
Peel off a small piece of red pepper epidermis and make a wet mount with water. Using the compound microscope, look for small, red chromoplasts within the cells. Next, look for breaks in the regions between the cells. These are the plasmodesmata.
Draw two adjacent pepper epidermal cells. Label the cell wall, middle lamella, plasmodesmata, and chromoplasts. You are encouraged to identify and label other cell components, such as the nucleus and nucleolus, if they are visible.
A potato is a modified part of the plant called a tuber. Much like an onion, a tuber is a part of the plant--this time the stem--adapted for storing starch. Potatoes synthesize and store these starches in organelles called amyloplasts (named for the starch amylose). In other cases, similar organelles are used to store lipids or other complex molecules. In general, these organelles are called leucoplasts (leuco- meaning white), because they lack pigments.
Using a razor blade or sharp probe, scrape a small amount of the interior of a potato (not the skin) onto a slide. Make a wet mount using one drop of IKI followed by one drop of water. The iodine in the IKI solution will surround the starch molecules, causing starches to appear to stain blue to purple. This will help you locate the amyloplasts within the potato cells, as they will absorb the IKI and the entire organelle is colored purple.
Draw a potato cell and label the cell wall, plasma membrane, cytoplasm, and amyloplasts.
3.09: Summative Questions
1. What were the earliest life forms on Earth and how did they obtain food?
2. What evidence do we have of this?
3. Explain how endosymbiosis influenced the evolution of eukaryotes.
4. Describe the relationship between Anabaena and Azolla. What type of symbiosis is this?
5. What is the difference between a chloroplast, chromoplast, and amyloplast? Where in plants would each of these different plastids most likely be found? Consider the function of each of these organelles in your answer.
6. Compare and contrast prokaryotes and eukaryotes. What features do they share? What sets them apart from each other? | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/03%3A_From_Prokaryotes_to_Eukaryotes/3.08%3A_Cell_Structures_and_Organelles.txt |
Learning Objectives
Content Objectives
• Learn the primary cell and tissue types found in plants (the -enchymas)
• See how tissues are composed of cells that work together to perform a similar function
• Understand how different tissues can work together to perform a united function as an organ
Skill Objectives
• Differentiate between parenchyma, collenchyma, and sclerenchyma
• Use Toluidine Blue stain to locate cell types in different plant tissues
• Identify the function and cell type of specialized cells
• Make a thin section of plant tissue where you can clearly see cell types
Contributors and Attributions
• Thumbnail: Phloem Tissue
04: Plant Cell Types and Tissues
1. Can unicellular organisms have tissues? Explain your answer.
2. Why would a nucleus be required for an organism to be multicellular? What does this organelle allow cells to do?
4.02: Introduction
Plant cells can be classified into three major categories based on their cell wall structure. Most plant cells are parenchyma cells (par- meaning equal), with an evenly-thickened primary cell wall. Some areas of a plant, particularly in young shoots, will require more flexibility, allowing them to bend without breaking. In these regions, you will often find collenchyma cells (coll- meaning glue), cells with strange, unevenly-thickened primary walls. Both of these cell types are alive at maturity. A third cell type, sclerenchyma cells (scler- meaning hard), develops a rigid secondary wall that is composed of lignin. This secondary wall provides structural support, but it also cuts off the exchange of water and other molecules permitted by the plasma membrane. Ultimately, this results in the death of the cell at maturity and loss of internal cell components.
Parenchyma Collenchyma Sclerenchyma
Plant cells performing a similar function can be assembled into tissues. This lab will introduce you to the three primary cell types found in plants, the tissues where you can find these cell types, and specialized cells that they can differentiate into. Specialized cells covered in this lab will be either parenchyma, collenchyma, or sclerenchyma (referred to as the “cell type”) and will have evolved for a specific function, such as conducting water. At the end of the lab, you will complete a table with the cell types, specialized cells, and functions of the major tissues within a leaf.
4.03: Identifying Cell Types and Tissues
Cell Types
Make a thin section of a celery petiole or the main celery stalk. This needs to be very thin to see the features you are looking for, so make a few samples to look at! If you would like to stain your specimen, place the specimen on a slide and add a small drop of Toluidine Blue. Wait a few seconds for the dye to penetrate into the sample, then rinse by adding water to the slide and either soaking up or draining off the excess liquid. When the water is mostly clear, add another drop or two of water and a coverslip.
View your specimen under the compound microscope.
You should be able to see several cell types in your specimen. Most of the cells will be parenchyma. A great place to look for textbook parenchyma cells is the outermost layer of the plant, the epidermis.
You will find collenchyma cells in dense clusters near the epidermis in a region called the cortex, forming the strings that you would find in your celery. They appear to have an almost checkerboard-like pattern, due to the unevenly thickened primary walls. In Toluidine Blue, primary walls stain purple.
Some specialized cells can be found in the vascular tissue, organized regions of cells that are transporting water, sugars, and other chemicals throughout the plant body. The xylem is the tissue responsible for conducting water. Specialized cells in the xylem tissue called tracheids and vessel elements have evolved specifically for this ability by forming hollow tubes with lignified secondary walls. Tracheids evolved first and are narrow with tapered ends. Vessel elements evolved in the most recent group of plants, the Angiosperms, and are usually much wider than tracheids. In Toluidine Blue, the lignin in the secondary wall stains bright aqua blue.
The image above shows three different types of cells with secondary walls found in wood pulp. A vessel element is shown in the center with a tracheid running parallel just above it. Note the pits in the walls of both of these cells and the large holes (perforation plates) on the ends of the vessel element only. Criss-crossing the rest of the slide are many thin fibers. All of these cells are dead at maturity and provide structural support due to the lignin in the cell walls.
In the image above, you can see the pits in the walls of a tracheid.
Unlike the xylem, conducting cells in the phloem tissue are alive so they may transport sugars and communication signals in any direction. These cells, sieve tube elements and companion cells, are more similar to parenchyma. The sieve tube elements conduct sugars and have specialized to do this by having reduced cytoplasm contents: sieve tube elements have no nucleus (or vacuole)! These cells are controlled by small, adjacent cells called companion cells. If you look closely, you can also see some sclerenchyma bunched together in the phloem. These are the phloem fibers. Their thick secondary walls should stain the same color as the tracheids and vessel elements.
In the image above, you can see clusters of thick walled fibers, large open sieve tube elements, and small companion cells containing nuclei.
Draw a cross section of the celery petiole, labeling parenchyma in the epidermis, collenchyma in the cortex, and sclerenchyma in the vascular tissue. Make notes about the differences in the cell wall for your future study.
The central region of the celery petiole is called the pith. What type of cells are present in this region?
While collenchyma tissue tends to have one job--flexible support--parenchyma and sclerenchyma can fill a diverse set of roles. In a developing pear, there is a high density of a second type of sclerenchyma cells called sclereids (the first type of sclerenchyma cells were fibers). These cells cause young pears to be tough and unpalatable, as the seeds inside are still developing. As the seeds mature, the pear ripens, making more parenchyma cells for storing large amounts of sugar, while the tough sclereids are slowly outnumbered by the larger, juicier cells. The grit that you feel when eating a pear are these remaining sclereids.
What other cellular changes might occur to signal that a pear is ripe?
Make a squash mount of the flesh of a pear (not the skin) by scraping off a small amount with a razorblade. Do not take a slice or a chunk, just a tiny bit of pulp (consider chopping it up on the slide). Again, I recommend staining with Toluidine blue, as this should make the thick secondary walls of the sclereids appear a bright aqua blue. Sclereids tend to occur in clusters, surrounded by large parenchyma cells. You may need to gently squish your coverslip down a bit to help disperse these clumps. Coverslips are fragile, so ask your instructor what they recommend before doing anything that might result with glass in your fingers.
When you find a sclereid, you should see lines running through the secondary wall. These are channels where the plasmodesmata extended through to connect to other cells. After the cell dies, only the empty channels (called pits) remain.
Draw a sclereid, located in the ground tissue of a pear. Label the secondary wall, pits, an adjacent parenchyma cell, and the primary wall of that parenchyma cell.
Is this sclereid alive or dead? What about the parenchyma cells around it?
What is the compound in the secondary wall that stains differently from the primary wall?
Tissues in the Leaf
When cells of the same type work together to perform a collective function, the collection of cells is called a tissue. For example, the epidermis is a collection of parenchyma-like cells working together to separate the internal environment of the plant from the exterior. The epidermis also contains specialized cells. Trichomes are outgrowths from the epidermis that look like hairs. These can protect the plant from sun damage by being white and reflective, trap evaporating moisture on the plant’s surface, secrete sticky substances, and be unpleasant for herbivores.
A second type of specialized cell in the epidermis is the guard cell. Guard cells are shaped like parentheses and flank small pores in the epidermis called stomata (sing. stoma). When the plant has adequate water, the guard cells inflate and the stoma is open, allowing water vapor to escape through transpiration. When the plant is low on water, the guard cells collapse, closing the stoma and trapping water inside. However, for the plant to perform photosynthesis, it must have access to carbon dioxide and be able to release oxygen. Both of these gases are exchanged through the stomata.
The image above is from the lower epidermis of a Nerium leaf. These plants live in harsh, dry environments and have many adaptations to prevent water loss. This is a pocket on the lower side of the leaf where stomata are located. You can see three different sets of guard cells, currently closed, appearing slightly darker than the other epidermal cells. Surrounding these stomata and filling the pocket are trichomes.
How does the location of the trichomes relate to prevention of water loss?
View a leaf under the dissecting scope. Can you find trichomes, guard cells, or other specialized epidermal cells?
Peel off the lower epidermis of the leaf, similar to how you removed it from the onion. It may help to break the leaf slowly, hopefully getting a piece of the epidermis that you can peel off. It will look like a transparent layer of skin. Make a wet mount of the epidermis and view it under the compound microscope. Draw what you see below, labeling any specialized epidermal cells.
What cell type (-enchyma) are these cells most similar to?
When multiple tissues work together to perform a collective function, this collection of tissues is called an organ. While we are familiar with the concept of organs in animals, it can sometimes be surprising to consider this aspect of plants.
An example of an organ in a plant is the leaf. A leaf is surrounded by epidermal tissue, protecting the interior environment, and allowing for the exchange of gases with the environment. The xylem tissue, found in the veins of the leaf, provides the water needed for specialized parenchyma, mesophyll cells, to carry out photosynthesis. Phloem tissue runs alongside the xylem tissue, transporting sugars made during photosynthesis to other areas of the plant for either immediate use or storage. Together, these tissues allow the leaf to function as an organ specialized for photosynthesis.
View a prepared slide of a leaf cross section. Draw what you see below. Identify and label as many tissues, cell types, and specialized cells as you can.
Summary Table of Cells and Tissues in the Leaf Organ
A simple tissue contains only a single cell type, while a complex tissue contains multiple cell types. The leaf organ is composed of both simple and complex tissues. In the table below under Tissue Type, try to identify whether it is a simple or complex tissue. If it is a simple tissue, identify which cell type it is composed of.
Tissue
Specialized Cells
Tissue Type
Function
Epidermis
1. Guard cell
2. Trichome
Mesophyll
Mesophyll cells
Xylem
Tracheids & vessel elements
Phloem
1. Sieve tube elements
2. Companion cells
3. Phloem fibers
Why didn’t I include a stoma among the specialized cells in the epidermis?
4.04: Summative Questions
1. What is the function of the xylem tissue and which specialized cells are involved in this function? Was this a simple or complex tissue in the specimens you viewed?
2. What is the function of the phloem tissue and how do the specialized cells in this tissue contribute to this function? Was this a simple or complex tissue in the specimens you viewed?
3. What is the function of the epidermis and how do the specialized cells in the epidermis contribute to this function? Was this a simple or complex tissue in the specimens you viewed?
4. One of the characteristics of living things is a hierarchical (nested) organizational structure. Organize the following terms into a nested structure: specialized cell, organ, organism, tissue, organelle.
5. Do the same with the following terms: chloroplast, leaf, guard cell, epidermis, plant.
6. Compare and contrast the function and appearance of parenchyma cells in the cortex of a plant. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/04%3A_Plant_Cell_Types_and_Tissues/4.01%3A_Formative_Questions.txt |
Learning Objectives
Content Objectives
• Learn the process of mitosis and how this type of cell division is used for growth, repair, and asexual reproduction
• Understand that mitosis is a type of cell division that results in clones of the original cell
• Learn the two phases of cell division: mitosis is the division of the genetic material, while cytokinesis is the division of the rest of the cytoplasm and the cell wall
• Understand that prokaryotes cannot undergo mitosis because they lack a nucleus
Skill Objectives
• Differentiate between chromatin and chromosomes in a prepared slide
• Locate and identify important cell structures, including the nucleus, nucleoli, nuclear envelope, and cell wall
• Identify cells in different stages of mitosis based on chromosomal organization
Contributors and Attributions
• Thumbnail: A cell in anaphase
05: Multicellularity and Asexual Reproduction
1. What is the difference between growth and reproduction?
2. How would you determine whether an organism is multicellular or a colony of unicellular organisms? What information would you need to make this determination?
5.02: Introduction
The first organisms on Earth were likely unicellular prokaryotes. These organisms could reproduce themselves by increasing the amount of cytoplasm, making a copy of their single loop of DNA, and forming a new cell wall to divide the original cell into two identical cells. This form of cell division is called binary fission (binary meaning two, fission meaning to split apart). This is a form of asexual reproduction, where an organism reproduces an exact genetic copy of itself. This is also sometimes called clonal reproduction.
In binary fission, one cell enlarges and the DNA is replicated, dividing into two cells.
As eukaryotic cells evolved, cell division became more complex. There were now organelles to replicate and organize. Perhaps more importantly, division of the DNA became more complicated. Instead of a single loop of DNA, eukaryotic DNA occurs in linear strands that are then wound around histone proteins. This combination of DNA and proteins is called chromatin, because it appeared dark when stained. When eukaryotic cells divide, they further condense the chromatin into tightly wound complexes called chromosomes. This organizational structure is necessary, as eukaryotic cells need instructions to make and maintain all of the new cell components, often resulting in large amounts of DNA. For example, there are approximately six feet of DNA that need to be contained within the nucleus of each cell in your body. Six feet! If you lined up all of the DNA from each of your trillion or so cells, it would stretch across the solar system...twice! How to organize this massive amount of DNA so that it can be replicated and divided into two identical sets as a cell divides?
5.03: Anatomy of a Chromosome
Chromosome literally means “colored body”. These densely packed structures are composed of highly organized DNA and have distinct regions we can make reference to. An unreplicated chromosome looks a bit like a cucumber that has been constricted in the middle (or two breakfast links, still connected). The central constricted region is called the centromere. When the chromosome is replicated, it will become two identical sister chromatids that are attached at the centromere region. You can count the number of chromosomes present by counting the number of centromeres. Two attached sister chromatids are one chromosome, but the instant they separate, two centromeres are visible and the sister chromatids each become individual chromosomes. This will become important later.
The main body of the chromosome above and below the centromere are called arms and the end of each arm is the telomere. Telomeres are of interest to many scientists studying aging, as it is possible that the degeneration of telomeres during replication eventually leads to the suite of symptoms we connect with the aging process. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/05%3A_Multicellularity_and_Asexual_Reproduction/5.01%3A_Formative_Questions.txt |
Mitosis is the division of the nucleus in eukaryotic cells to make two identical nuclei. This is accompanied by cytokinesis (cyto- meaning cell, kinesis meaning movement), division of the cytoplasm, to result in division of the entire cell into two identical daughter cells.
A unicellular eukaryote might do mitosis to reproduce asexually, making an identical copy of itself. This is common practice in many groups of organisms. For example, molds are fungi that are reproducing asexually--each mold spore is genetically identical to the others. Many unicellular algae will also reproduce this way.
Multicellular eukaryotes also do mitosis -- that is how they became multicellular! Multicellular eukaryotes primarily use mitosis for growth and repair of damage, though some also reproduce asexually (such as strawberries and mint). To see mitosis occurring in a plant, the best place to look are the growing tips, as most plants experience apical growth (growth from the tips).
Obtain a prepared slide of an onion root tip (Allium cepa). Cells toward the apex (pointed end) are likely to have been caught in a stage of active division.
Prophase
Mitosis begins with prophase. During this portion of mitosis, the chromatin condenses into a more organized state -- the chromosome. The nuclear envelope begins to break down and any nucleoli dissolve. The cell begins to produce a network of microtubules called the spindle (in animal cells, the production and organization of the spindle is organized by a set of organelles called centrioles that assemble together into a centrosome). These networks begin to form on opposite sides of the cell.
You can identify cells in prophase by looking for the following:
1. The nuclear region has expanded and moved to the center of the cell
2. DNA appears as distinct structures (sausage-like), still contained in the center of the cell
3. At the end of prophase, no nucleolus is present
The image shows a cell in early prophase, surrounded by cells in interphase. The nucleoli are still visible in all of these cells as dark dots, but the cell in early prophase has condensed its chromatin into darkly stained chromosomes, whereas the cells in interphase still have the lightly staining grey chromatin. The circular organization of the chromosomes indicates that the nuclear envelope has not finished breaking down yet.
Compare the cell in early prophase (left) to a cell in late prophase (right). As the nuclear envelope finishes breaking down, the chromosomes become more spread out. Additionally, no nucleolus is visible. During this time, the spindle fibers attach to the kinetochores and are about to start pushing and pulling the chromosomes into the middle of the cell.
Draw an onion cell in prophase. Label the cell wall, plasma membrane, nuclear envelope (or where it would be), and chromosomes. If a nucleolus is still present or you can distinguish a forming spindle, label these as well.
Metaphase
As the cell transitions into metaphase, referred to as prometaphase, the spindle fibers attach to sticky regions called kinetochores that flank the central region of the chromosome, the centromere. The centromere holds the two identical pieces of the chromosome, the sister chromatids, together. The spindle fibers contract and expand to push and pull the chromosomes into a line in the center of the cell, the metaphase plate. This is where the new cell wall will form
You can identify cells in metaphase by looking for the following: Chromosomes are found in the center of the cell, they appear to be connected, often forming a straight line.
There are two cells in this image not currently in interphase. The cell on the top is in metaphase.
Which stage of mitosis is the cell on the bottom in? How can you tell?
Draw an onion cell in metaphase. Label the cell wall, plasma membrane, chromosomes, spindle, and metaphase plate.
Anaphase
Anaphase begins when the two networks of spindle fibers start to contract. During this process, sister chromatids of each chromosome are pulled apart, each becoming an individual chromosome the moment they are separated. These chromosomes are pulled to either side of the cell in preparation to form two new nuclei.
You can identify cells in anaphase by looking for the following:
1. There should be two distinct groups of chromosomes
2. individual chromosomes can still be distinguished.
This image depicts a cell in anaphase. Are the dark lines in this cell chromatids or chromosomes? How can you tell?
Draw an onion cell in anaphase. Label the cell wall, plasma membrane, chromosomes, spindle, and metaphase plate.
Telophase
The final stage of mitosis is telophase. During this stage, a nuclear envelope begins to reform around each group of chromosomes and the chromosomes decondense into chromatin. The spindle fibers begin to break down and any nucleoli reform.
You can identify cells in telophase by looking for the following:
1. There should be two distinct darkened regions, but individual chromosomes cannot be distinguished.
2. The formation of the new cell wall, which occurs during cytokinesis, is still incomplete and the area between the two forming nuclei appears blurry.
This image shows a cell on the left that is currently in telophase, forming two new nuclei. On the right side of the image there are three cells that have probably recently finished dividing and have entered interphase.
Draw an onion cell in telophase. Label the cell walls, plasma membranes, forming nuclear envelope, and chromatin. (It is likely that the DNA is in a transitional state between chromosomes and chromatin)
Cytokinesis
The formation of the new cell wall and division of the cytoplasm is called cytokinesis. This stage of cell division happens concurrently with telophase, but is not part of mitosis, as that refers strictly to the division of the nucleus.
In the image below, label the new cell wall that is forming during cytokinesis.
Below is a diagram of mitosis. Label each stage and the relevant structures. Make note of important events. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/05%3A_Multicellularity_and_Asexual_Reproduction/5.04%3A_Mitosis.txt |
As the onion root tip grows deeper into the ground, the cells at the tip continue to divide. One of the resulting cells will remain in place and differentiate into a mature cell, such as a vessel element or a sieve cell. The other cell will remain meristematic, continuing to divide as the root grows deeper into the soil. The word meristem, unfortunately, doesn’t have a useful root to learn. It is perhaps ironic that the word meristem cannot be divided up into interpretable segments, as the term is derived from a Greek word that means “to divide”.
Cells that remain in the meristematic region will remain in the cell cycle, entering a period called interphase, which is composed of three distinct phases. G1 (gap 1), is when cells duplicate the contents of the cytoplasm, including the organelles and cytosol. At the end of G1, they will reach a checkpoint. If duplication of cell contents was successful, they will pass into S-phase (synthesis). During S-phase, DNA is replicated and each chromosome becomes a set of sister chromatids. After S-phase, there is another checkpoint to ensure that replication of cell contents, specifically the DNA, was successful. Finally, the cell enters G2 (gap 2), where it passes a final checkpoint before division. The cell enters cell division again, completing mitosis and cytokinesis to produce two identical daughter cells.
Cells that differentiate exit the cell cycle into G0 (resting). In this stage, cells can begin to express different genes and specialize for specific tasks. You will see these cells in the upper portion (away from the tip) of the onion root. Note that these cells are much larger than the actively dividing cells in the meristematic region of the root.
Why are so many checkpoints necessary during this process? What do you think would happen if there weren’t checkpoints?
5.06: Meristems and Tissues in the Root
The Root Apical Meristem (RAM, for short)
When you are viewing the onion root tip, you are viewing the beginning of the formation of tissues of the plant’s root system. This formation of tissues begins with the root apical meristem (RAM). Just below the tip of the root, there is a region of small, densely packed cells that are actively dividing. These cells make up the RAM. To the outside (below, in your slide) of the root tip, the RAM cells produce an ephemeral tissue called the root cap. Cells in the root cap protect the root apical meristem, secrete chemicals to attract beneficial bacteria and fungi, and secrete a mucilage that makes it easier for the root tip to penetrate the soil substrate. As the root tip grows, these cells are sloughed off into the environment and so the root cap must be continually produced by the RAM. To the inside (above, in your slide), the RAM produces cells that begin to form the three primary meristems. Remember, a meristem is a region of cells whose function is to produce more cells!
Based on the root tips you have looked at today, how might you identify meristematic regions in a plant organ under the microscope? What would you look for and why?
Primary Meristems and Their Primary Tissues
Primary meristems divide to form the primary tissues. The protoderm is the primary meristem that produces the epidermis. The procambium is the primary meristem that produces the vascular tissues (xylem, phloem, and any associated tissues). The ground meristem is the primary meristem that produces the ground tissue. Depending on the type of plant or the part of the plant, the ground tissue can be divided into distinct regions or not. The Zea mays root tip in the image below belongs to a monocot, so the ground tissue in this particular sample is divided into the pith (appearing as a column of tissue in the center of the root tip, bordered on either side by vascular tissue) and the cortex (located between the epidermis and the vascular tissue on either side of your slide).
In the image above, label the RAM, protoderm, procambium, ground meristem, pith, cortex, and vascular tissue.
Where in the root above would you be likely to find cells in the G0 phase of the cell cycle?
Where would you be likely to find cells in G1, S-phase, or G2?
5.07: Multicellularity and Clonal Reproduction
Many unicellular eukaryotes undergo mitosis for asexual reproduction. This is often called clonal reproduction, because offspring are identical to the parent cell. Because prokaryotes do not have a nucleus, they cannot undergo mitosis, but they can still do asexual reproduction.
How do prokaryotes reproduce asexually?
On the right is a colony of the cyanobacterium Nostoc. These unicellular organisms reproduce asexually to form colonies of individuals that live together in a secreted mucilaginous sheath (the bubble surrounding them). Much like Anabaena, some of these individuals are converted into heterocysts and are unable to feed themselves. How can we distinguish between a collaborative community of individuals and a multicellular organism?
The root tips you have been viewing are a small portion of the larger organism. How did life become multicellular from its unicellular origins? There are many theories about how this happened, but it is well-supported that multicellularity evolved multiple times in the tree of life. One theory for how multicellularity first occurred is through errors in cytokinesis, resulting in two identical cells that were still conjoined. This can be induced in yeasts (unicellular fungi), resulting in branching structures. These yeasts are referred to as snowflake yeasts. Investigate further here: http://www.snowflakeyeastlab.com/experimental%20evolution.htm
5.08: Summative Questions
1. Mitosis produces identical cells. What can this process be used for?
2. Compare and contrast binary fission and mitosis. Why can’t prokaryotes do mitosis?
3. What is the difference between mitosis and cytokinesis?
4. What are the major events that happen in each stage of mitosis?
5. How does the theory of multicellularity described for snowflake yeast relate to the plasmodesmata you have seen forming connections between plant cells? | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/05%3A_Multicellularity_and_Asexual_Reproduction/5.05%3A_The_Cell_Cycle.txt |
Learning Objectives
Content Objectives
• Understand what influences the weird behavior of water, particularly tonicity, adhesion, and cohesion
• Learn how transpiration drives the movement of water through a plant and which environmental factors influence transpiration rates
Skill Objectives
• Predict the movement of water in different situations and explain why it moved the way it did
• Draw and describe the flow of water as it moves through a plant from the soil environment to the atmosphere
06: Roots and the Movement of Water - How is water moved through a plant
1. What is easier, drinking water through a wide tube or drinking water through a thin tube? Why do you think that is?
2. Why do water drops form a three dimensional, domed structure when dripped onto a surface, rather than spreading out?
6.2: Introduction
Plants are nature’s great water filters. They absorb water from the soil through their roots (if they have roots), use this water to maintain homeostasis, and whatever is left evaporates from open stomata across the epidermis of the plant. Each water molecule that leaves the plant is electrically charged and, due to these charges, tugs on the molecule behind it. The vast majority of water absorbed by most plants will exit via this process, creating a continuous vacuum of water through the plant, from the soil to the atmosphere. This evaporation of water from plant tissues is called transpiration. This process is highly dependent on the current environment, but to provide some perspective, an acre of corn transpires about 3,500 gallons of water per day.
Why do plants transpire if they are losing so much water everyday? Why not close their stomata to prevent water from escaping? There are many reasons why plants have evolved to transpire instead of retaining water.
1. Transpiration drives the flow of water and dissolved nutrients through the plant. If plants stop releasing water through the stomata, they will stop pulling in the nutrients dissolved in that water essential for plant function.
2. Transpiration provides evaporative cooling. As water leaves the plant tissues into the atmosphere, it takes energy with it in the form of heat. Much like when we sweat, this allows the plant to cool and maintain homeostasis. This is particularly valuable in hot environments.
3. Leaving stomata open is required for photosynthesis (except in certain plants, more on this in Lab 10: Photosynthesis). Carbon dioxide enters the plant through the stomata and oxygen is released as a waste product. If the stomata are closed, the plant cannot form sugars.
→ At the beginning of this lab as a class, set up and number 4 potted plants of similar size and type. On each plant, put a bag over one of the branches (or main stem) and tape it closed around the base. Note: the bags should be the same type or the mass of each bag should be recorded beforehand. Water plants 1, 2, and 3 until the soil is saturated. Put plant 1 under a light or next to a sunny window, plant 2 in the dark, and plants 3 and 4 somewhere away from a window but where light is still present. You will check on these later in the lab. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/06%3A_Roots_and_the_Movement_of_Water_-_How_is_water_moved_through_a_plant/6.1%3A_Formative_Questions.txt |
Because all atoms are constantly in motion and because this motion is random, atoms tend to disperse relatively evenly across an environment because this arrangement has a higher probability of occurring. This process is called diffusion. When you drop food coloring into a glass of water, the molecules in the food coloring bounce off of each other and begin to gradually disperse away from each other. The water molecules do the same until the molecules of food coloring and the molecules of water are relatively evenly dispersed throughout the glass. The molecules don’t stop moving, but due to the random motion, the mixture appears to remain evenly mixed.
Osmosis
Osmosis is the diffusion of water across a semipermeable membrane. Because the membrane is semipermeable, water can move through the membrane, but other dissolved compounds may not be able to, for example dissolved ions like Na+ and Cl-. This has some interesting implications.
Tonicity
To think about osmosis, it is important to consider the concentration of water molecules relative to the amount of stuff dissolved in it on either side of the membrane. This concept is called tonicity -- how much stuff is dissolved in the solution relative to the other side.
Water will move from areas of high water concentration to areas of low water concentration. Even if you have the same volume of water on both sides of a membrane, the water will move to the side of the membrane with more dissolved solutes, which can also be interpreted as relatively less water.
Hypotonic
How does this influence plant structure and function? Plants use tonicity to provide increased structural support by keeping plant cells in a turgid state (swollen with water). Plants sequester dissolved solutes in the central vacuole, making the solution hypotonic.
Considering your knowledge of root words, what do you think the term hypotonic means?
Because the concentration of water is relatively low inside the central vacuole (due to the high concentration of solutes), this causes water to move from the exterior environment, where it is at a higher concentration, into the central vacuole. The cell wall keeps the plasma membrane from exploding as the central vacuole swells with water and pushes the cytoplasm outward. Think of it like blowing up a balloon inside a cardboard box--the tension of the inflated balloon provides structural support to the box. The cell above is in a hypotonic solution.
Isotonic
For animal cells, which lack a cell wall, being in a hypotonic solution causes cells to swell and burst. Our cells remain in an isotonic solution (iso- meaning equal or identical), with the concentration of dissolved solutes (and therefore water) approximately the same in the intracellular and extracellular fluids. The cell in the diagram is in an isotonic solution. Plant cells in this state are slightly wilted.
Hypertonic
The third possibility is that the concentration of solutes is higher in the solution than inside the cell, a hypertonic solution (hyper- meaning over or above). This means that the concentration of water is relatively higher inside the cell than outside, causing the water to move out. This is why salt can dehydrate (pull water out of) things, including organisms. If a plant that is not adapted to saltwater is submerged in a solution of saltwater, or perhaps there is too much salt in the soil, water will exit the plant cells to enter the solution. The cell in the diagram on the right is in a hypertonic solution.
To observe the effects of tonicity on plant cells, prepare a wet mount of an Elodea leaf (or similar freshwater plant). For the initial preparation, use pond water or water from the tank the Elodea was growing in. Observe at 400x magnification.
Draw a few cells in the space below, labeling the cell wall, plasma membrane, chloroplasts and tonoplast. The tonoplast is the membrane of the central vacuole. Though you cannot see it directly, you can infer the location of the tonoplast by where the chloroplasts are--they line the outside of it.
Remove the coverslip, soak up as much of the water as you can, and add a few drops of saltwater (sodium chloride, NaCl, solution) to your slide. Quickly place back under the microscope and view at 400x. Once you see a change in the cells, draw a few of them. Label the cell wall, plasma membrane, chloroplasts and tonoplast.
What happened to the cell? Was the solution hypotonic, isotonic, or hypertonic?
In your drawing, use arrows to depict the movement of water either entering or exiting the central vacuole.
Soak up as much of the saltwater as you can, and this time add a few drops of distilled water (water that has had the dissolved solutes removed) to your slide. Quickly place back under the microscope and view at 400x. Once you see a change in the cells, draw a few of them. Again, label the cell wall, plasma membrane, chloroplasts and tonoplast.
What happened to the cell? Was the solution hypotonic, isotonic, or hypertonic? Explain the movement of the water molecules in your answer. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/06%3A_Roots_and_the_Movement_of_Water_-_How_is_water_moved_through_a_plant/6.3%3A_The_Behavior_of_Water.txt |
When water evaporates from plant tissues, it is called transpiration. Ninety percent of water that evaporates from terrestrial surfaces occurs via transpiration--plants are the world’s greatest water filters! Water is absorbed by (most) plants through specialized organs called roots. The earliest plants, the bryophytes, don’t have roots. Instead, these plants rely on the absorption of water across the entire plant body and dispersal of this water by osmosis. For this lab, we will focus on the later groups of plants--the tracheophytes--that have specialized tissues for water absorption and transportation throughout the plant.
In Plant Cell Types and Tissues lab, you learned about cell types and tissues. The image above is a specialized cell called a tracheid. What tissue would you find this cell in? Is that tissue simple or complex?
Based on your knowledge of root words, what does the term tracheophytes mean?
Early plants have tracheids, while later groups of plants have an additional type of water conducting cell: vessel elements.
The image above is a cross section through the xylem of a corn root. You can see large open areas (vessel elements) surrounded by smaller, more densely packed cells (tracheids).
How would these two cell types differ in the ability to take up and transport water?
Testing the Relationship Between Tube Diameter and Water Movement
1. Put some water in a shallow dish or petri plate, at least enough to coat the bottom. Add a drop of food coloring and mix thoroughly. You can also mix the dye into the water before adding it to the dish.
2. Obtain glass tubes of different diameters (capillary tubes recommended). Measure and record the diameter of each tube in the table below. Note: The diameter is the longest distance across the opening of the tube.
3. Place the bottom of one of the tubes into the water, leaving space between the bottom of the tube and the bottom of the dish so water can move into the tube.
4. Mark the height of the water on the tube with a pen, remove it from the water, then measure the distance from the bottom of the tube to the line you drew.
5. Repeat steps 3 and 4 for each tube and record your data in the table below.
6. Next to the table, make a graph that shows your results.
Diameter of Tube (mm)
Maximum height of water (mm)
Is there any correlation between tube diameter and the height that the water traveled up the tube? If so, explain the relationship.
Vessel elements are large-diameter conducting cells in the xylem, while tracheids have a much smaller diameter. How would this influence capillary action and adhesion? Use examples from the tube experiment to help explain your answer.
Transpiration in Action
Water moves through the dead water-conducting cells in the xylem much like it moves through a tube. Transpiration acts like suction from the top of the tube, but as you saw in the previous experiment, other forces aid in the movement of the water: cohesion, adhesion, tension, and capillary action. All of these forces work to pull water into the plant through the root hairs, into the xylem, and out through the stomata.
You set up four plants at the start of lab. What were the conditions for each plant?
Plant 1 -
Plant 2 -
Plant 3 -
Plant 4 -
Check on the plants and, before doing anything, simply observe the appearance of the bags. Describe your observations below.
Turn each plant on its side and carefully remove the bags. Try not to let any condensation in the bag escape. Use a scale to obtain the mass of each bag. Note: if you used different types of bags, adjust your end mass measurements by subtracting the initial mass.
Mass of bag (adjusted)
1:
2:
3:
4:
6.5: Summative Questions
1. Were there any differences in the amount of water in the bags from plants 1 and 3 in your transpiration experiment? If so, what could explain these differences (which variable was changed between these treatment groups)?
2. What about between plants 2 and 3, considering the same questions as above?
3. Plants 3 and 4?
4. What did plant 3 represent in the experiment? Explain your answer.
5. What is the role of water in a plant -- why do plants need it?
6. Open stomata result in water loss through transpiration. Why don’t plants just close their stomata to prevent water loss?
7. What environmental factors influence the amount of transpiration in plants?
8. Cohesion, adhesion, tension, and capillary action act on water as it moves through the plant body. Define each of these terms and explain how they participate in the movement water through the plant.
9. How does the diameter of a conducting cell in the xylem influence the ability of the plant to take up water? | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/06%3A_Roots_and_the_Movement_of_Water_-_How_is_water_moved_through_a_plant/6.4%3A_Transpiration_and_Cohesion_-Tension_Theory.txt |
Learning Objectives
Content Objectives
• Learn the pathway of water through a plant and the structures involved in this pathway
• Learn the anatomy of a root and different types of root systems
Skill Objectives
• Draw and describe the flow of water as it moves through a plant from the soil environment to the atmosphere
• Identify specialized cells, tissues, and meristems in the root
• Distinguish between different root systems and explain the advantages of each type
07: Roots and the Movement of Water - Root structure and anatomy
1. What is the function of a root?
2. How does its structure contribute to that function?
7.2: Introduction
An essential part of a plant’s survival is obtaining access to water. Early plants did this by having small, creeping forms that grew in areas that would stay moist, never far from any surface. These plants did not have roots or the ability to transport water with xylem tissue. Instead, they absorbed and lost water across their tissues. As you can imagine, this would be a limiting state for plants and, soon after they moved onto land, plants evolved true roots with vascular tissue. Lignified tissues in roots would provide increased strength and stability for burrowing into the substrate (likely not yet a true soil) and access to water stored underground. Vascular tissue throughout the plant would allow water absorbed through the roots to be transported to other areas of the plant, meaning that tissues could elevate out of the water, getting increased access to sunlight.
In this lab, you will learn the general developmental pathway for tissues in the root, as well as the different anatomical organization of two groups of flowering plants: monocots and eudicots. Monocots, like corn and other grasses, germinate from seed with a single first leaf (called a cotyledon). Eudicots germinate with two leaves. Though this seems like a trivial distinction, these groups differ in many areas of growth and development.
7.3: Root Structure and Anatomy
The root system of a plant serves two primary functions: water absorption and anchorage. Roots with a higher surface area will be better adapted to absorbing water because they have more area to interact with the soil environment. Increased interaction with the soil environment can also contribute to increased anchorage, but there are always trade-offs. If roots become too fine, they will be easily broken and lose the anchorage function. Additionally, finer roots can also lose more water if the soil environment becomes dry.
Root Development
In plants, both roots and shoots grow from the tip or apex of the plant. New cells are produced in these growing tips by meristems, groups of undifferentiated cells whose function is to divide by mitosis to produce new cells. Root growth begins at the root apical meristem (RAM). This meristem divides in two directions, producing a root cap to the outside of the root to protect the growing tip and the primary meristems to the inside: the protoderm, ground meristem, and procambium.
Primary meristems produce the primary tissues in the root:
• Protoderm → Epidermis
• Ground meristem → Cortex (and pith in monocots)
• Procambium → Primary xylem and primary phloem
These primary tissues will then either differentiate into specialized cells or, as is the case in many eudicots, become meristematic and produce secondary tissues. More on this in Lab Secondary Growth.
Observe a long section of Zea mays (corn) root tip. This root tip can be divided into three regions based on what is occurring in that stage of development:
Locate the root cap, RAM, protoderm, ground meristem, and procambium. These can be found at the very tip of the root, where the cells are small and densely clustered. If you trace the columns of cells back to their origin, they will coalesce at the RAM. This area of meristematic activity is called the zone of division.
As you move up in the root, the cells begin to get larger, developing into primary tissues. This region is called the zone of elongation.
Further up the root, the larger cells begin to differentiate into specialized cells. In the xylem and phloem, you can find sclerenchyma with secondary walls. In the epidermis, you can see elongated cells called root hairs projecting outward. This region is called the zone of maturation.
Locate each of the zones described above and draw a root tip below. In the zone of division, label the root cap and the following meristems: RAM, protoderm, ground meristem, procambium. In the zone of elongation, label the following primary tissues: epidermis, cortex, pith, primary phloem, primary xylem. In the zone of maturation, label the following specialized cells: root hair, sieve tube element, companion cell, vessel element.
Root Development Flowchart
Below is a flowchart of eudicot root development in primary growth. There are two lines that will later lead to the secondary meristems. With this information and the filled in boxes, you should be able to determine which meristems and tissues go in the empty boxes.
Fill in the diagram below with the following terms: cortex, ground meristem, pericycle, primary phloem, and protoderm. Choose a different color to represent meristems and tissues, then color the boxes accordingly.
How would this flowchart be different in a monocot? Draw a flowchart of monocot root development in the space below.
Root Anatomy
Monocots
Observe a cross section of the zone of maturation in a Zea mays root. Locate the primary tissues and specialized cells that you found in the long section and label them in the image below.
Eudicots
The development of roots in eudicots differs slightly from monocots. Monocots develop a ring of vascular tissue with ground tissue (pith) in the center. In eudicots, the vascular cylinder is closed. The primary xylem develops as an X (or sometimes Y) shape in the center with pockets of primary phloem in the spaces between the arms.
Encircling the conducting tissues is a meristematic layer called the pericycle. This single cell layer is responsible for producing lateral roots. Unlike root hairs which emerge on the outside of the root (exogenous, exo- meaning outside), lateral roots emerge from the internal tissues (endogenous, endo- meaning inside).
Observe a cross section of a Salix (willow) root producing lateral roots. Locate the primary xylem, primary phloem, pericycle, cortex, and epidermis. Label these features in the image below:.
Just outside the pericycle is the innermost layer of the cortex called the endodermis. The endodermis regulates what enters and exits the vascular cylinder. When the root is young, the epidermis contains many root hairs whose function is to absorb water. This water is transported through the cortex and into the xylem. As the root matures, the root hairs are lost from the epidermis. These areas of the root now function for anchorage and transportation of water. Because these areas are no longer taking up water, the vascular cylinder is sealed off by a waxy layer of suberin that covers the endodermis called the casparian strip. The casparian strip forms in stages with the areas closest to the arms of the xylem forming last. These are the last areas to allow for the passage of water, giving them the name passage cells.
Observe a cross section of a Ranunculus (buttercup) root. Locate the pericycle, endodermis, casparian strip, and passage cells. The suberin in the casparian strip often stains similarly to the secondary walls in the xylem. Draw and label the cross section in the space below.
7.4: Types of Root Systems
Plants that have adapted to different environments might develop different root systems in response to the stressors in that environment. Observe the different root systems available in lab and try to classify them as one or more of the following:
Netted or Taproot System
In soils where water is readily available for most of the year, plants might develop a netted root system where many similar diameter roots capture as much water and nutrients as possible to outcompete their neighbors. In climates where there are droughts or freezes, plants might develop a taproot system, where a larger central root can burrow deeper into the soil profile, accessing water reserves that other plants cannot.
Storage Roots
A larger diameter root can also store water and/or sugars for long periods. This type of root is called a storage root. A large central root, such as in the middle left on the following page, could be both a taproot and a storage root.
Adventitious Roots
Adventitious roots emerge from stem tissue. This can happen when there is an underground stem, such as in the system at the top of the diagram on the following page, or to serve as a prop root, as in the center of the diagram.
In the figure above, label any adventitious roots, prop roots, and storage roots. Label each system as either netted or taproot (except the topmost root system, which is an underground stem).
7.5: Summative Questions
1. Compare and contrast the arrangement of tissues in monocot and eudicot roots. Use drawings as part of your explanation.
2. What is the function of the pericycle?
3. Why does the Casparian strip form more slowly in some areas? What are these areas called?
4. What is the difference between a root hair and a lateral root?
5. How does the structure of a root hair contribute to its function?
6. What type of environmental conditions might select for a netted root system? Explain your answer.
7. What about a taproot system? Explain your answer.
8. Why would a plant evolve to produce roots from stem tissue? What benefits does this provide the plant? | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/07%3A_Roots_and_the_Movement_of_Water_-_Root_structure_and_anatomy/7.1%3A_Formative_Questions.txt |
Learning Objectives
Content Objectives
• Learn the anatomy of a shoot and the sequence of development
• Compare shoot anatomy in monocots and eudicots
• Learn the morphology of a woody stem
• Understand that a node has a predictable arrangement of organs
Skill Objectives
• Identify specialized cells, tissues, and meristems in the shoot
• Distinguish between monocot and eudicot stems
• Identify the features of a woody stem and determine the age of the stem
Contributors and Attributions
• Thumbnail: Zea mays vascular bundle
08: Shoot Anatomy and Morphology
1. What is the function of meristematic tissue?
2. What part of a plant do you think the following things you’d get at the grocery store are:
• Onion
• Potato
• Ginger
• Lettuce
• Broccoli
8.2: Introduction
The shoot of a plant is responsible for two major functions: photosynthesis and reproduction. In most plants, the leaves carry out photosynthesis, while the stems provide stability to elevate those leaves above potential competition. In annual plants, whose entire life cycle from germinate to death is completed in a single year, the epidermis on the stem is often photosynthetic, as well.
Tissues in the shoot are derived from the shoot apical meristem (SAM). Just like in the root, the SAM produces three primary meristems, which produce the primary tissues:
• Protoderm → Epidermis
• Ground meristem → Cortex and pith (simply ground tissue in monocots)
• Procambium → Primary xylem and primary phloem
These primary tissues will then either differentiate into specialized cells or, as is the case in many eudicots, become meristematic and produce secondary tissues. More on this in Lab Secondary Growth.
8.3: Shoot Development
Coleus Shoot Tip
Though it looks a bit alien, this is a section through a growing tip of a plant. In the center, where the alien’s head might be, is a region of small, densely packed cells. This is the SAM of the apical bud. On either side of the SAM, like two upraised arms, are the leaf primordia. These are the early stages of developing leaves. Through the center of these leaf primordia is a darker region of small cells. This is the procambium, which will develop into the vascular tissue. Lining the outer edge of the SAM and the youngest portions of the leaf primordia is the protoderm. As the protoderm matures into the epidermis, it produces hair-like projections called trichomes. Between the protoderm and the procambium is the ground meristem.
On either side of the growing tip are two other darkened lumps of densely packed cells. These bud primordia will develop into axillary buds, producing either branches or flowers. Each bud primordium has its own SAM.
Observe a prepared slide of a Coleus shoot tip long section. Locate the meristems, tissues, and specialized cells. Draw what you see in the space below, label any important features.
Flowchart of Shoot Development
Below is a flowchart of eudicot shoot development in primary growth. There are two lines that will later lead to the secondary meristems. With this information and the filled in boxes, you should be able to determine which meristems and tissues go in the empty boxes.
Fill in the diagram below with the following terms: cortex, epidermis, fascicular cambium, ground meristem, pith rays, procambium and primary xylem.. Choose a different color to represent meristems and tissues, then color the boxes accordingly.
How would this flowchart be different in a monocot? Draw a flowchart of monocot shoot development in the space below. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/08%3A_Shoot_Anatomy_and_Morphology/8.1%3A_Formative_Questions.txt |
Monocots are a group of flowering plants that produce a single first leaf (cotyledon) as their seeds germinate. Eudicots (frequently referred to simply as dicots) produce two cotyledons. In addition to this feature, monocots and eudicots can be distinguished by several anatomical and morphological features. One of these features is the arrangement of tissues in the stem. In monocots, the vascular tissue is arranged in distinct bundles that are scattered throughout the stem. The xylem is located on the side of the vascular bundles that face the center of the stem and can be identified by the large, hollow vessel elements that stain differently due to their secondary walls. In the half of the bundle that faces the exterior is the phloem, which contains sieve tube elements and their accompanying companion cells. The phloem is capped by a clump of fibers called the phloem fibers. The vascular bundles are arranged throughout the ground tissue in concentric circles with xylem facing inward.
Vascular bundles in monocots are called closed vascular bundles, as they will not go on to form secondary tissues (no residual procambium).
In the above of the vascular bundle on the left, label the xylem, phloem, a vessel element, sieve tube element, and companion cell.
8.5: Eudicots
In eudicot stems, the vascular tissue is arranged into a ring (the vascular cylinder) that separates the ground tissue into two distinct regions. The region of ground tissue contained within the vascular cylinder is called the pith. The region of ground tissue that is between the vascular cylinder and the epidermis is called the cortex. Similar to the monocots, the primary xylem is located in the region of the vascular tissue facing the center of the stem, while the primary phloem is produced toward the exterior of the stem.
Observe a cross section of a young Helianthus (or other eudicot) stem. Locate the epidermis, cortex, primary phloem, primary xylem, pith, and pith rays. Label these tissues in the image above.
What cell types (-enchymas) can you identify in the Tilia cross section? Which tissue(s) do you find each cell type in?
This image shows the epidermis and cortex of a young Tilia stem in primary growth. The epidermis is the outermost layer of cells and it is covered in a thin layer of wax, the cuticle. Just inside the epidermis is the cortex. Because this is from a young, growing stem, the outer cortex is full of collenchyma cells.
In the image on the right, label the cuticle, epidermis, cortex, a collenchyma cell, and a parenchyma cell.
Why would the stem be covered in wax?
8.6: Shoot Morphology
Shoots are composed of nodes. Each node has a leaf and an axillary bud, which emerges from the leaf axil. The leaf is located just below the bud, which will become either a branch or a flower. The identity of each organ is determined by its location in the node.
In the diagram below, label the bolded features, as well as the photosynthetic part of the leaf (the blade) and the stem of the leaf (the petiole). Label the bolded features in the diagram below.
Scars and Features of a Woody Shoot
When a leaf or bud falls off, it leaves a scar behind. Leaf scars are crescent shaped and have small, circular bundle scars within them where vascular bundles traversed the plant tissues. A leaf scar will be located below a bud, branch, or bud scar.
Plants that grow in temperate regions generally have a growing season. Each year, the newly emerging growth is within the terminal bud which is protected by terminal bud scales. When these scales fall off as the new growth emerges, they leave a region of terminal bud scale scars behind. Regions between terminal bud scale scars represent one year of growth.
On woody shoots, a layer of bark has replaced the epidermis (more on this in lab 8). To continue to exchange gases with the exterior environment, the bark layer develops small tears called lenticels. These can be small circular or elongated scars.
Label the bolded features in the diagram below. Draw in leaf scars where they are missing!
Leaf Arrangement
As shoots develop, the leaves are arranged in a particular order which can differ from plant to plant. In some plant species, one leaf emerges from the branch at a time (left) so leaves appear to alternate from side to side. This type of leaf arrangement is called alternate. In other plants, two leaves are formed on either side of the stem at the same time. This type of leaf arrangement is called opposite (middle). If three or more leaves are produced at the same region on the stem, the leaf arrangement is whorled (right).
Observe the shoots available in lab and identify the leaf arrangement and age of each. Record your findings in the table below.
Shoot
Leaf Arrangement
Age (years)
8.7: Summative Questions
1. Can you identify the leaf arrangement if the leaves have fallen off of a branch? Explain your answer.
2. Compare and contrast the organization of tissues in the shoot of monocots and eudicots. Use drawings as part of your explanation.
3. How does the shoot protect itself from water loss? What type of biomolecule helps in this process?
4. Why are the vascular bundles in monocots considered “closed”? What does this tell you about monocot development?
5. Why were the outer layers of the cortex of the young Tilia stem composed of collenchyma cells? What is the function of this cell type?
6. How could you tell that those cells were collenchyma and not sclerenchyma cells, as both of these cell types have thickened walls? | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/08%3A_Shoot_Anatomy_and_Morphology/8.4%3A_Monocots.txt |
Learning Objectives
Content Objectives
• Learn the organization of tissues within the leaf
• Understand how the organization of these tissues changes in adaptation in different environmental conditions
• Explain how the relationship between surface area and volume relates to water retention in plant tissues
• Describe the steps in the process of science
Skill Objectives
• Identify tissues and structures within a variety of leaves
• Identify whether a leaf is mesophytic, xerophytic, or hydrophytic based on leaf anatomy
• Predict the drought tolerance of a leaf using anatomical and morphological features
• Use the process of science to ask and answer scientific questions
• Collect, analyze, and interpret data
Contributors and Attributions
• Thumbnail: Mesophytic Leaf
09: Leaf Anatomy
1. What is the primary function of a leaf?
2. How does the structure of a leaf contribute to performing this function?
9.2: Introduction to Leaf Anatomy
Leaves are specialized organs for performing photosynthesis. A leaf is often a relatively large, flat surface used to optimize sunlight capture. However, surfaces are areas that water can evaporate from, so a large amount of surface area exposed to sunlight results in increased transpiration. The anatomy of a leaf has everything to do with achieving the balance between photosynthesis and transpiration in the environment in which the plant grows. Plants that grow in moist areas can grow large, flat leaves to absorb sunlight like solar panels because sunlight is likely more limiting than water. Plants in dry areas must prevent water loss and adapt a variety of leaf shapes and orientations to accomplish the duel tasks of water retention and sunlight absorption. In general, leaves adapted to dry environments are small and thick with a much lower surface area to volume ratio.
Note
There is an experiment at the end of this lab that you should read through and make plans for as a class at the start of the lab.
9.3: Leaf Anatomy
Mesophytic Leaf Anatomy
View a prepared slide of a Ranunculus leaf. This small, herbaceous flowering plant is more commonly referred to as a buttercup. It grows in many environments, but tends to prefer shaded, cool spots with plenty of moisture. It is a good example of a "standard" leaf, not specially adapted to either wet or dry environments. This type of plant is called a mesophyte (meso- meaning middle, -phyte meaning plant), preferring moderate climatic conditions.
The outer layer of cells on both the upper and lower surface of the leaf is the epidermis. Can you find any pores (gaps) in the epidermis? These pores are called stomata and allow carbon dioxide (\(\ce{CO2}\)) to enter the leaf for photosynthesis. Oxygen (\(\ce{O2}\)), which is produced during photosynthesis as a waste product, is released through the stomata. A third gas, water vapor (\(\ce{H2O}\)), also escapes through the stomata, though this has both beneficial and detrimental effects for the plant.
Look to either side of a stoma (this is the singular version of stomata) to see the flanking guard cells. These cells regulate the opening and closing of the stoma by either inflating and opening when there is high water content in the leaf, or collapsing and closing the stoma when water content in the leaf is low. This prevents water vapor from escaping the plant if it has too little water. However, it also prevents \(\ce{CO2}\) from entering, stopping the formation of sugars in the plant and cutting of its source of energy. The flow of water vapor out of the leaf helps pull water up from the roots, see lab 5a for a full description of transpiration.
Beneath the upper epidermis is a layer of elongated cells full of chloroplasts. This is the palisade mesophyll, which has specialized for capturing incoming sunlight, rotating chloroplasts to the top of the leaf and then allowing them to regenerate by cycling them toward the leaf's center. Just below the palisade mesophyll is the spongy mesophyll. The spongy mesophyll is full of air pockets (hence the name spongy) that allow \(\ce{CO2}\) to move into the leaf to the palisade mesophyll, as well as allowing oxygen to diffuse from the palisade mesophyll through the spongy mesophyll and out the stomata.
You may see circles of densely packed cells that both stain a different color than the mesophyll cells, as well as have a different organizational structure. These are veins of vascular tissue running through the leaf. The xylem is on the top, staining differently from the rest of the cells due to its lignified secondary walls. The phloem is on the bottom of the bundle, supported by a cluster of fibers (sclerenchyma) that increase structural support for the veins. The xylem is transporting water and dissolved minerals from the roots up to the leaf, while the phloem is transporting sugars made in the leaf to other regions of the plant.
Draw a cross section of a mesophytic leaf, labeling each structure or tissue with its name and function. Consider simplifying the image to use as an easy reference.
In the leaf you are viewing, are there more stomata on the upper or lower epidermis? Can you think of any reasons why this might be?
How does the structure of the spongy mesophyll contribute to its function?
Hydrophytic Leaf Adaptations
Hydrophytes (hydro- meaning water) are plants adapted to growing in water. The structure of a hydrophytic leaf differs from a mesophytic leaf due to selective pressures in the environment -- water is plentiful, so the plant is more concerned with staying afloat and preventing herbivory.
Observe a prepared slide of a hydrophyte, such as Nymphaea, commonly called a water lily. Note the thin epidermal layer and the absence of stomata in the lower epidermis. In the spongy mesophyll, there are large pockets where air can be trapped. This type of parenchyma tissue, specialized for trapping gases, is called aerenchyma. Look for sharp-looking, branched cells traversing the leaf’s mesophyll. These will stain differently from the parenchyma cells because they have a thick secondary wall. These sclerenchyma cells are called astrosclereids and provide the leaf structural support, as well as prevention of herbivory.
Why are there no stomata on the lower epidermis of a Nymphaea leaf?
Draw a cross section of the Nymphaea leaf, labeling each structure or tissue with its name and function.
Xerophytic Leaf Adaptations
Xerophytic (xero- meaning dry) plants are adapted to dry conditions. California is a great place to view xerophytic plants due to the long dry season that also corresponds to the warmest season. If you are from California, this may seem perfectly normal. However, in most other places, the warm season coincides with the wet season, a much easier climate for plants to navigate. The coincidence of dry and warm seasons occurs in 5 distinct regions of the world, all on the west coast of their continent and all in a narrow latitudinal band between 30-60 degrees on either side of the equator. This type of climate is named after one of these 5 regions -- the Mediterranean.
Warm to hot temperatures cause increased transpiration. Dry conditions have the same effect, doubling the water stress on plants in Mediterranean climates where it is both warm and dry at the same time.
Pines
Pines evolved during a period in Earth’s history when conditions were becoming increasingly dry. Pine needles have many adaptations to deal with these conditions.
1. The epidermis of the leaf seems to be more than one cell layer thick. These subsequent layers of epidermis-like tissue under the single, outer layer of true epidermis are called the hypodermis (hypo- meaning under, dermis meaning skin), which offers a thicker barrier and helps prevent water loss.
2. The epidermis itself is coated on the outside by a thick layer of wax called the cuticle. Because waxes are hydrophobic, this also helps prevent water loss through the epidermis.
3. The stomata are typically sunken, occurring within the hypodermis instead of the epidermis. Sunken stomata create a pocket of air that is protected from the airflow across the leaf and can aid in maintaining a higher moisture content.
4. There are two bundles of vascular tissue embedded within a region of cells called transfusion tissue. The transfusion tissue and vascular bundles are surrounded by a distinct layer of cells called the endodermis (much like in a root, but not suberized).
5. Finally, the overall shape of the leaf allows for as little water loss as possible by decreasing the relative surface area, taking a rounder shape as opposed to a flatter one. This low surface area to volume ratio is characteristic of xerophytes.
Observe a prepared slide of a pine needle cross section. The strange, invaginated cells between the hypodermis and the endodermis are the mesophyll. Draw in the other half of the pine needle above. Identify and label the structures involved in water retention.
Note
There are several canals that appear as large, open circles in the cross section of the leaf. These are resin canals. The cells lining them secrete resin (the sticky stuff that coniferous trees exude, often called pitch), which contains compounds that are toxic to insects and bacteria. When pines evolved, not only was the Earth becoming drier, but insects were evolving and proliferating. These resin canals are not features that help the plant survive dry conditions, but they do help prevent herbivory. In addition to prevention of herbivory, resin can aid in closing wounds and preventing infection at wound sites.
Oleander
Oleander (Nerium oleander) is another plant that has specifically adapted to survive drought conditions, possibly native to the Mediterranean though its precise origin is unknown. Oleander also has defenses against herbivory, making the entire plant extremely toxic, even to humans.
Instead of sunken stomata, the epidermis in oleander recesses and creates a pocket that is lined with trichomes. The stomata are located at the base of these pockets, called stomatal crypts. The trichomes help capture evaporating moisture and maintain a relatively humid environment around the stomata. These stomatal crypts are located only on the underside of the leaves, where they experience less sun exposure and therefore less transpiration. The upper epidermis is free from stomata and, instead, is coated by a thick cuticle.
The image below shows a close-up of the upper epidermis. What is that thick layer on top of the epidermis? How does it compare to the same layer in a mesophytic leaf?
Observe a prepared slide of a cross section of a Nerium leaf. Draw and label the structures mentioned in the paragraph above, as well as any additional features you see from the description of xerophytic adaptations in pines.
Which adaptations do pines and oleander share? Which features found in the pine leaf are absent in the oleander leaf?
Corn
Corn (Zea mays) is not necessarily a xerophyte, but it is adapted to deal with high temperatures. One of these adaptations, C4 type photosynthesis, will be covered in the photosynthesis lab. The one you will identify is something that lets the leaf alter the amount of exposed surface area. When moisture is plentiful, the corn leaves are fully expanded and able to maximize photosynthesis. When moisture is limited, the leaves roll inward, limiting both moisture loss and photosynthetic capacity. This is accomplished by the presence of bulliform cells in the upper epidermis. These clusters of enlarged cells are swollen with water when there is abundant water available. As the water content in the plant decreases, these cells shrivel, causing the upper epidermis to curl or fold inward at these points. This adaptation to sun exposure can be found in many other grasses, as well (corn is a member of the Poaceae, the grass family).
Observe a prepared slide of a cross section of a Zea mays leaf. Look for clusters of enlarged cells in the upper epidermis. Label the bulliform cells in the image above.
Why aren’t there bulliform cells in the lower epidermis as well? What would happen to a leaf with bulliform cells on both sides?
Where are the stomata located on a leaf of Zea mays? | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/09%3A_Leaf_Anatomy/9.1%3A_Formative_Questions.txt |
The object of this activity is to use characteristics of leaf anatomy that you have observed in the previous sections of this lab to predict how drought tolerant a leaf will be. To assess this, you will measure the amount of water lost by individual leaves when exposed to an arid (dry) environment.
Materials needed
• An assortment of leaves of different shapes and sizes preferably collected from different environments. Note: These leaves should be collected at the same time to account for moisture loss, which will begin once removed from the plant.
• A ruler
• A digital scale
• A camera
• 1mm$^2$ graph paper and pencils (optional - a computer with the program ImageJ)
• A calculator
• A contained, arid environment (e.g. an incubator at 30-37C)
Observation: Obtain a set of leaves of different shapes and sizes. Make observations about these leaves, including aspects that the leaves possess or lack, that you think might contribute to drought tolerance. Record your observations below.
Question: The question that has been posed for this experiment is “Which characteristics of leaves are associated with drought tolerance?”
Hypothesis: To narrow the realm of focus, you will be looking specifically at the surface area to volume ratio of these leaves. For your hypothesis, make a statement as to how you think the surface area to volume ratio will influence moisture retention. A null hypothesis (H0), which predicts no relationship between the variables of interest, has been included below.
H0: The ratio of surface area to volume will have no correlation to the moisture retention in leaves when exposed to an arid environment.
H1:
Initial Data Collection
Step 1: Lay the leaves out as flat as possible on a solid background, such as a sheet of white paper, with a ruler aligned to one side of the paper. This will allow you to determine scale in your images. Make a label with your name (or group name), date, time, and "T0". T0 will denote "time zero", the state of the leaves before the start of the experiment.
Step 2: After you have arranged your leaves, ruler, and label, take a picture of them.
Step 3: Use the digital scale to obtain the mass of each leaf and record these values in Table $1$.
Note
The density of water is 1 g/cm$^3$. Notice that density is a measurement of mass divided by volume (D = m/v). Because leaves are 90% water, we can assume that they have approximately the same density, a 1:1 ratio of mass to volume. With this information, we can use the mass of the leaf as an approximation of the volume of the leaf.
Step 4: Calculate the surface area of each leaf using ONE of the following methods:
1. Upload your picture to the computer and open it with ImageJ. Obtain the surface area and record these values in Table $1$.
or
1. Trace each leaf on 1mm$^2$ graph paper. Count the number of squares fully contained within the tracing. Try to approximate the number of squares that are partially filled in by matching them to other squares that are also partially filled and counting these as one square. Take the number of completely filled in squares, add the number of approximated squares, then multiply this by 2 to get the total surface area (this accounts for both sides). Record these values in Table $1$.
Note
Because the measurement of volume is using cm, it might be more useful to convert mm2 to cm2. Divide the number of mm$^2$ by 100 to get cm$^2$. To do this, place a decimal at the end of the number of mm$^2$, then move it two places to the left.
Step 5: Use the values in $1$ to calculate the surface area to volume ratio for unit volume of each leaf by dividing the surface area by the volume.
Table $1$: Volume and surface area of different leaves
Leaf ID
Volume (equal to mass, cm$^3$)
Surface Area (cm$^2$)
Surface Area : Volume Ratio for Unit Volume
Example:
3.4
120. mm$^2$ /100 = 1.2 cm$^2$
1.2 / 3.4 = 0.35
Experiment: To test the drought tolerance, the leaves will be placed into a contained arid environment. After a set time, you will remove the leaves from the arid environment and determine the amount and rate of moisture loss from each leaf.
Step 1: Arrange the leaves on a tray and place in the contained arid environment. Set a timer or note the time. After 1 hour, remove the tray and take it back to your lab table.
Step 2: Use the digital scale to obtain the mass of each leaf in grams and record these values in Table $2$.
Step 3: Calculate the percent of water lost in each leaf by using the formula below:$((\text{Initial mass of leaf } - \text{ Mass of leaf after 1 hr in arid environment}) \div \text { Initial mass of leaf })\times 100 \nonumber$
*Order of operations: Do the subtraction in the innermost parentheses first, then divide the result by the initial mass, then multiply the result of that by 100.
Step 4: Calculate the rate of water loss from each leaf per minute by using the formula below:$((\text{Initial mass of leaf } - \text{ Mass of leaf after 1 hr in arid environment}) \div 60 \text { minutes} \nonumber$
Table $2$: Water retention of leaves after 1 hour
Leaf ID
Mass after 1 hour (g)
% Water Loss
Rate of Water Loss (g/min)
Example:
0.9 g
((3.4-0.9) / 3.4) x 100 =
(2.5 / 3.4) x 100 = 73.5
(3.4-0.9) / 60 =
2.5 / 60 = 0.04
Analysis:
To interpret data, it helps to visualize it. In the space below, make a graph with Surface Area : Volume Ratio for Unit Volume on the x-axis and Percent Water Loss on the y-axis. Graph these values from tables 1 and 2 for each leaf. Do your data seem to follow any particular pattern?
Add at least 15 more data points from your classmates. Do your data seem to follow any particular pattern? Attempt to draw a line of best fit through your data points.
Interpretation and Conclusions
Did water loss correlate to the surface area : volume ratio? Explain.
Are there any correlations between water loss and any of the other observed characteristics?
Consider any sources of error in this experiment and how you could improve upon the design.
9.5: Summative Questions
1. Did the results of the experiment support your hypothesis or the null hypothesis? Explain your answer.
2. Design a leaf for a xerophytic plant. Consider the dual and often contrasting objectives of performing photosynthesis while limiting moisture loss. Label important features and briefly explain their function with regard to drought-tolerance. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/09%3A_Leaf_Anatomy/9.4%3A_Process_of_Science_-_Predicting_Drought_Tolerance_in_Leaves.txt |
Learning Objectives
Content Objectives
• Understand why environmental pressures select for different plant adaptations
• Recall the identifying features of stems, roots, and leaves
Skill Objectives
• Identify whether a modified organ is a stem, leaf, or root based on location
• Identify the modification type and explain the function of this modification
• Predict modifications you might see in a plant knowing the environmental stressors
Contributors and Attributions
• Thumbnail: Prop Roots
10: Plant Adaptations
1. What do you think are the most important selection pressures on plants?
2. Which of these do you think is the most important for determining plant anatomy and morphology? Explain your reasoning.
10.2: Introduction
In labs Roots and the Movement of Water - How is water moved through a plant? - Leaf Anatomy, you learned the typical anatomy, morphology, and organization of plant tissues and organs. However, over the course of each different plant’s evolutionary history, environmental pressures can lead to modifications of these features in predictable ways that we can then classify. For example, plants that frequently encounter drought will experience selection for the ability to access water when there is no water in the environment. The plants that evolve water-storage will have better survival in this environment and are more likely to successfully reproduce. However, animals in this dry environment might then seek out their water-filled tissues. Plants that either hide or protect these water reserves are more likely to survive and experience less herbivory. Over time, as the environment continues to select for these defenses, they improve. And thus, over long periods of evolutionary time, one lineage of plants evolves into cacti, while another related lineage evolves in a different environment to be carnations.
There are many selective pressures on plants. As you saw in lab Leaf Anatomy, climate has a strong selective effect on the anatomy and morphology of plants, particularly water availability and access to sunlight. Herbivory is another strong selective pressure, as plants cannot run away from their predators. Instead, they must evolve other deterrents while balancing the energy costs required for these adaptations with energy devoted to reproduction. A third strong selective pressure is nutrient availability, which is often determined by the soil environment. For example, serpentine soils have low levels of nitrogen and calcium. Plants in serpentine habitats often evolve the ability to trap and digest insects to absorb the missing nutrients. These “carnivorous” plants are not heterotrophic, because they do not use the trapped insects as an energy source.
10.3: Organ Modifications
Roots
Root tissue is derived from the root apical meristem. In some plants, environmental stressors will select for plants whose root tissues perform functions other than water absorption and anchorage. These organ modifications have specific names, depending on what function they serve.
1. Storage roots: In most roots, surface area is maximized for water absorption. In a storage root, the volume becomes more important. Cells in the cortex are enlarged and contain leucoplasts.
1. Pneumatophores: Gases diffuse 10,000x more slowly in water than in air. In plants that grow in saturated soils, such as mangroves, roots cells need to access oxygen to perform cellular respiration at a rate that they cannot accomplish through water. Pneumatophores are roots that emerge above the surface of the saturated zone to “breathe” (pneumo- means lung) and exchange gases with the environment.
2. Adventitious roots: Unlike most roots, adventitious roots emerge from stem tissue. A root apical meristem is derived from tissues in the stem, then root tissues are formed from the RAM, as normal. Adventitious roots can be produced from nodes on horizontal or climbing stems or in response to environmental stressors.
1. Prop roots: Prop roots are adventitious roots with the specific function of providing stability to a plant. This might happen in unstable soils, on climbing plants, or in plants that have a shallow root system.
Stems
Stem tissue is derived from the shoot apical meristem. In many plants, there is a central stem that lateral stems emerge from. You can distinguish stems from roots by the presence of nodes. You can distinguish lateral stems from leaves by location within the node: stems emerge above the leaf.
Just like with roots, stems can be adapted to a particular function in response to environmental stressors.
1. Cladode: A cladode is a stem that has increased surface area to perform photosynthesis. This is usually because the leaves have been modified to some other purpose and are no longer performing photosynthesis. In essence, the stem is imitating a leaf.
2. Succulent: In the case of succulence, it is the volume of the stem that increases. Stem tissues develop large, specialized cells called hydrenchyma to store extra water. The plant can access this water for metabolism in periods of drought.
3. Tuber: Some stems are modified for storage of starches instead of water. A tuber is an underground stem that can be identified by its nodes (often referred to as “eyes” on a potato tuber). Each node is capable of producing a new shoot.
4. Corm: A corm is also modified for storage of starches. A corm is swollen tissue at the base of the shoot with linear nodes travelling across it. From these nodes, papery leaves emerge.
1. Rhizome: Some plants produce horizontal stems that are used for asexual reproduction. A rhizome is a horizontal stem that is underground. Roots emerging from the nodes of rhizomes are adventitious.
2. Stolon: Similar to a rhizome, a stolon is a horizontal stem used for asexual reproduction. Unlike a rhizome, stolons are formed above the soil surface.
3. Thorn: A thorn is a lateral branch that has been modified to protect the plant, usually from herbivory. Thorns have a subtending leaf or leaf scar.
1. Stem tendril: A tendril is a lateral branch that has been modified for climbing. A stem tendril will have a subtending leaf or leaf scar.
Leaves
Leaf tissue is derived from the shoot apical meristem. You can distinguish leaves from stem tissue by location within the node: leaves emerge below the axillary bud, lateral stem, or flower. Under normal conditions, the primary function of a leaf is photosynthesis. However, environmental stressors can select for the following modifications:
1. Succulent: Much like in stems, leaves can also be modified for water storage in environments where there is drought. You can distinguish a succulent leaf from a succulent stem because the stem will have nodes (the leaf will not).
1. Bulb: Leaves can also be modified to store starch. A bulb, like an onion, is composed of fleshy leaves that surround a short, central stem.
1. Spine: A leaf modified to be sharp for protection is called a spine. You can distinguish a spine from a thorn by the location within the node.
2. Leaf tendril: Sometimes leaves will be modified for climbing. You can distinguish a leaf tendril from a stem tendril by the location within the node.
1. Trap: In environments where nutrients are low, some plants have evolved to capture insects for access to nitrogen, phosphorus, and calcium. There are a few ways plants can achieve this. One is to modify a leaf into some sort of trap.
2. Phyllode: Another instance of leaf imitation is when the petiole of the leaf becomes flat and leaf-like to perform photosynthesis. Similar to a cladode, this is usually in response to a modified leaf. The pitcher plant on the right shows a phyllode, leaf tendril, and trap all in one leaf.
1. Stipular spines: Another sharp armament against herbivory can be a replacement of a stipule with a spine. These can be distinguished from other sharp modifications, as they emerge in pairs at the base of a leaf.
Other Plant Adaptations
In addition to the major organs, plants can have adaptations to specific tissues, cells, or molecules produced by the plant.
1. Prickles: Similar to thorns and spines, prickles are a protective adaptation. In this case, it is a modification of the epidermal (and sometimes cortex) tissue. Prickles can emerge anywhere on the shoot, unlike spines and thorns, which are restricted to nodes.
1. Trichomes: Trichomes are epidermal cells that have been modified as hairs. While they can still serve a protective function, they tend to be much smaller and less rigid than prickles.
1. Raphides: Some plants have molecular, internal sharp armaments. In plants like Tradescantia, calcium oxalate is crystallized into needle-like structures called raphides.
Chop up the leaf or stem of a Tradescantia and make a squash mount to see raphides within the plant tissues.
1. Latex: About 10% of flowering plants have evolved latex production, a sticky substance exuded when plants are damaged. This latex can prevent infection of wounds. Many groups of plants have independently evolved the production of latex gum up the mouthparts of herbivorous insects. In some plants, the latex also includes toxic compounds to aid in defense. For example, in milkweeds (Asclepias) this latex contains neurotoxins.
10.4: Determining Plant Modifications and Function
Observe the specimens of different plant adaptations available in the lab. For each specimen, make note of the organ, tissue, or other component of the plant that was modified and the function of that modification. Record your findings in the space below.
10.5: Color-coded Organs
In the diagram below, identify the organ modification(s). Choose a color for stems, another for roots, and a third for leaves. Make a key to indicate which color represents which type of organ, then color the organs of each plant with the correct color.
10.6: Summative questions
1. Categorize the plant modifications described above as an adaptation in response to the following stressors. Briefly state how each modification helps the plant deal with the particular stressor.
1. Water availability:
2. Access to sunlight:
3. Nutrient availability:
4. Defense against herbivory
2. How do you differentiate between a stem and a root? What about a stem and a leaf? Consider both macroscopic and microscopic options. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/10%3A_Plant_Adaptations/10.1%3A_Formative_Questions.txt |
Learning Objectives
Content Objectives
• Learn the sequence of development of meristems and tissues in secondary growth
• Understand the tradeoffs (costs and benefits) of secondary growth
Skill Objectives
• Identify anatomical features of woody stems
• Determine the age of a tree based on annual growth rings
• 11.1: Formative Questions
• 11.2: Introduction
In secondary growth, primary tissues and residual meristematic tissues produce secondary meristems, which then produce secondary tissues. Whereas primary tissues allow for vertical growth, secondary tissues allow for lateral growth: they allow stems and roots to become wider.
• 11.3: Secondary Tissues in the Root
In roots, the formation of both secondary meristems involves the pericycle. The pericycle and some residual procambium join together to form the vascular cambium, a secondary meristem that produces vascular tissue. The other secondary meristem, the cork cambium, is initially formed solely from the pericycle. Each of these secondary meristems divides in two directions to form a different secondary tissue to the inside and outside of the meristematic layer, respective to the center of the plant.
• 11.4: Flowcharts of Development
• 11.5: Secondary Tissues in the Shoot
Though the secondary tissues are all the same, the sequence of secondary meristem development in shoots is a bit different than in roots. Shoots have no pericycle, so the secondary meristems must be formed from different tissues. In shoots, the vascular cambium is formed from residual procambium within the vascular bundles (fascicular cambium) joined by tissue in the pith rays (interfascicular cambium).
• 11.6: Dendrochronology
Dendrochronology is a process used to determine the order and timing (chronology) of events using information in tree rings (dendro- refers to trees). These rings form in response to environmental conditions, storing information on climate and historic occurrence of fires in an area. Essentially, you bore a hole into a living tree and extract a thin section column of the wood. This becomes your reference specimen, as you date each annual ring, tracing back from the present date.
• 11.7: Summative Questions
Contributors and Attributions
• Thumbnail: Tilia stem cross section.
11: Secondary Growth
1. What are some factors that limit how tall a plant can grow?
2. What are some factors that limit where a plant can grow?
11.2: Introduction
The two questions you answered above are essential to understanding the evolution of secondary growth. In secondary growth, primary tissues and residual meristematic tissues produce secondary meristems, which then produce secondary tissues. Whereas primary tissues allow for vertical growth, secondary tissues allow for lateral growth: they allow stems and roots to become wider. How might this impact the ability of a plant to grow taller?
In addition to growing wider, secondary growth exchanges the living epidermis for a thick layer of dead, waterproofed cells called cork. The cork and a few other layers of tissue comprise something called the periderm, or perhaps more familiarly called bark. Consider the tradeoffs between having a living exterior with guard cells vs. a thick layer of waterproofed dead cells. How might this impact the ability of the plant to interact with the outer environment? How might this impact the ability of the outer environment to interact with the interior of the plant?
Variations on this type of growth appear in a few places, but the evolution of gymnosperms (conifers and their relatives) is when the more typical secondary growth appears in evolutionary history. As you will see in later labs, the gymnosperms evolved during a period in Earth’s history when inland seas were drying out and plants were migrating further from the water. This group of plants is specialized for growing tall (think Coast redwoods) or living in harsh, low-water environments. As a general rule, monocots do not undergo secondary growth, so this lab will only address eudicots.
11.3: Secondary Tissues in the Root
In roots, the formation of both secondary meristems involves the pericycle. The pericycle and some residual procambium join together to form the vascular cambium, a secondary meristem that produces vascular tissue. The other secondary meristem, the cork cambium, is initially formed solely from the pericycle.
Each of these secondary meristems divides in two directions to form a different secondary tissue to the inside and outside of the meristematic layer, respective to the center of the plant.
The vascular cambium produces secondary xylem to the inside of the root and secondary phloem to the outside.
The cork cambium produces phelloderm, a storage tissue, to the inside of the root and cork, a protective layer of dead, suberized cells, toward the outside. These three layers -- phelloderm, cork cambium, and cork -- are referred to as a periderm. As the first periderm layer is formed, it separates the epidermis, cortex, and endodermis from the conductive tissues of the root. The epidermis and cortex cells die and are shed as secondary growth proceeds.
The pine root below has one layer of periderm fully formed. Can you label the tissues and meristems described above? What are the large holes in the root?
11.4: Flowcharts of Development
Below are two flowcharts. The upper represents the pathway of development in the shoot, while the lower represents the development of the root. Both of these include secondary growth. Fill in the missing pieces of each flowchart. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/11%3A_Secondary_Growth/11.1%3A_Formative_Questions.txt |
Secondary Vascular Tissue
Though the secondary tissues are all the same, the sequence of secondary meristem development in shoots is a bit different than in roots. Shoots have no pericycle, so the secondary meristems must be formed from different tissues. In shoots, the vascular cambium is formed from residual procambium within the vascular bundles (fascicular cambium) joined by tissue in the pith rays (interfascicular cambium).
This layer of cells will divide to produce secondary xylem toward the inside of the stem and secondary phloem toward the outside. The conducting cells in the secondary xylem are dead at maturity. As the stem matures, these dead cells become separated from the living cells and the plant would be unable to direct the transport of materials into and out of the center of the plant. To solve this, plants have pathways of living cells that traverse the xylem and phloem called rays. Xylem rays radiate outward from the center of the stem until they reach the vascular cambium. At this point, they become phloem rays. Some of these will balloon out into large deltas of cells in the phloem tissue, while others will remain narrow strips of cells.
The secondary phloem becomes densely packed with layers secondary phloem fibers that provide structural support for the stem to grow tall.
Vessel elements in secondary xylem
In the secondary xylem, different diameter conducting cells form in wet and dry conditions. Larger diameter cells form in the wet season when water is plentiful. As the seasons transition to drier whether or approach freezing conditions, cells are smaller. Because most climates have a wet and dry season each year, you can track years of growth by the formation of these annual growth rings in the secondary xylem.
On the left is a pine stem, on the right is an oak. In the pine stem, the cells are a more similar diameter because they are all tracheids. On the right, The largest diameter cells are vessel elements. How old are each of these stems?
Resin canals
In answer to the previous question about the roots, those giant holes in the pine stem (as in the roots) are resin canals. These canals transport a sticky substance called resin. Much like the latex produced in some flowering plants, resin acts as a defense compound and a wound sealant.
Resin canals are also present in some groups of angiosperms, particularly tropical trees. Why might these structures occur more commonly in tropical plants?
The cork cambium is formed directly from the cortex. Layers of periderm are continually formed and shed until the cortex runs out. At this point, secondary phloem de-differentiates and forms the new layers of cork cambium.
Because the periderm is suberized and dead, it cannot have living guard cells to regulate pores and control gas exchange with the environment. Because the cells on the inside are still alive and respiring, gas exchange is still necessary. This is accomplished through tears in the periderm called lenticels (shown below).
In a mature woody stem, the layers of periderm are referred to as the outer bark and the secondary phloem is the inner bark. The secondary xylem is called wood. Secondary xylem that is still actively involved in conducting water is called sapwood, while inactive xylem is called heartwood. The cross section below is too young to have these last two features.
In the stem cross section above, label the following: pith, secondary xylem, xylem ray, vascular cambium, secondary phloem, phloem ray, cortex, phelloderm, cork cambium, cork, and epidermis. Next, indicate where the oldest and youngest xylem cells are located.
Note: the newest cells will be located closest to the meristem that produced them!
Where is the vascular cambium?
11.6: Dendrochronology
Dendrochronology is a process used to determine the order and timing (chronology) of events using information in tree rings (dendro- refers to trees). These rings form in response to environmental conditions, storing information on climate and historic occurrence of fires in an area. Essentially, you bore a hole into a living tree and extract a thin section column of the wood. This becomes your reference specimen, as you date each annual ring, tracing back from the present date. Your reference specimen can then be used to determine the timing of events in dead trees, potentially tracing back thousands of years.
Imagine that the sample below is from your reference specimen. The example core shows features you might see in a tree core and how to interpret them.
How old is this tree? At which dates did rainy years occur? Which periods represent droughts? When did fires occur? Make notes on the annual rings with the date and notable events.
Using your reference specimen, date the events in the following dead trees of unknown age from the same forest. It will be easiest to do this by cutting out each core and aligning it next to the reference. There is no text on the bottom of this page on the opposite side, so cut away!
Based on the dendrochronology you just performed, what did you determine about the history of this forest and climate in this region? Be as specific as possible in your answer.
What is the oldest date that you calculated on the tree core present?
11.7: Summative Questions
1. How is the production of secondary meristems different in the shoot and the root?
2. What are the tradeoffs (costs and benefits) to secondary growth?
3. What is the function of a lenticel?
4. Would roots in secondary growth need lenticels? Why or why not?
5. What is the function of a xylem ray?
6. How could you distinguish phloem fibers from other phloem cells? What cell type are phloem fibers?
7. What type of questions could you answer with dendrochronology if you didn’t have a living reference specimen to connect the cores to the current date?
8. Girdling is a process used to slowly kill a tree and/or to create a standing dead tree (a snag). Both the outer and inner bark of the tree is removed all the way around a section of the trunk. Often, this also removes or damages the vascular cambium.
9. Explain why girdling would kill the tree. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/11%3A_Secondary_Growth/11.5%3A_Secondary_Tissues_in_the_Shoot.txt |
Learning Objectives
Content Objectives
• Understand that colors we see are determined by reflected light and relate this to the absorption spectrum of pigments
• See how photosynthesis converts electromagnetic energy into chemical energy
• Know the stages where products and reactants of photosynthesis are produced or used, respectively
• Relate the process of photosynthesis to the structure of a leaf
Skill Objectives
• Use thin layer chromatography (TLC) to determine which pigments are present in plant tissues
• Determine the Rf values of pigments on a TLC strip
• Determine the relative polarity of a pigment based on the polarity of the TLC solvent
• Identify anatomical structures in leaves involved in photosynthesis and explain their function in this process
Contributors and Attributions
• Thumbnail: Diagram of a chloroplast
12: Photosynthesis and Plant Pigments
1. What is the role of photosynthesis in the global cycling of nutrients?
2. Would it be better to wear a white shirt or a black shirt on a particularly hot day? Why is this?
12.2: Introduction
To understand photosynthesis, we must first understand a bit about light. Light is a form of electromagnetic energy, meaning it travels as a wave and does not require a medium to move through. Light, unlike sound, can travel to us from the sun through the vacuum of space. Once it gets to Earth, there are particles for it to either reflect off of, be absorbed by, or pass through. Depending on the length of the light wave--think about stretching or compressing a slinky--and whether it is reflected or not, our eyes will either interpret it as different colors, or we will not be able to see it at all. In fact, our eyes only pick up a tiny fraction of the light spectrum.
When we look at objects, we may see them as being a certain color. This is because that object reflects that particular wavelength of light back at us. Objects that are white reflect the entire spectrum back at us. Objects that are truly black absorb all of the light, reflecting nothing back. Objects that are a particular color, such as green, absorb all of the other wavelengths of visible light except green, reflecting that green out to our eyes. Pigments are molecules that reflect specific colors.
12.3: Part 1 - Pigments
The molecule chlorophyll a has a specific shape. This shape causes wavelengths of light that we see as a dark bluish green to be reflected back. Change the shape of that molecule by adding only two atoms, making it chlorophyll b, and the light that is reflected back is now less blue and more yellow.
What does this have to do with photosynthesis? Organisms that perform photosynthesis do so by absorbing light and converting it into usable energy. This absorbed light is not reflected back. This means that the color of the pigment(s) that the organism has will determine the wavelengths of light that the organism can use. For example, plants have two types of chlorophyll molecules, chlorophyll a & b. Each of these reflects green light, meaning that green light cannot be used for photosynthesis. To help capture a bit more of the spectrum, plants have accessory pigments called carotenoids that reflect yellow, orange, and red light, absorbing a portion of the green part of the spectrum.
Chlorophylls tend to mask most other pigments in plants, so to see these other pigments, we need to separate them. You will use a process called thin layer chromatography to extract pigments from leaves, then dissolve them in a solvent.
Thin Layer Chromatography (TLC)
Note
Below is a list of suggested materials. Spinach is suggested for the leaves, as it is easy to acquire and rich in pigment. However, consider comparing the pigments in different types of leaves for a more interesting experiment.
Materials needed:
• Mortar and pestle
• Sand or similar material
• Soft leaves (e.g. spinach)
• Extraction solvent: 3 parts propanone to 2 parts ethoxyethane (diethyl ether)
• Capillary tubes
• Test tubes and corks
• TLC paper strips (cut to size of test tube--about 1 cm less in length)
• Chromatography solvent: 5 parts cyclohexane, 3 parts propanone, and 2 parts ethoxyethane
• Pencil
• Forceps
Procedure for Thin Layer Chromatography
1. Extracting the pigments.
1. Prepare your TLC strip by drawing a line across the paper in pencil 2 cm from the bottom of the strip and set aside. Important note: Handle the strip as little as possible so oils from your hands do not interfere with the process.
2. Under a hood or in a well-ventilated room, put some of the leaves into the mortar with a little bit of sand (to help break the tissue apart) and some extraction solvent.
3. Grind the leaves with the pestle until they have turned to mush. You may need to add more extraction solvent as it soaks into the leaf tissue.
2. Thin layer chromatography.
1. Pull the mush to one side of the mortar. Place the end of a fine capillary tube into the liquid, then transfer the liquid in the tube to a point in the center of your line.
2. Repeat this process, drawing more liquid from the mortar, until you have a small, concentrated dot of pigment. For best results, keep the dot as concentrated in one place as possible.
3. Under a hood, add 1 cm of chromatography solvent to your test tube and place this in a test tube rack (label your tube if multiple people are using the same rack).
4. Add your strip to the tube with the line you drew in pencil sitting about 1 cm above the level of the solvent, then cork the tube.
3. Calculating Rf values and determining polarity.
1. Check on your TLC strip regularly and have a pencil with you. When the solvent has traveled up the TLC strip about 1 cm from the top of the strip, remove the strip from the test tube and draw a line in pencil at the edge of the solvent front.
2. Allow the strip to dry, then measure the distance from the original line you drew (where the pigment started) to the solvent front.
3. Next, measure the distance from where the pigment started to the farthest point that each pigment traveled.
4. Calculate the Rf value of each pigment: divide the distance the pigment traveled by the distance the solvent traveled.
Both the chromatography solvent and the extraction solent you used are nonpolar compounds, meaning they lack residual charges. Nonpolar compounds dissolve well in nonpolar solutions, while polar compounds do not. Pigments that are more nonpolar will dissolve better in this solvent, traveling farther up the strip. More polar pigments that have residual charges (like water) will not interact much with the solvent, staying closer to the bottom of the strip. High Rf values from TLC using a nonpolar solvent means the pigment is more nonpolar. Lower Rf values mean the pigment is more polar.
Draw or tape your TLC strip and label as many pigments as you can (see the next page for more information on pigments). Record the Rf values of each pigment next to its label.
Which pigment is more polar, chlorophyll a or chlorophyll b? How can you tell?
How many pigments were present in your leaf sample?
Which pigments were the most nonpolar (least polar, highest Rf values)?
If there were polar pigments in the leaves and you used a nonpolar solvent to extract the pigments from the leaf, would they dissolve and be present in the solution you used to run your TLC experiment? How might this impact your results?
12.4: Pigments and Evolutionary Adaptations
Cyanobacteria were potentially the first organisms to do oxygenic photosynthesis -- the variety of photosynthesis that produces oxygen as a waste product. To do this, cyanobacteria use the pigment chlorophyll a. This is the only pigment directly involved in photosynthesis, but other pigments called accessory pigments can absorb wavelengths of light, then transfer this energy to chlorophyll a. Cyanobacteria have accessory pigments called phycobilins that allow them to absorb more of the blue and red portions of the spectrum of light. The pigment phycocyanin, a blue phycobilin, resulted in the cyanobacteria’s more common name: blue-green algae.
Which part of the spectrum of light do you think phycocyanin absorbs?
The red algae were the first lineage of organisms to have true chloroplasts, derived from the endosymbiosis of a cyanobacterium, and so they have the same pigments: chlorophyll a and phycobilins. In particular, the red algae derive most of their coloring from a red phycobilin called phycoerythrin. This pigment reflects red, meaning it absorbs the blue portion of the spectrum.
Red algae are almost entirely marine and have been found deep in the water column. The record for algal depth was set by a coralline red alga found 268 m (about 880 feet) below the surface. Why is this so impressive? As light travels through water molecules, much of it is reflected back or refracted off course. High-energy blue light is more likely to penetrate deeper into the water column, so it is essential that the algae are able to capture it. Because both chlorophyll a and phycocyanin would reflect much of this blue light instead of absorbing it, red algae make use of phycoerythrin to capture it, making them appear red.
Green algae are more likely to live in shallow marine waters, freshwater, or even terrestrially. This group of organisms has lost the phycobilins present in its red algal ancestors and has adapted chlorophyll b and carotenoids, instead. Carotenoids can help mitigate damage caused by solar radiation, as well as absorb some of the green parts of the spectrum. These pigments are reds, oranges, and yellows..
As you saw in your TLC strip, chlorophyll b appears more yellow while chlorophyll a appears more blue. What portion of the spectrum is chlorophyll b absorbing that chlorophyll a is not?
What color were the carotenoids on your TLC strip?
Where might you see carotenoids in your daily life?
Brown algae have chlorophyll a and chlorophyll c, as well as the carotenoid fucoxanthin, which gives them their golden brown color. Compare the available specimens of cyanobacteria, red algae, green algae, and brown algae. What do the colors of these organisms tell you about where they live?
Which pigment do all of these organisms have? | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/12%3A_Photosynthesis_and_Plant_Pigments/12.1%3A_Formative_Questions.txt |
Photosynthesis is a process used to harness the energy from sunlight to ultimately bond carbon atoms from carbon dioxide into molecules of glucose. This occurs in two major phases--the light dependent phase and the light independent phase--and requires chlorophyll a. Both phases take place inside of the chloroplasts of eukaryotic photosynthesizers or across folded membranes within prokaryotes. For simplicity’s sake, we will focus on the eukaryotes and describe these processes in relation to chloroplast anatomy (sorry, bacteria, back seat again).
Anatomy of a Chloroplast
Chloroplasts are surrounded by at least two membranes. The two membranes were derived from the original cyanobacterium that was engulfed but not digested (Keeling, 2004). Inside of the inner membrane, the chloroplast has a jelly-like matrix, much like the cytosol of the cell, called the stroma. The stroma surrounds a series of folded membrane-bound structures, thylakoids, that resemble pancakes and are stacked (again, much like pancakes) into columns called grana (granum, singular). The thylakoids appear dark green, as the thylakoid membrane is full of chlorophyll molecules. Enclosed inside each thylakoid is a region called the thylakoid space.
Label the bolded structures in the diagram of the chloroplast. Indicate the origin of each membrane of the chloroplast.
Light-dependent Phase: The electron Transport Chain
This phase is called the light-dependent phase because to begin this process, a particle of light, called a photon, of the correct wavelength must be absorbed by a molecule of chlorophyll a, embedded in a complex called photosystem II (PSII).
The energy from the photon causes an electron to be knocked off of the chlorophyll molecule and this electron is shuttled to an adjacent protein complex. More on this in a second.
The chlorophyll molecule needs to replace the electron it lost so that it is ready to respond to the next photon. A water molecule is split at the base of PSII, releasing two electrons and producing oxygen and 2 protons (\(\ce{H+}\)) into the thylakoid space. Make note of each time H+ is added to the thylakoid space, as this creates a buildup of positive charges, which repel each other.
Back to that first electron. The protein complex it is shuttled to is a proton pump. When the electron enters the protein complex, it effectively switches on the proton pump, causing \(\ce{H+}\) to be pumped from the stroma and into the thylakoid space.
The electron then jumps to another proton pump, causing another H+ to be pumped from the stroma into the thylakoid space.
This electron has depleted much of its energy, so it must enter another photosystem (PSI) to be reenergized by another photon.
The re-energized electron is then transported to an enzyme called NADP+ reductase, which reduces (adds electrons to) a molecule of NADP+ to create high-energy NADPH. This requires two electrons, but this process involves a continuous flow of electrons, so another one arrives shortly after the first.
This is the end of the electron transport chain. However, there is another component to this phase. Inside the thylakoid membrane, \(\ce{H+}\) are building up, creating a storage of energy across the thylakoid membrane as they repel against each other. Even though they are tiny (a single proton), they cannot pass freely through the membrane due to the positive charge.
Instead, an enzyme called ATP synthase allows the \(\ce{H+}\) passage to the other side. The flow of \(\ce{H+}\) through the enzyme causes it to spin, much like a turbine, converting the stored energy into kinetic energy (movement).
ATP synthase converts this kinetic energy into chemical energy by using it to add phosphate groups onto molecules of ADP. This creates unstable, high energy ATP molecules that can be used to power other cellular processes.
Apply it: Work with your labmates to design a model that illustrates the light-dependent phase of photosynthesis. Consider the materials available in lab (including, potentially, other students), as well as whether your model will have moving parts. What is the best way to communicate this process?
Below is an example of a model of the electron transport chain in photosynthesis. Can you talk your way through what is happening? Try explaining this process to a partner.
Light-independent Phase: The Calvin Cycle
The light-independent phase takes place in the stroma of the chloroplast. During this phase, called the Calvin Cycle, the chemical energy stored in the NADPH and ATP produced during the light-dependent phase is used to build molecules of glucose. This happens in three major stages.
1. Carbon Fixation: Carbon dioxide (\(\ce{CO2}\)) enters the plant through the stoma, diffuses into the cells, then into the chloroplast. An enzyme called RuBisCO (a much easier way to say Ribulose-1, 5-bisphosphate carboxylase/oxygenase) bonds \(\ce{CO2}\) to a 5-carbon molecule called RuBP. This breaks into two 3-carbon molecules of 3-PGA.
2. Reduction: NADPH donate electrons and ATP donates a phosphate group to convert each 3-PGA into G3P, two of which are required to make glucose. For every six G3P made, only one goes on to make glucose, the rest must be recycled.
3. Regeneration: To keep the cycle going, RuBP must be regenerated. Using energy and phosphate donated by more ATP, five G3P are recycled to make three more RuBP.
This process can happen during day or night, but it requires a steady input of ATP, NADPH and \(\ce{CO2}\).
The reactants above are required to complete this process. What are the products of the light independent phase?
For each of the products and reactants in the chemical reaction for photosynthesis written below, identify where it was either produced or consumed.
Table \(1\): Summary Table for Photosynthesis
Phase
Where does it take place?
Reactants
Products
Light dependent
Light independent
12.6: C4 and CAM Photosynthesis
The enzyme RuBisCO needs a high \(\ce{CO2}\) environment to function efficiently. If the ratio of oxygen to \(\ce{CO2}\) gets too high, RuBisCO will bind oxygen instead and waste energy in the process. This is called photorespiration and accounts for a large amount of yield loss for crops in hot areas. Why does photorespiration happen?
If it is too hot or dry, plants often close their stomata to prevent water loss. This prevents \(\ce{CO2}\) from entering the leaf, as well as prevents \(\ce{O2}\) from exiting. Oxygen builds up inside the leaf and photorespiration happens instead of the Calvin cycle. Though this wastes energy for the plant, preventing water loss is often a larger priority. However, some plants have evolved special ways of performing photosynthesis that prevent or limit photorespiration.
C4 Photosynthesis is for Plants Adapted to Hot Environments
In most photosynthesis, the first product of the Calvin cycle is a 3 carbon compound, so this type of photosynthesis is called C3. For plants adapted to particularly hot environments, the first compound formed has 4 carbon atoms, hence C4 photosynthesis. In these plants, RuBisCO is restricted to the bundle sheath cells of the leaf. Carbon dioxide is converted into an acid and transported into the bundle sheath cells where it will be converted back into \(\ce{CO2}\). This keeps the concentration high where RuBisCO is active, preventing photorespiration.
CAM Photosynthesis is for Plants Adapted to Dry Environments
CAM plants are often found in desert environments. It is too hot and/or dry to keep stomata open during the day, so they only open them at night. However, there is no light at night to do photosynthesis. To solve this, CAM plants have evolved to take in \(\ce{CO2}\) at night and store it in the central vacuole in the form of an acid. This is where CAM gets its name: Crassulacean Acid Metabolism. During the day, the acid is converted back into \(\ce{CO2}\), and the Calvin cycle can take place alongside the electron transport chain.
In both of these types of photosynthesis, compounds must be formed, transported, and broken back apart again. Each of these tasks costs energy to perform, but it outweighs the energy lost by photorespiration.
12.7: Summative Questions
1. What is the energy input for photosynthesis (i.e. what powers the electron transport chain)?
2. At what stage in photosynthesis is water used? What is the function it serves in this process and what is it converted into?
3. At what stage in photosynthesis is \(\ce{CO2}\) used? What is the function it serves in this process and what is it converted into?
4. Zea mays is a C4 plant. In the cross section of a Z. mays leaf below, identify where RuBisCO is and where the Calvin cycle is occuring.
1. What other features does this leaf have that indicate it might be adapted to an environment where water was limiting? | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/12%3A_Photosynthesis_and_Plant_Pigments/12.5%3A_Part_2_-_Photosynthesis.txt |
Learning Objectives
Content Objectives
• Understand how the process of cellular respiration converts glucose into usable energy, carbon dioxide, and water.
• Explain why some organisms do fermentation instead of cellular respiration, even though this produces less usable energy
Skill Objectives
• Identify stages in cellular respiration where ATP is used and where it is produced
• Calculate net ATP produced from a single glucose molecule
• Make predictions about the rate of fermentation under different conditions
13: Cellular Respiration and Fermentation
1. If a plant is in the light for 24 hours, what will happen to its mass: will it increase, decrease, or stay the same? Consider the processes occurring within the plant. Explain your answer.
13.2: Introduction
Part 1: Carbohydrates in Food
When you eat food, you are acquiring nutrients that you need to maintain homeostasis--to keep your internal conditions regulated to the specific parameters required for your survival and function. From these foods, you might get vitamins that your body cannot synthesize itself, amino acids needed to build proteins, and, in anything that is food in the scientific sense, carbohydrates that you can break down to use as energy.
Carbohydrates can be simple sugars--monosaccharides like glucose--to complex starches, often made of long chains of those same monosaccharides bonded together to form a much larger molecule called a polysaccharide. Some of these polysaccharides have bonds that our bodies have enzymes to break down, such as Amylose. Amylose is a starch composed of glucose monomers attached together by a glycosidic bond, forming a relatively straight line. In your mouth, you produce an enzyme called amylase that breaks the glycosidic bond, eventually turning the long chain of amylose into many molecules of glucose. Amylase functions best at pH 6.7-7.0 [connect to maintenance of homeostasis]. Other polysaccharides are bonded together in ways that we do not have enzymes for, like cellulose, the primary component of plant cell walls. This is why plants contain “dietary fiber,” which passes right through your system and “keeps you regular.”
Part 2: Converting Carbohydrates into Usable Energy
A molecule of glucose stores about 3,000 kJ of chemical energy. Chemical energy is the potential energy stored in the bonds that hold atoms within a molecule together. Everytime you break one of these bonds, that energy is released. Molecules of glucose are relatively stable, so this energy is safely stored inside the molecule. Because of this, we can transport glucose, dissolved in our blood, to areas in our body where it is needed or to our liver, where it can be stored for later in the form of glycogen. Plants can do the same through phloem cells, transporting glucose to areas of the plant where it can be stored long term as starch, such as the roots.
Because the glucose molecule is stable, the energy is trapped inside it. To access this energy, the glucose molecule must be broken apart. You can make this happen by applying large amounts of energy, such as heat from a fire--this is what happens when you burn wood or paper. These materials are plant-based, composed primarily of plant cell walls containing large amounts of cellulose. As we saw above, this cellulose is composed of long chains of glucose molecules. The wood itself doesn’t spontaneously catch fire, but when you apply enough energy, such as by igniting a small portion of it with a lighter or a match, you can cause some of those molecules to break apart. This releases energy in the form of light and heat, which can then provide the energy to break more molecules apart. This process is called combustion.
What was required to make this reaction happen? Carbohydrates, oxygen, and an initial energy input. A similar process happens inside most eukaryotic organisms. However, to avoid catching on fire, we break down the glucose molecule piece by piece, releasing small amounts of energy at a time, and using that energy to build molecules of something called ATP (adenosine triphosphate).
Unlike glucose, ATP is an unstable molecule. It has three negatively charged phosphate groups, all attached end-to-end. The negative charges on these phosphate groups repel each other, much like the two negative sides of different magnets. As the number of negative charges increases, the repulsion increases exponentially. Because of this, the third phosphate group holds a large amount of energy in its bond to the second. This phosphate group can be “donated” to a reaction (called phosphorylation), breaking the bond and releasing the stored chemical energy to power the reaction. The ATP loses its third phosphate group and becomes ADP (adenosine diphosphate), a relatively low-energy molecule. Our cells use ATP to power most of the work done in our bodies.
Key Point
While heterotrophic organisms like animals and fungi must consume other organisms to obtain carbohydrates, plants and other autotrophs synthesize their own carbohydrates. However, just like heterotrophs, autotrophic organisms still need to do cellular respiration to access the energy stored in those carbohydrates. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/13%3A_Cellular_Respiration_and_Fermentation/13.1%3A_Formative_Questions.txt |
Step 1: Glycolysis
When glucose is transported into the cytoplasm of cells, it is broken down into two molecules of pyruvate. This process is called glycolysis (glyco- for glucose and -lysis, meaning to break apart). Glycolysis involves the coordinated action of many different enzymes. As these enzymes start to break the glucose molecule apart, an initial input of energy is required. This initial energy is donated by molecules of ATP.
Though two molecules of ATP are used to get glycolysis going, four more molecules of ATP are produced during the reaction, resulting in the net production of two ATP per molecule of glucose. In addition to ATP, two molecules of NAD+ are reduced to form NADH. When a molecule is reduced, electrons have been added to it. Electrons have a negative charge, so this is termed “reduction”. When NAD+ is reduced to NADH, two high energy electrons derived from breaking the bonds of glucose are added to it. One of those negatively charged electrons is balanced by the positive charge (+) on NAD+. The other is balanced by adding a proton (\(\ce{H+}\)) to the molecule. Because NADH carries two high energy electrons, it is often referred to as an electron carrier.
Table \(1\): Summary Table of Reactants and Products of Glycolysis
Reactants (What went in?)
Products (What came out?)
1 Glucose
2 NAD+
2 ADP (net)
Alternate Pathways: Fermentation
At this point, cells make a check: Is there oxygen present or not? If not, some organisms can go through a process called fermentation. In fermentation, glycolysis is the only part of glucose breakdown that a cell can do. Thus, only two net ATP can be gained from each glucose molecule. To continue doing fermentation, the cell must regenerate the NAD+ needed to do glycolysis.
There are two primary pathways to regenerate NAD+. In your body, cells can regenerate NAD+ by producing lactate and \(\ce{H+}\). This is called lactic acid fermentation, though lactic acid is never actually produced, so it is a bit of a misnomer. Some bacteria, like Lactobacillus, are also capable of doing this type of fermentation.
Making Yogurt With Lactic Acid Fermentation
Put some milk into a flask and add a spoonful of yogurt. Mix thoroughly, cover, and place in a warm environment. Over the next 24 hours, the sugars in the milk will be fermented into lactate and H+ ions. This thickens the milk and adds acidity, making yogurt.
Why did you add the spoonful of yogurt to start it? What did this add to the milk that would allow fermentation to occur?
Why did you need to put it into a warm environment?
Do you think the type of milk you use affects the time it takes to get yogurt? Explain your answer.
Another pathway to regenerate NAD+ is to produce \(\ce{CO2}\) and ethanol (alcohol). Certain yeasts, like Saccharomyces cerevisea, perform this type of alcohol fermentation when they do not have access to oxygen.
Experimental Design: Rates of Alcoholic Fermentation
For this experiment, you will be attempting to answer the following question:
“What factors influence the rate of fermentation by yeast?”
You will have the choice between one of two independent variables. The independent variable is the variable you are asking questions about. It should be the only thing that you establish as different between your treatment groups:
1. Type of juice used
2. Temperature of fermentation chamber
Why would it be important to only have one independent variable?
For the dependent variable, you will measure the circumference of a balloon that has been placed over your fermentation flask. The dependent variable is the one that you measure to determine the influence of the independent variable on your different treatments.
Why would the circumference of the balloon be a good indicator for the rate of fermentation? What will the balloon be collecting?
Choose an independent variable and make a prediction on how it will influence the dependent variable. This is called your hypothesis.
To measure the effects of the independent variable on the rate of fermentation, you’ll need to establish treatment groups and controls. Your treatment groups should have some modification to the independent variable (such as different temperatures or different types of juice). Choose three different variations of your independent variable and record them below:
T1:
T2:
T3:
Controls are established to account for any additional variables within the experiment, as well as to give you information on whether your experiment actually worked. A positive control should always work (give expected results). In this experiment, fermentation should definitely occur in the positive control for this experiment. If fermentation does not occur in the positive control, you know something about your experiment is off and you could be getting false negatives. Think of a positive control you could use for your experiment and record it below:
Positive control (C+):
A negative control is just the opposite. In a negative control, you want to see no change from initial conditions. If there is a change in the dependent variable in the negative control, this gives you a baseline of variability that you can expect in the absence of changes to the independent variable. For example, in this experiment, a negative control should have no fermentation occurring and thus, no inflation of the balloon. However, gases can expand in higher temperatures, so a negative control placed in a warmer environment would have expansion of the balloon. This gives us some baseline data for how temperature will affect the gases in the balloon without the influence of fermentation. Think of a negative control you could use for your experiment and record it below (you may need more than one):
Negative control(s) (C-):
Now that you have designed the experiment, it is time to run it and collect data on the dependent variable. Enter your results into the table below:
Table \(2\): Change in Balloon Circumference During Fermentation
Treatment Group
Initial Circumference (cm)
Final Circumference (cm)
Change in Circumference (cm)
Table \(3\): Change in Balloon Circumference for Controls During Fermentation
Control
Initial Circumference (cm)
Final Circumference (cm)
Change in Circumference (cm)
C+
C-
Once you have collected your data, you need to interpret it and draw conclusions about your hypothesis.
Does the data support your hypothesis or not? Explain your answer.
Were there any variables that could have influenced your experiment that you did not control for? If so, how could you adjust this experiment to account for these on a future trial?
Consult as a class about your findings. Did everyone perform the experiment the same? Did they get the same results?
Compare the results of the class for the two different independent variables. Which had a stronger influence on rate of fermentation: the type of juice or the temperature? Attempt an explanation for these findings.
Connecting back to cellular respiration, how much ATP is produced from one molecule of glucose if a cell only goes through glycolysis?
Why do organisms ferment instead of going through the process of cellular respiration?
We used the balloon to collect gases produced during fermentation. However, the balloon was necessary for another reason (hint: it relates to the previous question). Why else did we cover the flask with the balloon?
Step 2: The Link Reaction
If oxygen is present, cellular respiration can continue (more on this in the next section). The two molecules of pyruvate are transported into the matrix of the mitochondrion. During transport, each pyruvate is converted into a 2-carbon molecule called acetyl-\(\ce{CoA}\). The other carbon atom from each pyruvate molecule exits the cell as \(\ce{CO2}\). The electrons from this broken bond are captured by another molecule of NAD+, reducing it to NADH.
Step 3: The Citric Acid (Krebs) Cycle
The acetyl-\(\ce{CoA}\) enters a cycle which, much like glycolysis, involves the action of many different enzymes to release energy and transport it in energy-carrying molecules: ATP, NADH, and another electron carrier, \(\ce{FADH2}\). This cycle takes place within the matrix of the mitochondrion.
In the space below, draw a mitochondrion in a cell. Show where glycolysis, the link reaction, and citric acid cycle happen. It may help you to include the compounds involved in each stage, such as glucose, NAD+, \(\ce{CO2}\), and others.
Step 4: Oxidative Phosphorylation
This stage of cellular respiration has two steps. During the electron transport chain, our electron carriers power a series of proton pumps that move \(\ce{H+}\) ions from the mitochondrial matrix to the space between the inner and outer mitochondrial membranes. During chemiosmosis, an enzyme called ATP synthase allows the protons to flow back into the mitochondrial matrix, using the physical flow of the protons to turn ADP into ATP.
The Electron Transport Chain
NADH and \(\ce{FADH2}\) drop off their electrons at a protein complex within the inner mitochondrial membrane. This effectively “turns on” this protein complex, which pumps a \(\ce{H+}\) from the mitochondrial matrix to the intermembrane space. The electrons are then passed down a line of protein complexes, much like a current of electricity, powering these complexes to each pump a \(\ce{H+}\) from the matrix into the intermembrane space. This is appropriately named the electron transport chain.
At the end of the electron transport chain, the low energy electrons need to be picked up to make space for more electrons. An oxygen atom picks up two electrons and, to balance the charge, two \(\ce{H+}\) from the matrix, forming a water molecule (\(\ce{H2O}\)). In cellular respiration, oxygen is the terminal electron acceptor, because it picks up the electrons at the end (the terminus) of the electron transport chain. This job is so important that, as you saw above, if oxygen is not present, this part of cellular respiration will not occur.
The diagram above shows the interaction between the citric acid cycle and the electron transport chain inside the mitochondrion. There are two functioning electron transport chains and a single ATP synthase that uses the \(\ce{H+}\) gradient established by those \(\ce{H+}\) pumps to make ATP.
Chemiosmosis
Why are the protein complexes pumping \(\ce{H+}\) into the intermembrane space? The intermembrane space is relatively small. As more \(\ce{H+}\) are added to this area, the intermembrane space becomes increasingly positively charged, while the matrix becomes increasingly negatively charged. This is similar to how a battery stores energy--by creating an electrochemical gradient. The positive charges repel each other and would “prefer” to be balanced across both sides of the membrane. However, they cannot directly pass through the membrane. Even though they are small, \(\ce{H+}\) ions carry a full charge, making them too polar to pass through the nonpolar tails of the phospholipid bilayer that composes the mitochondrial membranes.
An enzyme called ATP synthase allows the \(\ce{H+}\) to move back into the matrix. This enzyme is structured much like a waterwheel or turbine -- the flow of protons through the enzyme physically rotates it, converting the potential energy stored in the electrochemical gradient into kinetic energy (movement)! This kinetic energy is used to force another phosphate group onto ADP, converting the kinetic energy back into chemical energy, which is stored in the bonds of ATP
Experiment: Understanding the Relationship Between Cellular Respiration and Photosynthesis
1. Put 50 mL of tap water into a flask (Flask A) and add a pH indicator, such as phenol red. Record the initial color of the solution in the table below.
2. Get out a stopwatch and have a partner ready to time you. When they say go, use a straw to blow into the water + indicator. As soon as it changes color, your partner should stop the stopwatch and you should stop blowing through the straw. Record the time below.
3. Repeat step 1 to set up another flask (Flask B).
4. This time, before you blow into the straw, do 30 seconds of intense exercise, such as running around outside, jumping jacks, burpees, or something else fun. As soon as you are done, have your partner time you as you blow through the straw into the new flask. Stop as soon as the color changes and record the time below.
5. Put a sprig of Elodea (or other aquatic plant) into Flask B, cork both flasks so no gases can get in or out, and set them both under a light source.
6. Observe these flasks over the course of the lab and make note of any color changes.
Table \(4\): pH Changes Measured by Indicator Color
Initial Color
Time to Change
Changes Under Light Source?
Flask A
Flask B
Why did the color of the indicator change when you blew into the liquid? What caused the change?
Did the pH of the solution go up or down?
Was there any difference in timing of color change between your two trials? If so, what might explain this difference?
Explain what you observed after you added the plant to Flask B.
13.4: Concept Mapping - Connecting Ideas Visually
Either individually or as a group, create a concept map on a separate piece of paper or whiteboard that relates the processes of photosynthesis and cellular respiration. The words in the word bank below should go into bubbles. Arrange these bubbles in a way that helps communicate relationships between the words, then connect the bubbles with lines that have a verb or action phrase attached to them. It might help to start by organizing the words into related groups. You can use words more than once.
For example, you could group \(\ce{CO2}\) and ATP together, then draw a line connecting them to RuBisCO and Calvin Cycle. The verb for the connecting line could be “used in”. You could then draw a line from those two that said “produces”. What would that line connect to?
Word Bank: Put these terms in bubbles
\(\ce{O2}\)
\(\ce{H2O}\)
\(\ce{CO2}\)
\(\ce{H+}\)
ATP
Glucose
Photon
Electrons
Chloroplast
Stroma
Thylakoid membrane
Mitochondrion
Matrix
Inner mitochondrial membrane
Cytoplasm
High energy electron carriers
NADH
NADPH
\(\ce{FADH2}\)
Glycolysis
Krebs cycle
Electron transport chain
Chemiosmosis
RuBisCO
Calvin cycle
13.5: Summative Questions
1. Why do plants need to use cellular respiration if they can do photosynthesis?
2. If a plant is in the dark for 24 hours, what will happen to its mass: will it increase, decrease, or stay the same? Explain your answer.
3. Explain how the action of your respiratory system relates to the process of cellular respiration.
4. Which stage of cellular respiration produces the most energy?
5. How much energy is produced during fermentation? Why do some organisms undergo fermentation instead of respiration?
6. What factors influenced fermentation the most in your class experiment? Develop a hypothesis for why this might be.
7. Both glucose and ATP store chemical energy. Why can’t glucose be used to directly power cells instead of ATP?
8. All experiments should have controls, if possible. Explain what a positive control and a negative control can tell you about your experiment. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/13%3A_Cellular_Respiration_and_Fermentation/13.3%3A_Cellular_Respiration.txt |
Learning Objectives
Content Objectives
• Learn the process of meiosis and how it leads to genetic diversity
• Understand the difference between gametes and spores
• See the three different life cycles of multicellular organisms
• Learn the terms sporophyte and gametophyte and the processes occurring within and between these life stages
Skill Objectives
• Identify stages of meiosis and differentiate them from mitosis
• Distinguish between different life cycles based on the ploidy of the multicellular stage(s)
• Identify key stages and features within life cycles and know their general functions
Contributors and Attributions
• Thumbnail: Lillium anthers
14: Meiosis Fertilization and Life Cycles
1. What do you consider to be the stages of a human life cycle?
2. At which point does your life as a human start? When does it end? What makes it a cycle?
14.2: Introduction
If the only type of cell division eukaryotes underwent was mitosis, the only source of genetic variation we would have would be through mutations. Mutations still provide the foundation for differences, but another type of cell division allows organisms to mix and match their own DNA. In meiosis, the sets of chromosomes that an organism inherited from each parent interact and recombine to form novel combinations. The haploid cells or organisms produced through this process can then fuse with another haploid cell or organism that has gone through the same process of recombination.
The result is that these sexually reproducing populations are composed of genetically distinct individuals. As environmental conditions change, genetically diverse populations have more options within the gene pool to adapt to those conditions. This increases the overall likelihood of survival of that species. As always, there are trade-offs to each type of reproduction.
Asexually reproducing organisms can mass-clone themselves and often reproduce quickly, investing very little in the survival of individual clones and instead relying on the probability that some might survive. Often, these organisms are found in high-resource environments (like mold on a sugary fruit) or colonizing a new area because they do not need a partner to reproduce.
Sexually reproducing organisms generally require a compatible partner to reproduce with, though some species can “self”, combining their own products of meiosis with each other. This process still results in an increase in diversity, as the offspring can still differ from the parent due to the novel recombinations that occur during meiosis. Finding a partner might be a passive process, such as release into the ocean, or a more active one, such as courtship rituals performed by birds of paradise.
Organisms that undergo sexual reproduction have a defined life cycle that describes the events from fertilization to meiosis and back again. The type of life cycle an organism has can offer insight into its evolutionary history, as well as its ecology. In this lab, you will learn the process of meiosis and the three generalized life cycles of multicellular eukaryotes.
14.3: The Production of Genetic Diversity
Mitosis is a type of cell division in which the original cell makes a duplicate copy. If this were the only form of cell division, the production of genetic diversity would be limited to mutations. Most lineages of organisms might not be able to adapt to the rapidly changing conditions of Earth’s many environments. This is not to say that asexually reproducing organisms cannot be successful. The most proliferate, widespread organisms on the planet, the Bacteria, are asexual and cannot undergo meiosis because they lack a nucleus. Most multicellular eukaryotic organisms (and some unicellular eukaryotes) have evolved to use sexual reproduction to create diversity through meiosis and random fertilization.
Meiosis = PMAT x 2
If a diploid cell undergoes meiosis, the product will be four genetically distinct haploid cells. In humans, this process is used to make eggs and sperm. Meiosis is almost like doing mitosis twice. This description will focus on the differences between the phases of meiosis and mitosis, rather than describing everything that occurs in each stage. For a refresher of the other events, see lab 4 (Multicellularity & Asexual Reproduction).
Meiosis I
• Prophase I: During prophase I, homologous chromosomes pair together. These chromosomes have the same genes in the same order but are derived from different parents. Due to their similarity, when they pair, they can overlap and trade segments of the chromosomal arms. The result is a mixture of genes from each parent on each chromosome. This process is called crossing over and it will happen differently in each cell that undergoes meiosis.
• Metaphase I: In metaphase I, homologous chromosomes line up across from each other on the metaphase plate. This is another source of variation, as they may align differently in each cell with a different combination of genes on either side of the metaphase plate. This is called independent assortment.
• Anaphase I: During anaphase in mitosis, sister chromatids are separated from each other. In meiosis, pairs of homologous chromosomes that are separated.
• Telophase I: Telophase I is similar to telophase in mitosis. The only difference is that the two daughter cells that emerge are genetically distinct from each other.
Meiosis II
Meiosis II is nearly identical to mitosis. It consists of prophase II, metaphase II, anaphase II, and telophase II. It is in anaphase II that sister chromatids are pulled apart. At the end of telophase II, four haploid cells have been produced, each genetically distinct.
Meiosis in Lilium Pollen
Anthers are one of the sexually reproducing structures in flowers, producing haploid pollen that will fertilize haploid eggs.
The above image shows a cross section through Lillium anthers. There are four circular areas, one of which is circled in the picture. These are the pollen sacs. It is inside these pollen sacs that meiosis occurs, so look within these areas for cells in different stages of meiosis.
Pollen divides synchronously, so most cells should be in approximately the same stage. However, meiosis is a continuous process, so some cells may be in anaphase while most are in metaphase. Additionally, dividing pollen cells remain attached, so you will be able to tell if you are in meiosis I or meiosis II by how many cell walls have formed within the dividing cell. In meiosis I, there should be one cell until cytokinesis occurs during telophase I, producing two joined cells. In meiosis II, there will be two cells stuck together until cytokinesis during telophase II, producing four cells.
The image on the left shows two cells in metaphase I (left and middle) and one cell in prophase I (upper right corner). The image on the right shows two “cells”, both in metaphase II and showing that each “cell” is now actually two cells stuck together.
Note the wall that travels down the center of each of the cells in the photo on the right. During which phase of meiosis would this wall have been formed?
Observe slides of Lilium anthers at different stages of cell division and find each phase of meiosis. Draw and label each phase below. Make sure to note where crossing over and independent assortment are happening.
Meiosis I:
Meiosis II:
At what point did the number of sets of chromosomes in each cell go from 2 sets to 1 set? Said another way, at what point did the ploidy change from diploid to haploid? | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/14%3A_Meiosis_Fertilization_and_Life_Cycles/14.1%3A_Formative_Questions.txt |
Haploid cells that must fuse together are called gametes. Gametes do not grow by mitosis, they must fuse to another haploid cell to produce a zygote, the first diploid cell in an organism. This process is called fertilization and it is composed of two distinct stages: plasmogamy and karyogamy. During plasmogamy, the cytoplasm of the two gametes combines together (plasm- referring to cytoplasm, -gamy meaning marriage). In most organisms, karyogamy is simultaneous with plasmogamy and the nuclei of each gamete fuse together into a single diploid nucleus. When you learn about fungi, you’ll see that plasmogamy and karyogamy can be separated by long periods, resulting in a strange condition called being dikaryotic (having two nuclei).
Each gamete produced during meiosis is genetically distinct, resulting in a pool of genetic options. During sex, organisms don’t choose which gametes are combined, so any combination of genes from the two parents is possible. This is called random fertilization and exponentially increases the potential for diversity within a population.
How many different zygotes could you make by combining the following two sets of gametes produced by meiosis?
Parent 1 eggs Parent 2 sperm
1 1
2 2
3 3
4 4
14.5: Life Cycles
A life cycle describes the events from the start of life to the reproduction of new life. For humans, we each begin as a zygote and grow by mitosis to become a multicellular. At some point, reproductive cells in our bodies undergo meiosis to make either eggs or sperm. If we fuse our eggs or sperm with another human, this forms a new zygote and the cycle continues. We can classify the type of life cycle that multicellular organisms have based on the ploidy of the multicellular stage, whether it is haploid or diploid.
Diplontic: The multicellular stage is diploid
Humans have a diplontic life cycle because the multicellular stage is diploid. The zygote grows by mitosis into a diploid, multicellular organism. Part of this multicellular organism undergoes meiosis to produce haploid cells called gametes within structures called gametangia (gametangium, singular). These gametes fuse to create a diploid zygote. Because gametes are produced by meiosis in this life cycle, each gamete is unique.
This is also sometimes called gametic meiosis because meiosis results in the production of gametes. In the life cycle diagram, the gametes are alone on the haploid portion.
Haplontic: The multicellular stage is haploid
For organisms with a haplontic life cycle, such as fungi and some of the green algae, the multicellular stage is haploid. In this case, as soon as the diploid zygote is formed, it undergoes meiosis to produce haploid spores inside structures called sporangia (sporangium, singular). Spores can be distinguished from gametes, not by how they are produced, but based on what they do afterward: spores grow, gametes fuse.
These spores then grow by mitosis to produce multicellular haploid organisms. These haploid organisms produce haploid gametes by mitosis. In this life cycle, all of the gametes produced by an organism are identical to each other and to the parent organism. These haploid gametes can then fuse to form a zygote.
This is also called zygotic meiosis because the zygote undergoes meiosis. In the life cycle diagram, the zygote is alone on the diploid portion.
Haplodiplontic: There are two multicellular stages, one haploid and one diploid
For a few marine algae and all plants, the haplodiplontic life cycle becomes more complex. To complete one life cycle, there are at least two multicellular individuals. The diploid zygote grows by mitosis to become a multicellular diploid organism, the sporophyte. A sporophyte produces haploid spores through meiosis in sporangia. These spores then grow by mitosis into multicellular haploid organisms, the gametophyte. A gametophyte produces haploid gametes by mitosis in gametangia. Gametes fuse to produce a diploid zygote.
This complex life cycle is also referred to as alternation of generations and, occasionally, sporic meiosis because meiosis produces spores. However, I avoid using this term because meiosis also produces spores in a haplontic life cycle.
14.6: Life Cycle Diagram Fold Out
To help you remember the different life cycles, you are going to make a fold out. When the page is completely unfolded, you’ll see the haplodiplontic life cycle. When you fold down only the top ¼ of the page, you will be looking at the diplontic life cycle. When you fold up only the bottom ¼ of the page, you will be looking at the haplontic life cycle. Make notes on this fold out to help you remember which organisms have these different life cycles and other important aspects highlighted by your instructor.
1. Remove the last page of the lab and position it so the haplodiplontic life cycle is facing you, right side up. You can find a copy of the folding diagram to print here.
2. Fold the paper in half by bringing the top of the page down to line up with the bottom of the page: “hamburger style”. Then unfold it again.
3. Now fold it again so that the top of the page lines up with the fold you made in the middle. If done correctly, you should be looking at a diplontic life cycle. If what you are looking at matches this description, write “Diplontic Life Cycle” at the top of the folded page.
4. Unfold the paper again, then fold the bottom half up so that the bottom of the page lines up with the middle fold. If you are looking at the haplontic life cycle, write “Haplontic Life Cycle” on the bottom of the folded paper.
5. Label the important structures and stages in the diagram, including gamete, spore, and zygote.
6. On thick arrows, label that mitosis is occuring.
7. Choose a color to represent haploid and another to represent diploid. Make a key and color in the structures in the diagram to match the appropriate ploidy.
14.7: Summative Questions
1. Label the following diagram of meiosis. Note important events that are happening at each stage and whether the cells are currently haploid or diploid.
1. At which stage(s) in meiosis is genetic variation introduced? Explain how this occurs.
2. Can genetic variation be introduced after meiosis? Explain.
3. What is the difference between plasmogamy and karyogamy? What do the words plasmogamy and karyogamy mean? | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/14%3A_Meiosis_Fertilization_and_Life_Cycles/14.4%3A_Fertilization.txt |
Learning Objectives
Content Objectives
• Learn the traits used to distinguish between true fungi and fungus-like organisms
• Understand how filamentous morphology relates to feeding by external absorption
• Compare feeding strategies and ecology of different fungal groups
Skill Objectives
• Use life history traits to differentiate between true fungi and fungus-like organisms
• Identify important structures in the haplontic Rhizopus life cycle. Know the function and ploidy of these structures.
• Interpret a phylogeny to draw conclusions about the relatedness of organismal groups
• Use stains to view microscopic features more effectively
15: Microfungi - Slimes Molds and Microscopic True Fungi
Note
This lab is one of a two-part series (Microfungi - Slimes, Molds, and Microscopic True Fungi and Macrofungi and Lichens) that can be modified or combined based on available specimens.
1. What is a fungus? What traits do you associate with being fungal?
2. Which group of organisms do you think fungi are most closely related to? Explain your reasoning.
15.2: Introduction
There are many groups of unrelated organisms that are referred to as ‘fungus’ or ‘mold’. Formerly, these groups were all believed to have been related, sharing the characteristics of being heterotrophic eukaryotes, generally with cell walls, usually decomposers or parasites, and often with the storage carbohydrate glycogen. These organisms were placed into groups ending with “-mycota”, meaning fungus. On closer inspection, we find that these groups differ on many important life history traits and very few of them actually belong to the true Fungi. However, if you take a mycology course, you will most likely study these unrelated organisms, as well. Similarly, fungi are still studied under the umbrella of botany as a relic of past classification, but they are more closely related to animals than they are to plants.
15.3: Fungus-Like Organisms
Myxogastria (formerly Myxomycota): Plasmodial Slime Molds
Myxomycetes (members of the Myxogastria) are fungus-like organisms called slime molds, but they are not members of Kingdom Fungi. In their feeding stage, myxomycetes form one large amoeba with many nuclei and no cell wall. This amoeba moves over damp, decaying material looking for bacteria to engulf and digest. When it dries out or runs out of food, it begins to make fruiting structures called sporangia (sporangium, singular). Inside these sporangia, the myxomycete will undergo meiosis, wall off individual nuclei and make haploid spores for aerial dispersal. Dispersal by spores, heterotrophism, and glycogen as a storage carbohydrate originally classified this group within Kingdom Fungi, but this is the end of the similarities. The spores have cell walls made of cellulose, like plants. When these spores land, they will germinate into haploid cells (called swarm cells) that will fuse together to form a diploid amoeba. This means that, unlike true Fungi, they have a diplontic life cycle.
Observe the slime molds on display. If possible, make a wet mount of a small sample. If there are motile cells (cells that are actively moving around), look for the presence of 2 flagella. This also distinguishes myxomycetes from Kingdom Fungi, where only one flagellum is present. If a mature amoeba is present, can you see cytoplasmic streaming occurring?
Draw what you see and make observations below. Label the name, function, and ploidy of any identifying structures.
Other Slime Molds: Dictyostelia and Protostelia
Quite on theme for fungal classification, there are several different groups of unrelated organisms that are called slime molds. The two mentioned above, Dictyostelia and Protostelia, represent cellular slime molds and are relatively closely related to the plasmodial slime molds. Unlike the plasmodial slime molds, these groups exist as individual amoebae until it is time to leave the area. In the Dictyostelia, the amoebae assemble together to form elaborate fruiting structures where only some of the individuals will become spores and the rest will die. They are studied for this altruistic behavior.
Oomycota -- The Water Molds
Oomycetes are also fungus-like organisms with cell walls made of cellulose. Similar to myxomycetes, they have motile spores with 2 flagella. However, one of these flagella is "normal"-looking (called a whiplash flagellum) and the other is ornamented. This strange characteristic puts organisms into a group called the heterokonts (meaning "different flagella"). Like us, true Fungi are part of the opisthokonts (opisth- meaning rear, -kont meaning flagellum).
Also similar to myxomycetes, oomycetes have a diplontic life cycle. What does this mean?
If supplies are available, place a dead bug into a petri dish with some pond water. Observe the insect under a dissecting scope, then add a cover to your petri dish. Label “Saprolegnia Culture”, your initials, and the date. Set the covered dish aside to incubate until Lab Heterokonts, where you will be learning about heterokonts in more detail.
15.4: Kingdom Fungi - The True Fungi
Though many heterotrophic decomposers and parasites are fungus-like, an organism must be in Kingdom Fungi to be considered a true fungus. In addition to genetic relationships, organisms are classified into Kingdom Fungi based on the following traits:
• Eukaryotic
• Heterotrophic by absorption
• Morphology: Unicellular or a thallus composed of hyphae. If present, motile cells have only a single, whiplash flagellum (like us).
• Cell walls: Chitin
• Storage carbohydrate: Glycogen
• Life cycle: Haplontic (though this one gets a bit weird)
• Ecology: Many fungi are decomposers, primarily in terrestrial ecosystems. Other groups of fungi live in symbiosis with other organisms as parasites, mutualists, or commensalists. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/15%3A_Microfungi_-_Slimes_Molds_and_Microscopic_True_Fungi/15.1%3A_Formative_Questions.txt |
Unicellular fungi are called yeasts. Yeasts reproduce asexually by budding, where the cell produces an outgrowth that enlarges and is eventually pinched off, creating an identical copy of the parent cell. The parent cell is left with a small circular scar (called a bud scar) that, at the right angle, refracts the light of the microscope and appears to glow a brighter white.
Place a small drop of the yeast culture on a slide, dilute it with a drop or two of water, and add a cover slip. Observe your slide under the microscope at 400x (or 1000x, if possible) and look for yeast reproducing asexually. Can you see bud scars?
Draw what you see in the space below and label any important structures.
When fungi produce a larger vegetative body, it is called a thallus (a body plan that is not differentiated into tissues). The thallus is composed of microscopic strands called hyphae (pronounced high-fay). As a collective, these hyphae are called a mycelium (pronounced my-seal-ee-um).
Safety Note
Congo Red is carcinogenic, do not contact with skin. Do not wash slides with Phloxine B in the sink, as this compound is harmful to aquatic life. See your institution’s MSDS for safety information on these compounds before use.
Obtain a sample of mycelium, make a wet mount with 5% KOH and stain it with Phloxine B and/or Congo Red. The stains are optional, but usually the fungal mycelium lacks pigment. The stains are picked up by the hyphae and allow them to stand out from the background. Phloxine B stains only the contents of the hyphae, while Congo Red stains the contents and walls. You can combine these two stains together or use separately.
In your prepared slide, can you see divisions of the hyphae into compartments? These dividing walls are called septa (septum singular) and are only present in certain groups of fungi.
Draw a strand of the hyphae (also sometimes called a hyphal thread or filament), label the cell wall, septum (if present), plasma membrane, and any other features you see.
Molds
Molds are the asexual form of a fungus. Before the advent of DNA sequencing, we thought molds were fungi that only reproduced asexually and they were placed in their own group, the Fungi Imperfecti (also sometimes called the Deuteromycota). As we are sequencing different mold species, we are discovering that they are identical to sexually reproducing fungi that we have already identified. This is causing a lot of trouble for taxonomy, as now we have one species with two names.
Asexually reproducing fungi produce spores called conidia. Much like the yeasts, many conidia are formed from budding off of another cell. Make a wet mount of a mold by adding a drop of 5% KOH to your slide, scraping a small amount of mold onto a razor blade or other tool, and gently depositing it onto the droplet. GENTLY lay down a cover slip. You may also consider trying to view molds as a dry mount, as they tend to be extremely hydrophobic.
Look for conidia and conidiophores, structures where the conidia are produced. Draw what you find below and label any identifiable structures: | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/15%3A_Microfungi_-_Slimes_Molds_and_Microscopic_True_Fungi/15.5%3A_General_Morphology.txt |
Determining the ancestry and relatedness of groups of fungi is surprisingly difficult. DNA sequencing has led to continual rearrangements of the fungal tree of life and, at the time of writing, there is no one agreed upon picture of the history of fungi that the author is aware of. However, there are a few major groups that Kingdom Fungi is commonly broken into, and these will be discussed in the following section.
The image below is from an open-access publication (doi.org/10.1128/ mBio.01739-16) and presents one possible hypothesis for the relationships between groups of fungi. This hypothesis is called a phylogeny and is based on genetics, as well as physiological and morphological features. No single phylogeny is currently accepted by all mycologists.
Chytrids: includes Chytridiomycota and Blastocladiomycota from the above phylogeny
Chytrids comprise the oldest lineages of fungi. Unlike any other group within this kingdom, they are aquatic and have swimming spores (zoospores) with a single flagellum. Though many in this group are harmless decomposers, the most famous of the chytrids is Batrachochytrium dendrobatidis, a fungus that infects the skin of amphibians. This chytrid is contributing to a worldwide decline in many amphibian species (though there are numerous other contributing factors), particularly frogs.
If available view specimens of chytrids under the dissecting and compound microscopes. What features can you find?
Zygomycetes: includes all of the non-flagellated, early diverging fungi above, except Glomeromycotina
The zygomycetes are composed of at least two distinct lineages of fungi that all share a common structure during sexual reproduction, the zygosporangium. This is a large, ornamented, orange-to-brown structure where both fertilization and meiosis occur. Zygomycetes have no septations in their hyphae, which is referred to as being coenocytic. These fungi are commonly found on high-sugar substrates, such as rotting fruits or molding bread, or as insect parasites. Zygomycetes can reproduce asexually by producing mitosporangia (shown below), making haploid spores by mitosis.
Above are three mature zygosporangia produced during sexual reproduction of Rhizopus stolonifer. In the center of the image, two compatible mycelia (+ and -) have connected together and are currently forming gametangia. They will flood these gametangia with haploid nuclei, which will fuse within the zygosporangium to create diploid zygotes. Each of these will immediately undergo meiosis to produce haploid spores.
View specimens of zygomycetes available in lab and record your observations below. Which features would help you identify this group?
Glomeromycota: listed as Glomeromycotina in the fungal phylogeny
This single lineage within Kingdom Fungi forms relationships with the roots of almost all land plant species and thalli of the earliest plant lineages, who evolved before roots. This mycorrhizal (myco- meaning fungus, rhiza meaning root) relationship has existed for 400 million years and was likely involved in the movement of plants onto land. Glomeromycetes are called endomycorrhizal because the fungal hyphae enter inside of plant cells. The fungus enters the plant tissue, usually through the roots, and penetrates the cell walls of the cortex cells in the root. However, they hyphae do not go through the plasma membrane. Instead, they form highly branched, tree-like structures called arbuscles. This provides a large amount of surface area for the plant and fungus to interact with each other. How are they interacting?
These images are from an open access paper (doi: 10.1038/srep29733) studying fungal colonization of plant cells. In the image on the right, the fungal tissue was stained with a fluorescent dye. In the lower two (B and C), fungal hyphae were stained dark and are forming arbuscules within the plant cell walls.
Because it is a heterotroph, the fungus takes sugars from the plant. The fungal hyphae extend beyond the plant roots into the soil, where it can absorb water and nutrients that are transferred to the plant. Each partner gains a benefit from this relationship, so this is called a mutualistic symbiosis, or mutualism.
A symbiotic relationship refers to a shared relationship between at least two organisms of different species. This relationship can benefit both parties, as above, only benefit one, potentially causing harm to the other. What would you call this last type of symbiosis?
View a mycorrhizal root tip under the compound microscope, either as a prepared slide or from a fresh sample. If from a fresh sample, use 5% KOH + Phloxine B or Cotton Blue to stain the fungal tissue. Draw what you see below and label any distinguishing features of both the plant and fungus.
The Dikarya: Ascomycota and Basidiomycota
These two groups of fungi are referred to as the dikarya because some portion of their life cycle is dikaryotic. When two compatible mating types meet, they fuse together and begin the process of fertilization with plasmogamy (fusion of the cytoplasm). However, the nuclei do not fuse. Instead, a new mycelium is formed with two different haploid nuclei in each cell, making the ploidy n + n (as opposed to diploid, 2n). This state is called dikaryotic. Karyogamy (fusion of the nuclei) does not occur until the fungus is about to make spores. The two nuclei fuse and the zygote immediately undergoes meiosis, forming haploid spores.
Which type of life cycle would you classify this as and why?
Though there are microscopic species and life stages in each group (including the yeast you saw earlier), members of the Ascomycota and Basidiomycota both form macroscopic fruiting bodies. Because of this, they will be covered in more detail in Lab Macrofungi and Lichens.
15.7: Summative Questions
1. How would you classify a heterotrophic eukaryote with a filamentous thallus? What traits would you need to make your determination?
2. Fungal hyphae are much smaller in diameter than plant roots. How would this influence the ability of a mycorrhizal fungus to acquire water and nutrients from the soil, in comparison to the plant?
3. Compare and contrast yeast and conidia.
4. What classifies something as a mold?
5. Where do you most often see molds and how might this relate to the reproductive strategy of these organisms?
6. Describe what is happening in the Rhizopus life cycle diagram below. Label important structures and note where fertilization and meiosis are occuring. Choose a different color to represent haploid and diploid tissues. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/15%3A_Microfungi_-_Slimes_Molds_and_Microscopic_True_Fungi/15.6%3A_The_Fungal_Phylogeny.txt |
Learning Objectives
Content Objectives
• Learn the differences between groups of macrofungi
• Understand the relationships between fungi and other organisms
Skill Objectives
• Identify features of fungi both microscopically and macroscopically
• Differentiate between endomycorrhizal and ectomycorrhizal fungi
• Locate the photobiont in a lichen cross section
Contributors and Attributions
• Thumbnail: Amanita muscaria mushroom
16: Macrofungi and Lichens - True Fungi and Fungal Mutualisms
Note
This lab is one of a two-part series (Microfungi - Slimes, Molds, and Microscopic True Fungi and Macrofungi and Lichens - True Fungi and Fungal Mutualisms) that can be adjusted and combined based on available specimens.
1. What does the term symbiosis mean to you? How do you think this is different from the term mutualism?
2. What is the function of a mushroom? Do all fungi make mushrooms?
16.2: Introduction
Though both of the groups of fungi we will learn about today contain microscopic subgroups and/or life stages, we still refer to them as the macrofungi because they account for nearly all of the fruiting bodies that we can easily see with the naked eye.
Fungi make a diversity of fruiting structures, structures in which meiosis occurs to form spores. In the microfungi, you saw zygomycetes making their fruiting structures: zygosporangia. In the macrofungi, you will learn three different types of ascocarps and the anatomy of a basidiocarp, otherwise known as a mushroom. These structures are analogous to fruits produced on a tree. The rest of the fungal body (or, the rest of the tree, in this analogy) is the mycelium, buried within the substrate and busily acquiring food. Note: The mycelium is not the equivalent of the roots of the tree, but of the entire tree, including the roots, stems, and branches. Mushrooms are the ephemeral fruits of this structure, emerging for sexual reproduction.
16.3: Macrofungi
Ascomycota -- The Sac Fungi
This group of fungi is when the life cycle starts to get particularly strange and the fungus forms something called a dikaryon (di- meaning two, karyo- meaning nucleus). When two different hyphal bodies fuse, they do not fuse their nuclei, only the cytoplasm. The result is a single fungal body with two separate types of nuclei floating around inside it. Because it is neither haploid (n), nor truly diploid (2n), this condition is called being dikaryotic (n+n).
Ascomycota are distinguished from other groups of fungi by the following characteristics:
• Production of ascospores (usually 8) within an ascus, a normally elongate, sac-like structure
• Hyphae with simple septations
• The mycelium is primarily haploid. When two haploid (n) mycelia of the correct mating types meet, they can form a dikaryotic (n+n) fruiting body called an ascocarp.
There are three different types of ascocarps:
1. Apothecium - a (usually) cup-shaped fruiting body with asci lining the interior of the cup. Two exceptions are Helvella and Morchella (morels), where the cup has been inverted, no longer looking like a cup, and the asci cover the now-exterior surface.
2. Perithecium - a round fruiting body with a long neck, much like a bottle, with the asci inside the bottom of the bottle. Microscopic and quite similar to a Fucus conceptacle.
3. Cleistothecium - a spherical, enclosed fruiting body packed with rounded asci. These are also microscopic and can be beautifully ornamented.
Above is the spore producing surface of an apothecium. The long asci each contain 8 ascospores.
Observe the ascomycetes on display. What type of ascocarps do you see? Draw and label them below, indicating where the asci would be found.
Make a thin section slide of Xylaria using KOH for the wet mount. Draw a perithecium below. Label an ascus and ascospores.
Make a thin section slide of an apothecium. Draw it below, labelling an ascus and ascospores. Can you find septations in the hyphae?
If available, view a cleistothecium from the Erysiphales (powdery mildews) under both the dissecting and compound microscope. Draw it below, labelling an ascus and ascospores.
Basidiomycota -- The Club Fungi
Basidiomycota are the other group of dikaryotic fungi. This group includes the mushroom-forming species of fungi, as well as two groups of (mostly) plant parasites, delightfully referred to as the rusts and the smuts.
Basidiomycota are distinguished from other groups of fungi by the following characteristics:
• Basidiospores (usually 4) produced from a basidium--a normally squat, roundish structure with prongs called sterigmata that spores are produced on.
• Hyphae with complex septations. These specialized septations look like they are surrounded by parentheses and help the mycelium maintain its dikaryotic state.
• Clamp connections can sometimes be seen, though are only obvious in certain genera.
• The mycelium is primarily dikaryotic. The haploid spores germinate and must find another germinating spore of the correct mating type to form a dikaryotic mycelium. This can then produce fruiting bodies called basidiocarps, or as we commonly call them, mushrooms.
The image above shows the spore producing surface of a basidiomycete, Coprinus. The dark football-shaped structures are the haploid basidiospores. They sit atop small projections called sterigmata (sterigma, singular) that emerge from the top of the basidium. In most mushrooms, each basidium produces four basidiospores.
Based on the presence of four haploid spores, which type of cell division do you think formed these spores and why?
How could you explain the presence of 8 spores in ascomycetes?
Label the anatomy of an Amanita muscaria basidiocarp below, including the cap (pileus), universal veil scales, gills (lamellae), annulus, stipe, volva, and mycelium.
Not all basidiocarps look like Amanita muscaria. Observe the diversity of basidiomycetes on display. In the space below, draw and label a jelly mushroom, a coral mushroom, a bracket, and a puffball.
Make a thin section of a gilled mushroom, using 5% KOH for the wet mount. Try to find basidia and basidiospores. Can you see clamp connections? It may help to stain with Phloxine B. Draw and label what you see below.
The phylum Basidiomycota is composed of three major groups. All of the fungi above are from the subphylum Agaricomycotina. The two other groups are composed of primarily parasitic fungi, rusts (Pucciniomycotina) and smuts (Ustilaginomycotina). View the available prepared slides of these fungi and draw what you see below. Can you find basidia and basidiospores? Compare and contrast what you see with the Agaricomycetes. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/16%3A_Macrofungi_and_Lichens_-_True_Fungi_and_Fungal_Mutualisms/16.1%3A_Formative_Questions.txt |
The Mycelium
Like us, fungi are heterotrophic and digest other organisms to survive. Unlike us, they do this digestion outside of their bodies, then absorb the nutrients. Consider the structure of a hyphal filament and what you know about surface area to volume ratio. How does the structure of the mycelium allow for more efficient uptake of water and nutrients?
Because fungi must absorb their food, they tend to live on or in whatever it is they are eating. If a mushroom is growing out of a log, it is likely that it is eating that log or something contained within it. This structure and diversity is what makes them such important decomposers. However, fungi play many other important ecological roles.
Mutualisms
Mycorrhizae
As you saw in Lab Microfungi - Slimes, Molds, and Microscopic True Fungi, not all fungi are decomposers and parasites. Members of the Glomeromycota form a mutualistic relationship with plant roots called endomycorrhizae, penetrating inside the cell wall.
Some members of both the Ascomycota and Basidiomycota (as well as at least one zygomycete, Endogone) also form mycorrhizal relationships with plants, but the interface between the plant and the mycelium is a bit different. Instead of penetrating inside the cell wall, the fungal hyphae surround the plant cells and make a sheath on the exterior of the root. This type of relationship is called being ectomycorrhizal (ecto- meaning outer), because the hyphae are outside of the cells.
View specimens of fruiting bodies of ectomycorrhizal fungi (such as chanterelles, boletes, Russula, etc…). Compare and contrast these fungi with the glomeromycetes you saw in Lab Microfungi - Slimes, Molds, and Microscopic True Fungi.
In the diagram above, both ectomycorrhizal and endomycorrhizal fungi are shown interacting with the roots of a tree. In the root cross section, label which structures represent each type of mycorrhizal relationship. Next, show the direction of the flow of sugars with arrows. Do the same for the flow of water and nutrients.
Lichens
A lichen is a mutualistic relationship between several organisms. There is always a fungal partner, the mycobiont, and this is usually an ascomycete. There are a few basidiolichens, but they look very different from ascolichens.
The mycobiont is heterotrophic and so must eat sugars produced by other organisms. The sugars it consumes are harvested from the photosynthetic partner, the photobiont, which is either green algae, cyanobacteria, or less commonly, both (these lichens are called tripartite lichens)!
The photobiont produces glucose through photosynthesis, which the mycobiont harvests and converts into mannitol, a form of sugar that only the fungus can use. The photobiont only really gets to eat when the mycobiont is focused on maintaining the lichen thallus from desiccation (drying out) and sun damage. Though this relationship is technically a mutualism, it has some very parasitic components.
What does the photobiont get out of this relationship?
Lichens are incredibly diverse, but we can begin by classifying them into three different growth forms:
• Crustose lichens are firmly attached to the substrate and usually have to be chipped off. These lie flat, like a crust.
• Foliose lichens have a distinct upper and lower surface, like a leaf (think ‘foliar’).
• Fruticose lichens are shrub-like and usually attach to the substrate at a single point, like a plant with roots.
Observe the diversity of lichens on display. Draw a lichen from each of the three forms below. Label any ascocarps (or basidiocarps) that you find.
16.5: Summative Questions
1. What determines whether a fungus is considered microfungi or macrofungi?
2. How would you differentiate between an ascomycete and basidiomycete? Which features would be most helpful in making this determination?
3. Fungi can form many different types of symbiotic relationships with plants. Compare and contrast parasitic and mutualistic symbioses.
4. What is a lichen? How would you classify this in the tree of life and why?
5. How do lichens obtain food? Are they autotrophic or heterotrophic? Explain your reasoning.
6. Label the structures present in the diagram on the next page and identify the group of fungi that each of these is from. Choose a different color to represent haploid, diploid, and dikaryotic tissues. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/16%3A_Macrofungi_and_Lichens_-_True_Fungi_and_Fungal_Mutualisms/16.4%3A_Fungal_Ecology.txt |
Learning Objectives
Content Objectives
• Learn the characteristics used to classify an organism as a heterokont.
• Explain the process of secondary endosymbiosis and how it lead to photosynthetic heterokonts.
• Understand the important ecological roles of heterokonts
Skill Objectives
• Identify important features in the life cycles of Saprolegnia and Fucus. Know the functions of these features.
• Use life history traits to distinguish between different groups of heterokonts, fungi, and other algal groups.
Contributors and Attributions
• Thumbnail: Unfertilized oogonium
17: Heterokonts
1. How do autotrophs and heterotrophs differ in how they obtain energy?
2. How do autotrophs and heterotrophs differ in how they use energy?
17.2: Introduction
Heterokont means “different flagella”. Organisms in this group are eukaryotes that have a life stage with two different flagella, one whiplash and one ornamented. In addition to this shared feature, heterokonts have a diplontic life cycle. The earliest heterokonts were heterotrophic. These often function as decomposers or parasites in aquatic ecosystems, and primarily as plant pathogens in terrestrial ecosystems. The Irish potato famine was caused by one of these.
At some point in history, a heterotrophic heterokont tried to eat an ancestor of the red algae. This photosynthetic, red alga-like organism was not digested and, instead, evolved slowly over time into a four-membraned chloroplast. This process is called secondary endosymbiosis and is described in more detail later in the lab.
17.3: Oomycota -- The Water Molds
Oomycetes (a term used to refer to organisms in the phylum Oomycota) are a group of fungus-like organisms that rely on water for completion of their life cycle, hence the common name “water molds”. Members of this group share the following characteristics:
• Heterotrophic by absorption
• Morphology: Filamentous
• Cell wall composition: Cellulose
• Storage carbohydrate: Glycogen
• Life cycle: Diplontic
• Ecology: Many oomycetes are important decomposers in aquatic ecosystems, while others -- namely those in the genus Phytophthora -- are some of the most destructive plant pathogens. You can culture some of the more common decomposers (genus Saprolegnia) by putting pond water into a petri dish, adding a dead insect, covering it, and waiting for a few days.
Considering what you have learned about root words, how does the name Saprolegnia help you understand this organism’s ecology?
Saprolegnia Life Cycle
View prepared slides of Saprolegnia. This organism reproduces asexually by producing zoospores (zoospores are spores that swim, zoo- meaning ‘to live’) inside of an elongated sac called a zoosporangium (-angium meaning vessel, so a zoosporangium is what zoospores are produced inside of). These zoospores grow by mitosis into a diploid thallus, an undifferentiated body.
Look for asexual reproduction in the form of elongate zoosporangia releasing zoospores. The life cycle of Saprolegnia is diplontic. Are the zoosporangia and zoospores haploid or diploid?
The image above shows a mature zoosporangium releasing diploid zoospores. Each of these spores has two flagella, one ornamented and one whiplash.
Look for Saprolegnia's sexual reproducing structures, the globose oogonium and smaller, pad-like antheridia (singular, antheridium) that attach to the oogonium. Because these structures produce gametes--much like spores are produced in sporangia--the oogonia and antheridia are also referred to as gametangia (gametangium singular). The oogonium produces haploid eggs via meiosis. These eggs are fertilized by the haploid male nuclei produced by meiosis within the antheridium, creating a diploid, thick-walled zygote called an oospore.
The oospore will be released and grow by mitosis to create a new multicellular thallus, completing the life cycle.
Check your Saprolegnia cultures for presence of mycelium (fuzz around the dead insect). Make a wet mount of some of the mycelium and look at it under the compound microscope. It may help to stain the tissue, as you did in the fungal labs.
Do you see any features that belong to Saprolegnia? Do you see any that belong to a member of the true fungi?
Draw what you see in the space below and attempt to identify which group(s) of organisms you have in your culture.
Below is the life cycle diagram of Saprolegnia. Label an oogonium, antheridium, male nuclei, egg, oospore (zygote), zoosporangium, zoospore, and thallus. Indicate where meiosis and fertilization occur. Use a different color to indicate which tissue is haploid or diploid. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/17%3A_Heterokonts/17.1%3A_Formative_Questions.txt |
The photosynthetic lineages of the heterokonts derived their golden brown chloroplasts from secondary endosymbiosis. In this event, an ancestral oomycete engulfed a red alga. As in primary endosymbiosis, instead of being digested, overtime the red alga degenerated into a chloroplast, this time with 4 membranes -- the engulfing membrane from the oomycete, the red alga’s plasma membrane, and the two membranes of the original chloroplast within the red alga. Note: In many groups derived from secondary endosymbiosis, the chloroplast has lost one of these membranes.
The process of secondary endosymbiosis is shown on the following page. Add arrows to show the sequence of events. Label the heterotrophic oomycete, the photosynthetic red alga, and the origin of each membrane of the 4-membrane chloroplasts.
Note
The red algal cell would also have mitochondria. Feel free to add one in!
Secondary Endosymbiosis in Heterokonts:
Brown algae and diatoms are the result of this endosymbiotic event. Their chloroplasts have chlorophyll a, chlorophyll c, and fucoxanthin. Which pigment(s) are ancestral from secondary endosymbiosis (present in the red alga)?
Which pigment(s) were derived after this event (not present in the red alga)?
17.5: Phaeophyta - The Brown Algae
Brown algae are brown due to the large amounts of carotenoids they produce, primarily one called fucoxanthin. These organisms are exclusively multicellular and can get so large that they require special conductive cells to transport photosynthates from their blades down to the rest of their tissues. These conductive cells are called trumpet hyphae and have sieve plates and resemble sieve tubes found in flowering plants. You may not see these, but be on the lookout for them!
• Morphology: Multicellular thallus
• Cell wall composition: Cellulose
• Chloroplasts: 4 membranes, pigments are chlorophyll a, chlorophyll c, and fucoxanthin
• Storage carbohydrate: Laminarin
• Life cycle: Diplontic
• Ecology: Marine
View the variety of brown algae on display. In the space below, label the morphology of a brown algal thallus. Much like Saprolegnia, the body of an alga is termed a thallus because it is not differentiated into specialized tissues. Label the holdfast, stipe, gas bladder(s) [also called a float], and blade(s).
17.6: Fucus Life Cycle
Our model organism for the Phaeophyta life cycle is Fucus (rockweed), which, like its relative Saprolegnia, has a diplontic life cycle.
Observe the displayed Fucus thallus. Note the dichotomous branching (forking into two equal branches) and the swollen, heart-shaped reproductive tips of the branches. These swollen branch tips are called receptacles. The receptacles are covered in small bumps, each with a pore at the center of the bump called an ostiole. The bumps are conceptacles, chambers that house the male and female gametangia.
View prepared slides of a cross section through a Fucus receptacle to view inside the conceptacles. The male and female gametangia are housed within the conceptacle chamber. The antheridia are branched structures that look like small trees. These produce sperm with heterokont flagella.
The oogonia are globose structures divided into sections as eggs are produced. The eggs will be fertilized by sperm that swim in through the ostiole, forming a diploid zygote that will be released in the marine water. This zygote will grow by mitosis into a multicellular, diploid thallus. Label the bolded structures in the life cycle diagram on the following page.
In the Fucus life cycle below, label any important structures, indicate where meiosis and fertilization occur, and color the haploid and diploid tissues differently. Draw arrows that show where mitosis is occuring. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/17%3A_Heterokonts/17.4%3A_Photosynthetic_Heterokonts_-_Descendents_of_Secondary_Endosymbiosis.txt |
Diatoms are an incredibly diverse group of unicellular organisms containing anywhere from 20,000 to 2 million species.
• Morphology: Unicellular
• Cell wall composition: Silica frustule
• Chloroplasts: 4 membranes, pigments are chlorophyll a, chlorophyll c, and fucoxanthin
• Storage carbohydrate: Chrysolaminarin
• Life cycle: Diplontic
• Ecology: Everywhere! Marine, freshwater, and terrestrial.
We are still trying to figure out how to determine what a diatom "species" is and, so far, they have been classified based on the morphology of their frustules, a silica shell made from two distinct valves that enclose the plasma membrane. Using this classification, there are two major groups of diatoms: centric (have radial symmetry) and pennate (have bilateral symmetry). Draw and label an example of each type below.
Diatoms are a major component of the phytoplankton (phyto- meaning plant, plankton meaning ‘to wander’). These are photosynthesizing, microscopic organisms in aquatic environments and include members from many of the other groups covered in general botany, including cyanobacteria, Rhodophyta, and the green algae.
Would any members of Phaeophyta be considered phytoplankton? Why or why not?
Considering that phytoplankton are primary producers, what important roles might they have in aquatic and/or terrestrial environments?
In addition to morphology, diatoms can also be classified by where they occur. Free-floating diatoms are planktonic. Diatoms attached to other organisms (like giant kelp) are epiphytic. Benthic diatoms tend to dwell toward the bottom of a body of water. Try to find some planktonic diatoms by preparing slides from fresh samples of pond or seawater. Look for epiphytic diatoms by mounting a piece of algae and searching along its edges. Note: I have the best luck with epiphytic diatoms on intertidal red algae. In fact, you can usually find diatoms on prepared slides of Polysiphonia, our model organism from the Rhodophyta.
Draw what you find below. Label frustules and chloroplasts and identify whether the diatom is centric or pennate.
Diatoms primarily reproduce asexually by binary fission, similar to prokaryotes. During binary fission, the two valves of the frustule are separated and each new cell forms a new valve inside the old one. However, the new valve is always smaller. If diatoms only reproduce in this way, it results in a continual decrease in average size. When some minimal size is reached, this can trigger sexual reproduction. When diatoms sexually reproduce, they have a diplontic life cycle and produce a very large auxospore. Stay on the lookout for these strange, enlarged diatoms!
Note
Apparently, diatoms can produce auxospores through asexual or sexual reproduction.
17.8: Summative Questions
1. What traits could you use to differentiate between Oomycota and Kingdom Fungi? Specify which traits belong to which group in your answer.
2. Which traits could you use to differentiate between Phaeophyta and Bacillariophyta?
3. Water molds, brown algae, and diatoms do not initially appear to have anything in common. What traits could you use to classify these organisms into an evolutionary related group?
4. Why do the heterotrophic heterokonts have chloroplasts with extra membranes? Where did these membranes come from?
5. Could brown algae be considered phytoplankton? Why or why not? | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/17%3A_Heterokonts/17.7%3A_Bacillariophyta_-_The_Diatoms.txt |
Learning Objectives
Content Objectives
• Understand how differences in life history traits between red and green algae can be explained by differences in ecology
• Relate selection pressures to adaptations within each group
• Learn the life cycles of Polysiphonia and Spirogyra
Skill Objectives
• Use life history traits to differentiate between red and green algae
• Identify important structures in the Polysiphonia and Spirogyra life cycles. Know the function and ploidy of these structures.
Contributors and Attributions
• Thumbnail: Tetrasporangium
18: Red and Green Algae
1. Red and green algae share a common ancestor. How might natural selection have led to these two different lineages? What traits would natural selection be acting upon?
2. What role did meiosis have in the selection process described above?
18.2: Introduction to Red and Green Algae
The red and green algae are descendents of the primary endosymbiosis event that resulted in the first chloroplast. Their plastids have two membranes, an inner membrane that was the cyanobacterial cell wall and an outer membrane from the organism that first engulfed it. The story of these algal groups is the evolutionary history of all plants. As time passes in this story and groups specialize for their given environments, we will see changes in cell wall composition, photosynthetic pigments, storage carbohydrates, and life cycles. Make note of these important differences between groups, as they represent the primary ways we classify these organisms.
18.3: Phylum Rhodophyta - The Red Algae
The red algae represent a monophyletic group of organisms. Members of this group share the following characteristics:
• Morphology: Unicellular to multicellular, no flagellated stages. Cells of multicellular species are connected via incomplete cytokinesis, resulting in pit connections.
• Cell wall composition: Cellulose and galactans
• Chloroplasts: 2 membranes, pigments are chlorophyll a and phycobilins (primarily phycoerythrin, providing their red color)
• Storage carbohydrate: Floridean starch
• Life cycle: Alternation of generations with an extra diploid stage, the carposporophyte
• Ecology: Primarily marine (97% of species)
Selection Pressures and Drivers
An important aspect of understanding the life history traits of the Rhodophyta is understanding the challenges of living in a marine environment.
1. Access to sunlight: Most colors of light cannot penetrate into deeper water, as they are scattered by water molecules. The wavelengths of light that reach deepest into the ocean are blue and green. Many fish that live in the deep ocean are red. Because red light does not penetrate to the depths where they live, this makes them virtually undetectable by sight. Remember, we see things because of the light that bounces off of them. Red pigments reflect red light, so no red light, no reflected light. Red algae are using a similar strategy--absorb the wavelengths of light that are not red--with a different goal: to use that absorbed light to make food. The phycoerythrin in their chloroplasts reflects red light, giving them a red appearance, and absorbs the blue light that is able to penetrate to deeper areas in the water column.
2. Fertilization: The ocean is an expansive environment, often with large areas of open space between populations of organisms. In this environment, successful fertilization of an egg by a nonmotile sperm--red algae lack flagella--presents a challenge. Having multicellular haploid and diploid phases provides red algae more opportunities to produce gametes and spores. A diploid stage that clones the zygote, the carposporophyte, provides more opportunities to do meiosis from each fertilization event.
3. Salinity: Marine environments are relatively high in salinity. A possible adaptation for this is to have sulfated polysaccharides in the cell wall, such as the galactans present in Rhodophyta. This is a strategy present in (potentially all) marine algae and is inferred to be an adaptation for salinity-tolerance. See this open-access article for further information: https://doi.org/10.1371/journal.pone.0018862
Observing the Red Algal Life Cycle
Polysiphonia is the model organism for Rhodophyta. Red algae have an alternation of generations life cycle that has an extra diploid stage: the carposporophyte. The gametophytes of Polysiphonia are isomorphic (iso- meaning same, morph- meaning form), meaning they have the same basic morphology. Use prepared slides and the images in the following pages to label the life cycle diagram below.
In the diagram above, indicate where meiosis and fertilization occur. Color the haploid and diploid tissue differently, and draw arrows to show when mitosis is happening.
Observe a prepared slide of a Polysiphonia male gametophyte. The male gametophyte (shown on right) has elongated structures that emerge from the tips of the thallus branches. These are spermatangia, where spermatia are produced by mitosis. Label the bolded features in the life cycle diagram.
Female gametophyte stages shown below:
Observe a prepared slide of a Polysiphonia female gametophyte. The female gametophyte produces an egg that is contained within a structure called the carpogonium. This structure has a long, thin projection called a trichogyne (trich- meaning hair, -gyne meaning female). During fertilization, a spermatium fuses with the trichogyne and the nucleus of the spermatium travels down the tube to the egg. When the nucleus of the spermatium fuses with the egg, a zygote is produced. This zygote is retained and nourished by the female gametophyte as it grows.
The globose structures you see growing from the female gametophyte thallus are called cystocarps. A cystocarp is composed of both female gametophyte tissue (n) and carposporophyte tissue (2n). The outer layer of the cystocarp, the pericarp (peri- meaning around) is derived from the female gametophyte and is haploid.
The interior of the cystocarp consists of the carposporophyte, which is diploid, and produces elongated structures called carposporangia, inside of which it produces carpospores by mitosis. All of these--carposporophyte, carposporangia, and carpospores--are diploid. Label the bolded features in the life cycle diagram and color them according to their ploidy.
Observe a prepared slide of a Polysiphonia tetrasporophyte. The diploid carpospores are released into the ocean waters, where they will be carried on currents to another location. If the carpospore lands in an appropriate environment, it will grow by mitosis into a tetrasporophyte (2n). The tetrasporophyte produces tetrasporangia (2n) within the branches of the thallus. Each tetrasporangium produces four unique, haploid tetraspores by meiosis.
Tetraspores (n) are released and will grow by mitosis into either male or female gametophytes, completing the life cycle. Label the bolded features in the life cycle diagram. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/18%3A_Red_and_Green_Algae/18.1%3A_Formative_Questions.txt |
The nature of the evolutionary relationships between the green algae are still up for debate. As of 2019, genetic data supports splitting the green algae into two major lineages: chlorophytes and streptophytes. The streptophytes include several lineages of green algae and all land plants. Streptophytes and chlorophytes represent a monophyletic group called Viridiplantae (literally “green plants”).
Organisms that are classified as green algae share the following characteristics:
• Morphology: Unicellular to multicellular; two whiplash flagella on motile cells
• Cell wall composition: Cellulose
• Chloroplasts: 2 membranes, pigments are chlorophyll a, chlorophyll b, and carotenoids
• Storage carbohydrate: Starch
• Life cycle: Varies, but primarily haplontic. Some marine species have alternation of generations.
• Ecology: Freshwater, marine, and terrestrial species
Selection Pressures and Drivers
1. Sun Damage. Green algae represent a diverse group of organisms with diverse life history traits, many of which are shared with land plants. The development of carotenoids-- yellow, orange, and red pigments that act in both light harvesting and sun protection--offers this group increased access to sunlight while simultaneously protecting against UV damage. UV rays do not penetrate very far into the water column, so organisms moving into shallower waters or terrestrial environments would need to deal with this new challenge. Many terrestrial species of green algae appear orange, rather than green, due to the production of large amounts of carotenoids.
Observing the Life Cycle of Green Algae
Though green algae display a diversity of life cycles, many have a haplontic life cycle. A model organism for the green algae is Spirogyra. Spirogyra is a unicellular green algae that grows in long, filamentous colonies, making it appear to be a multicellular organism. Even though it is technically unicellular, its colonial nature allows us to classify its life cycle as haplontic. In the haploid vegetative cells of the colony, the chloroplasts are arranged in spirals, containing darkened regions called pyrenoids where carbon fixation happens. Each haploid cell in the filament is an individual, which makes sexual reproduction between colonies an interesting process. Note the nucleus suspended in the center of each cell in the colony on the right.
When two colonies of Spirogyra meet that are of a complementary mating type (+/-), sexual reproduction occurs.
The two colonies align, each cell across from a complementary cell on the other filament. A conjugation tube extends from each cell in one colony, inducing formation of a tube on the cells in the other colony. The conjugation tubes from each colony fuse together.
The contents of one cell will move through the conjugation tube and fuse with the contents of the complementary cell, resulting in a diploid zygote.
The zygote appears as a large, egg-like structure contained within the complementary cell. It has a thick wall that provides resistance to desiccation and cold, allowing colonies of Spirogyra to overwinter, when needed. The other colony is now a filament of empty cells that will be broken down by some decomposer.
When conditions are right, the zygote undergoes meiosis to produce another vegetative colony of haploid cells.
In the life cycle diagram below, indicate where meiosis and fertilization occur. Label a vegetative cell, chloroplast, pyrenoid, nucleus, conjugation tube, and zygote. Choose a color for the zygote to indicate that this structure is diploid.
What type of life cycle is this? | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/18%3A_Red_and_Green_Algae/18.4%3A_Phylum_Chlorophyta_and_Phylum_Streptophyta.txt |
The following resource may be useful for this activity: https://www.msnucleus.org/watersheds/mission/plankton.pdf
A pond is an ecosystem teeming with life from across the tree of life. If available, view a sample of pond water in a petri dish under the dissecting scope. Try to identify any major groups of organisms you see (at least to phylum level) and make a catalogue of these in the space below.
Use a dropper to place a few drops of the pond water onto a slide. Add a coverslip and view your sample under the compound microscope. Document and attempt to identify any additional microorganisms you find in the space below.
As a class or with your lab group, make a collaborative list of the organisms in the pond ecosystem. Next, make a conceptual model that arranges these groups by relatedness. One way to do this is by building a phylogeny, much like you saw in Lab Microfungi - Slimes, Molds, and Microscopic True Fungi, organizing the evolutionary trajectory of these groups with a branching tree diagram. You can also see an example of this in Lab Evolution of the Embryophyta. However, a branching tree diagram is not the only way to communicate relationships. Feel free to get creative! What traits (such as photosynthetic pigments or cell wall components) differentiate these evolutionary lineages? How could you incorporate these into your model?
18.6: Design an Experiment
You have learned about different groups of algae and the selection pressures that contributed to the differences between them. Use this information and your understanding of photosynthesis to design an experiment to test differences in photosynthesis between red and green algae. This is an open-ended prompt intended to give you some creative freedom to explore your own question. True science is rarely neatly packaged for you, so dig in and get messy!
Some suggested common lab materials to consider using for your experiment:
• Red, green, and blue transparent film
• Test tubes and flasks
• Saltwater and freshwater
• Baking soda (produces \(\ce{CO2}\) when mixed with an acid or dissolved in water)
• Potassium hydroxide (absorbs \(\ce{CO2}\))
• Full spectrum lamp, heat lamp, and other light sources
Record your experimental design in the space below. What are the essential components you will need to consider? If time and supplies allow, set up and carry out the experiment.
18.7: Summative Questions
1. If you were shown an algal species that produced large amounts of carotenoids, what would you infer about the ecology of this organism? Explain your reasoning.
2. If you were shown an algal species that produced a sulfated polysaccharide in its cell walls, what would you infer about the ecology of this organism? Explain your reasoning.
3. Many green algae have a haplontic life cycle. However, a few marine species exhibit alternation of generations. Why might you expect to see this in marine species?
4. Plants that specialize in shade-tolerance often have a dark red or purple pigment on the underside of their leaves. Why might this be?
5. What regions of the spectrum of light do chlorophylls absorb? Which part do they reflect? | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/18%3A_Red_and_Green_Algae/18.5%3A_Pond_Ecosystem_Relationships.txt |
Learning Objectives
Content Objectives
• View the different groups of land plants in the world around you and observe where and how they are growing
• Understand how the Earth has changed over time and how major lineages of plants evolved in response to these changes
• Understand the nested structure of classification and evolutionary relationships
Skill Objectives
• Distinguish between groups of plants using life history characteristics
19: Evolution of the Embryophyta
1. What do plants need to survive?
2. Which of these do you think is the strongest driver for plant evolution and adaptations in your current environment? Explain.
3. Do you see evidence of this in the plants around you?
19.2: Introduction to the Evolution of the Embryophyta
There are four major evolutionary groups of land plants: Bryophytes, Seedless Vascular Plants (SVPs), Gymnosperms, and Angiosperms. These groupings represent major changes in plant structure and life history characteristics over the course of time that coincide with major changes in the evolution of the Earth, as a whole. The classification “embryophytes” refers to the evolution of the embryo, a zygote that is retained and nourished by the female gametophyte as it grows.
Embryophytes share many common features, most corresponding to the selective pressures from the initial movement onto land. The embryo is one of these, providing higher likelihood of success for offspring in a new, harsh environment. In addition to the embryo, all plants have the same basic life cycle: alternation of generations. Much like the marine algae, the first plants were living in an environment where they needed to increase their chances of reproductive events. Multicellular stages on both sides of the life cycle increases the number of reproductive propagules. All plants are also multicellular, with tissues and multicellular gametangia.
Other adaptations to life on land include the desiccation-resistant compound sporopollenin. This is found in the cell walls of spores of early land plants and in pollen of seed plants, hence the name sporo - pollen - in. It is also found in the cell walls of a few green algae. On the exterior, plants are surrounded by a waxy cuticle that helps protect them from their outer environment.
Much like their green algal predecessors, plants store their carbohydrates as starch inside plastids, plastids with two membranes (the result of primary endosymbiosis), cell walls containing cellulose, and have the pigments chlorophyll a, chlorophyll b, and carotenoids. For this reason, along with genetic sequencing, land plants and green algae are grouped together in the Viridiplantae.
Phylogeny: A Hypothesis on Evolutionary Relationships
The diagram on the next page shows the nested relationships between lineages in the descendents of primary endosymbiosis. This diagram is a hypothesis for how these different lineages are related, based on a large amount of genetic information that was analyzed through the One Thousand Plant Transcriptomes Initiative (2019). In this project, they compared the genetic products (RNA transcripts) from over 1,000 species. The phylogeny summarizes their findings by grouping the organisms by who was most similar.
The red algae were most similar to the outgroup--a group thought to be prior to primary endosymbiosis--while the angiosperms were the least similar to the outgroup. It is most likely that these differences are because the angiosperms evolved later, after many differences had accumulated in their evolutionary history, differentiating them from the outgroup. The conclusions we draw about relatedness of major groups of plants and when these groups of plants evolved can also be supported by the fossil record. Rocks that fossils are found in can be dated and we can use morphological features found in those fossils to link them to extant plants.
We have no way of knowing the true evolutionary history of organisms, as we were not there to witness the events. However, we can use an assemblage of data to develop hypotheses about how the events occurred. The evolutionary groupings used in this lab manual are based largely on the findings from the following phylogeny.
As you work through the material today, reference the phylogeny and try to make sense of how it depicts the relationships described in the lab. Note the nested groups discussed. At the end, try to make your own simplified phylogeny.
Diagram below: Phylogenetic inferences of major clades; from One thousand plant transcriptomes and the phylogenomics of green plants by Leebens-Mack, J.H., Barker, M.S., Carpenter, E.J. et al. Nature 574, 679–685 (2019) [Open Access] | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/19%3A_Evolution_of_the_Embryophyta/19.1%3A_Formative_Questions.txt |
The Bryophytes (~23,000 extant species)
Bryophytes arose in a period of Earth’s history before soils had formed. The terrestrial surface was rocky and consisted primarily of crusts (microbial mats) composed of assemblages of prokaryotes. The exposure to sunlight would have been intense relative to the buffer provided by water. In addition, being surrounded by water would provide regulation of surrounding temperature and structural support. As green algae began to colonize the terrestrial surface, at least one of these lineages accumulated adaptations that were favorable to living on land--a waxy cuticle to prevent water loss, desiccation-resistant dispersal propagules called spores, and retention and feeding of the developing zygote. This lineage of green algae evolved into the ancestor of the bryophytes. This evolutionary group includes liverworts, mosses, and hornworts. These plants do not have true roots to absorb water, nor do they have vascular tissue to transport that water to other regions of the plant. Because of this, bryophytes tend to grow prostrate (close to the surface they are growing on) and stay quite small. They also tend to grow in moist areas where there is access to water and are reliant on water for the dispersal of gametes and fertilization.
Draw any bryophytes that you see. Describe the environment where you found them. Were there features that the locations had in common (e.g. shaded)?
What features did the bryophytes themselves share?
Tracheophytes
Seedless Vascular Plants (~20,000 extant species)
As bryophytes began to colonize the terrestrial surface, they produced organic acids during metabolism that aided in the breakdown of the rocky substrate. When they died, their organic matter mixed with the weathered rock, forming the Earth’s earliest soils. Formerly abundant to the first terrestrial colonists, access to sunlight became competitive as bryophytes proliferated. This led to selection for individuals that could lift themselves higher and transport water throughout their tissues. Eventually, this selection resulted in the evolution of vascular tissue -- pipes that could bring water up from the ground so that parts of the plant could be raised upward, and those parts raised upward could transport their photosynthates down to the lower parts of the plant. The cells in the xylem (water-transporting vascular tissue) contained lignin, the tough, decay-resistant compound that wood is made out of. This rigid molecule in the vascular tissue allowed for structural support, allowing plants to grow taller -- some over 100 feet! The vascular system also allowed for the specialization of organs: roots for water absorption, leaves for photosynthesis, and stems for structural support.
Seedless vascular plants also began to rely more on the sporophyte stage. The sporophyte became the larger, nutritionally independent stage of the life cycle. Branching sporophytes offered more sites for meiosis to occur, resulting in increased opportunities for variation, which could be interpreted as more options in an increasingly competitive environment.
Seedless vascular plants that you might see today include club mosses, spike mosses, ferns, and horsetails. Though many modern taxa are relatively small in stature, extinct members of each of these groups had arborescent (tree-like) forms. Imagine a 100 ft tall horsetail -- this would be Calamites, an extinct member of the Equisetopsida from the Carboniferous period. The true era of the SVPs was 300-400 mya. The climate was tropical, with warm, shallow seas extending inland. Rapid growth of tree-like trunks with woody tissues that were difficult to break down resulted in the accumulation of large amounts of carbon-rich biomass that would become the coal deposits that this period is named for.
Draw any SVPs you encounter. Describe the environment where you found them. Were there features that the locations had in common?
What features did the plants themselves share?
Seed Plants
Gymnosperms (~1000 extant species)
Toward the end of the Carboniferous period, major changes in the climate occurred. The current day European and North American continents slammed together, forming the Appalachian mountains (which were taller, at that time, than the present-day Himalayas). Fossil and geologic records show a tendency toward a drier climate, with evidence of glaciation and lowered sea levels. Inland seas were increasingly diverted into distinct river channels as woody debris channelled the movement of waterways. In short, the terrestrial surface began to dry out and there was much more of it. The ancestors of birds, reptiles and mammals were adapting eggs that could survive outside of the water -- plants were working toward a similar strategy. Dry conditions would have selected for plants with thicker cuticles, leaves with less surface area to evaporate from, propagules that could survive through dry periods to germinate when water was available, and those that could grow taller than the current canopy. Around this time, a group of animals likely took flight for the first time -- the insects! This would present both new challenges and new opportunities for plants.
The plants that would become the gymnosperms evolved xerophytic leaves to prevent desiccation in the dry air. Some would have the ability to grow wider (and thus taller) via the production of a new layer of secondary xylem, AKA wood, each year. These plants could also produce exterior layers of dead cells, unlike the living epidermis, called bark. Together, the production of bark and wood are part of a process called secondary growth. To increase the chances of fertilization in the absence of water, gametes began to be dispersed aerially via pollen. Perhaps most importantly, the zygote and female gametophyte were surrounded in a protective coating and dispersed as seeds. Both seeds and pollen develop within structures called cones.
The first fossil records of gymnosperms are from a period called the Permian, just after the Carboniferous. Gymnosperms used to have many more species, but it is likely that the event that wiped out most of the dinosaurs also represented the end for most of those lineages. Extant groups of gymnosperms include the conifers, cycads (similar in appearance to palms), gnetophytes, and single species from the ginkgophytes, Ginkgo biloba. Of the approximately 1000 species of gymnosperms alive today, about 600 of these are conifers, 58 of which are found in California. Many lineages of gymnosperms are currently threatened with extinction.
Why might so many conifers be found in California? Consider the unique climate of California -- Mediterranean -- and the climatic conditions during which gymnosperms evolved.
Draw any gymnosperms you encounter. What features of their habitat and morphology did they share, if any? Were they all conifers?
Consider the resin canals present in pine needles. What is their function and how does the presence of these canals reflect the conditions in which pines likely evolved?
Flowering Plants
Angiosperms (>370,000 extant species and counting)
At the end of the Permian period, there was the largest mass extinction this planet has ever experienced. It is estimated that 96% of species that lived at that time went extinct. This event signalled the downturn for some groups and opened up space for others to emerge. The exact timing of the emergence of angiosperms is unknown, so it is difficult to relate their evolution to specific climatic conditions. However, there is relatively new fossil evidence that may place flowering plants as early as the Jurassic period, 174 mya. This was the age of the dinosaurs and coincides with the emergence of the first feathered dinosaurs -- birds! Much like the insects, birds would present interesting opportunities for this new group of plants, working as both pollinators and seed dispersers.
Angiosperms can be distinguished from other plants by a set of specialized characteristics that allowed them to compete in an already full world. The (usually) easiest thing to identify about an angiosperm are its flowers. These collections of modified leaves allowed this group of plants to attract pollinators and increase the chances of successful fertilization. Once pollinated, the fertilized seeds are encased in a protective ovary whose structure can be specialized for different methods of dispersal, such as animal ingestion, animal attachment, flotation, or wind dispersal. This protective ovary and the encased seed(s) are more commonly called a fruit. Inside the developing seeds, angiosperms provide an additional food source to the developing zygote, the endosperm.
Competing with the gymnosperms for access to sunlight was perhaps hopeless, so the angiosperms adapted ways to work smarter, not harder. In the xylem, they evolved large diameter conducting cells for rapid water uptake called vessel elements, though this made them vulnerable to freezing conditions. In the phloem, sieve cells evolved into sieve tube elements, increasingly specialized for transportation of photosynthates.
As you might have guessed from the vast number of species, angiosperms occupy incredibly diverse habitats and span a range of morphologies, from tiny plants floating as a film on the surface of a pond to towering Eucalyptus trees dominating the forests of Tasmania, rivalling redwoods in height.
Draw a few of the angiosperms you see. What did they have in common? Did they tend to grow in similar environments?
Do you think any of these plants have the same pollinator? Why or why not?
19.4: Summative Questions
1. Which of these major evolutionary groups appears to be the most abundant in your area? Why might this be? Consider your local climate, topography, and animal species.
2. If your climate were to become drier, which group would you expect to see more of? What if it were to become wetter? Explain your reasoning.
3. Which adaptations do all plants share for living on land?
4. Which traits do land plants share with green algae?
5. Place the following groups into the correct nested structure: Viridiplantae, flowering plants, tracheophytes, seed plants, embryophytes.
6. On the back of this page, draw a phylogeny that uses green algae as the outgroup. The traits you listed that are shared with green algae will be the ancestral traits. Use the information included in this lab to indicate where new traits evolve on your evolutionary timeline. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/19%3A_Evolution_of_the_Embryophyta/19.3%3A_Major_Evolutionary_Lineages.txt |
Learning Objectives
Content Objectives
• Understand the difficulties organisms would face moving from an aquatic to a terrestrial environment
• Learn the characteristics that distinguish the bryophytes from green algae and from other land plants
• Learn the characteristics possessed by each group within the bryophytes
• Gain perspective on how the characteristics of early plants correspond with an Earth much different from today’s, including how these plants would begin to shape that environment into our current one
Skill Objectives
• Correlate features of bryophyte anatomy and life history traits with adaptation to life on land
• Distinguish between multicellular green algae and thalloid bryophytes
• Distinguish between groups within the bryophytes, particularly between thalloid liverworts and hornworts, and between leafy liverworts and mosses
20: Bryophytes
1. What new environmental stressors would an alga need to adapt to if it were to try to move onto land?
2. Choose one of the stressors you mentioned above. Try to come up with a few ways that you could modify the alga to be adapted to that stressor (changes in specialized cells, changes in tissue composition and types of biomolecules present, changes in organ structure, etc…). Explain how these modifications would help the alga adapt to those conditions. Drawings or diagrams may help.
20.2: Introduction to Bryophytes
Bryophytes arose in a period of Earth’s history before soils had formed. The terrestrial surface was rocky and consisted primarily of crusts (microbial mats) composed of assemblages of prokaryotes. The exposure to sunlight would have been intense relative to the buffer provided by water. In addition, being surrounded by water would provide regulation of surrounding temperature and structural support. As green algae began to colonize the terrestrial surface, at least one of these lineages accumulated adaptations that were favorable to living on land--a waxy cuticle to prevent water loss, desiccation-resistant dispersal propagules called spores, and retention and feeding of the developing zygote. This lineage of green algae evolved into the ancestor of the bryophytes. This evolutionary group includes liverworts, mosses, and hornworts. These plants do not have true roots to absorb water, nor do they have vascular tissue to transport that water to other regions of the plant. Because of this, bryophytes tend to grow prostrate (close to the surface they are growing on) and stay quite small. They also tend to grow in moist areas where there is access to water and are reliant on water for the dispersal of gametes and fertilization. There are approximately 23,000 known extant species.
The evolutionary relationships between bryophyte lineages are currently unresolved.* Members of this group have the following characteristics:
• Morphology: Multicellular, can be leafy or thalloid. Complex tissues, including an exterior protective layer. Root-like structures called rhizoids provide anchorage.
• Cell wall composition: Cellulose
• Chloroplasts: 2 membranes, pigments are chlorophyll a, chlorophyll b, and carotenoids
• Storage carbohydrate: Starch
• Life cycle: Alternation of generations. Gametophyte dominant: sporophytes grow from and are nourished by the female gametophyte.
• Ecology: Terrestrial, gametes are dispersed in water
*Note: As of 2019, much is unresolved on the early lineages of plants and who was first on land. Recent genetic analyses interpret bryophytes as being monophyletic, all deriving from a common ancestor that branched from the main line of plants. Read this open-access paper for further information: https://doi.org/10.1073/pnas.1323926111 | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/20%3A_Bryophytes/20.1%3A_Formative_Questions.txt |
An important aspect of understanding the life history traits of the bryophytes is understanding the challenges of living in a terrestrial environment.
1. Sun exposure. Sunlight provides the power that drives our biosphere, but some wavelengths of sunlight can be damaging to cellular structure and even DNA. High frequency wavelengths, such as ultraviolet (UV), X-rays, and gamma rays can penetrate outer protective layers like skin, through cell membranes, and causing damage to DNA, proteins, and other biomolecules. Fortunately for organisms on Earth, almost all of these wavelengths are filtered by the atmosphere before they reach us, though some UV rays still make it through. These last UV rays are filtered out for aquatic organisms, but terrestrial organisms need adaptations to protect against UV radiation. Humans have skin with melanin pigments. Terrestrial plants have an epidermis and carotenoid pigments.
2. Desiccation. Transitioning from a completely aquatic environment to a terrestrial one leads to challenges of drying out, also known as desiccation. Temperatures are more extreme outside of the water and evaporation from tissues into the relatively dry air is constant. Terrestrial plants quickly adapted a waxy covering on the epidermis, called a cuticle. This water-tight covering required the evolution of simple pores, and eventually stomata, to allow gas exchange with the outer environment. Because these plants lack vascular tissue, water can only be transported around the organism via osmosis. Thus, these plants must keep all tissues close to water access.
3. Lack of a soil environment. The first organisms to move onto land would have found a relatively barren, rocky landscape. Soils did not yet exist. The rocky substrate experienced physical weathering from rain and wind that would help break it down. Chemical weathering through acidic rain or the interaction of water with compounds in the rock could also assist in breakdown. However, up to this point, contributions from organic matter would be minimal. Bryophytes lack true roots, instead producing structures called rhizoids whose function is anchorage. There are genes present in bryophytes, as well as some fossil evidence, that indicate bryophytes likely had mycorrhizal relationships with fungi that helped them acquire nutrients in this new landscape.
View the bryophytes on display. Compare and contrast the overall morphology of bryophytes to the plants you can see outside. Why do you think the bryophytes all share such a similar growth form?
20.4: Anthocerotophyta - Hornworts
• Gametophyte Morphology:
• Exclusively thalloid, often with compartments of mutualistic cyanobacteria from the genus Nostoc
• Hornwort cells are monoplastidic, containing one large chloroplast in each cell
• Simple pores allow for gas exchange (no guard cells, meaning pores are permanently open)
• Sporophyte Morphology:
• Linear sporangium that lacks a seta
• Grows from a basal meristem
• Stomata present for gas exchange
If available, observe an Anthoceros gametophyte with sporophytes under the dissecting microscope. Look for simple pores, rhizoids, Nostoc colonies, sporangia, and stomata on the sporangia. Draw/label these features in the space below.
Which part of the sporangium is the oldest? How do you know?
Make a wet mount of a small piece of hornwort thallus or obtain a prepared slide. Can you tell how many chloroplasts are in it? Draw this cell in the space below and label the chloroplasts and any other features you recognize. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/20%3A_Bryophytes/20.3%3A_Selection_Pressures_and_Drivers.txt |
• Gametophyte morphology
• Leafy liverworts have leaves arranged in a flat plane with a set of smaller underleaves
• Thalloid liverworts have no leaves
• Liverwort cells have multiple chloroplasts
• Simple pores allow for gas exchange (no guard cells, meaning pores are permanently open)
• Asexual clones, called gemmae, are sometimes produced
• Sporophyte morphology
• Leafy liverworts produce single sporangium at the end of a seta (often fragile, transparent)
• Marchantia, a thalloid liverwort, develops complex structures called where gametangia are produced
Leafy Liverworts
Cut off the end of a leafy liverwort and make a wet mount or obtain a prepared slide to view under the compound microscope. Draw the specimen below and indicate the three different rows of leaves (two in one plane, and a row of smaller underleaves running beneath the stem).
Still under the compound microscope, observe one of the leaf cells. Can you tell how many chloroplasts are in it? Draw this cell in the space below and label the chloroplasts and any other features you recognize.
Thalloid Liverworts: Marchantia
If available, observe a Marchantia polymorpha gametophyte under the dissecting scope. Look for simple pores, rhizoids, archegoniophores, antheridiophores, and gemmae cups containing asexual clones of the gametophyte, called gemmae. Label the bolded features in the life cycle diagram.
Marchantia life cycle:
In the diagram above, indicate where meiosis and fertilization occur. Color the haploid and diploid tissue differently, and draw arrows to show when mitosis is happening.
Obtain a prepared slide of a Marchantia antheridiophore. The male gametangia, antheridia, are produced on the top of this structure. Each antheridium produces haploid, swimming sperm by mitosis. Label the bolded features in the life cycle diagram.
Obtain a prepared slide of an unfertilized Marchantia archegoniophore. This is the structure that produces the female gametangia, archegonia. Each archegonium produces a single haploid egg by mitosis.
A sperm will be transported by water to the archegoniophore, travel down the venter of the archegonium, and fertilize the egg. This forms a diploid zygote. Label the bolded features in the life cycle diagram.
Obtain a prepared slide of a fertilized Marchantia archegoniophore with sporophytes. The zygote will be retained within the archegonium and nourished through the placenta, an area of gametophyte tissue adjacent to the foot of the sporophyte.
The mature sporophyte produces haploid spores via meiosis, which will grow into gametophytes. The sporophyte has a seta (the stalk), sporangium (also called a capsule), spores, and elaters, which aid in spore dispersal. Label the bolded features in the life cycle diagram.
20.6: Bryophyta - Mosses
• Gametophyte Morphology:
• Exclusively leafy. Leaves are arranged in a spiral and usually have a costa.
• Simple pores allow for gas exchange (no guard cells, meaning pores are permanently open)
• Sporophyte Morphology:
• Complex sporangium at the top of a seta
• Stomata present for gas exchange
The Moss Life Cycle
If available, observe moss gametophytes with sporophytes under the dissecting scope. On the gametophytes, look for spirally arranged leaves, each with a costa, and rhizoids at the base. Female gametophytes will look tufted at the top. Within these tufts are hidden archegonia, each with a single egg. Male gametophytes will have a flat or cupped-looking top called a splash cup where antheridia produce sperm to be splashed out by rain drops. A sporophyte will grow from the top of a female gametophyte, emerging from one of the archegonia. Label the bolded features in the life cycle diagram.
Mnium life cycle:
In the diagram above, indicate where meiosis and fertilization occur. Color the haploid and diploid tissue differently, and draw arrows to show when mitosis is happening.
Obtain a prepared slide of a Mnium male gametophyte (antheridial head). The splash cup at the top of the gametophyte holds the male gametangia, antheridia. Each antheridium produces haploid, swimming sperm by mitosis. Label the bolded features in the life cycle diagram.
Obtain a prepared slide of an unfertilized Mnium female gametophyte (archegonial head). This is the structure that produces the female gametangia, archegonia. Each archegonium produces a single haploid egg by mitosis. The process of fertilization is the same as in the liverworts, described above. Label the bolded features in the life cycle diagram.
Make a wet mount of a moss sporophyte or obtain a prepared slide of a Mnium sporangium. These complex sporangia contain several different parts. When the sporophyte emerges from the archegonium, it tears off the venter and creates a sort of cap on the sporangium, called a calyptra. This calyptra is haploid, as it originated from the female gametophyte tissue. Under the calyptra is a tiny lid-like structure called an operculum that keeps the capsule closed until the spores have developed. When the spores have matured, the operculum pops off and reveals the peristome teeth, which aid in spore dispersal via hygroscopic movements in response to desiccation. Label the bolded features in the life cycle diagram.
This long section shows the developing sporangia (surrounding the grey areas), the operculum (covering the tip of the capsule), and the peristome teeth just below. The calyptra is not present.
If your instructor has found fresh moss sporophytes that have peristome teeth (not all mosses do), place one or two of these sporophytes in a petri dish with a lid on and observe it under the dissecting scope. Draw what you see below and label all parts of the sporophyte.
While looking through the dissecting scope and focused on the peristome, remove the lid of the petri dish. What happens to the peristome and what does this have to do with spore dispersal? If nothing happens, try lightly breathing on it and watching again through the scope.
20.7: Summative Questions
1. Describe three adaptations that bryophytes have specifically for life on land.
2. Explain why bryophytes tend to be so small in stature and are often found growing appressed to a surface.
3. How would you tell the difference between a leafy liverwort and moss? Describe at least two things you could look for.
4. How would you tell the difference between a thalloid liverwort and hornwort? Describe at least two things you could look for.
5. Considering what you learned about the cyanobacterium Anabaena’s relationship to the water fern Azolla, how might the Nostoc living inside the hornwort tissues influence the hornwort’s ability to survive on land?
6. What is the ploidy of the calyptra and where did this tissue come from? Does it have a function? | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/20%3A_Bryophytes/20.5%3A_Marchantiophyta_-_The_Liverworts.txt |
Learning Objectives
Content Objectives
• Understand the changing conditions on Earth during the period in which SVPs evolved and how those conditions selected for features found in SVPs
• Learn the characteristics that distinguish the SVPs from bryophytes
• Connect what you have learned about plant anatomy and photosynthesis to SVP structure
• Learn the sporophyte dominant life cycle of ferns
Skill Objectives
• Correlate features of SVP morphology and life history traits with adaptation to the changing environment
• Distinguish between mosses and lycophytes
• Distinguish between microphylls and megaphylls
• Identify SVPs based on strobilus anatomy
• Identify structures involved in the fern life cycle and describe their functions
Contributors and Attributions
• Thumbnail: Equisetum sporangiophore
21: Seedless Vascular Plants
1. When bryophytes evolved on land, they didn’t have to compete with other plants. What were the major selective pressures for the bryophytes?
2. If seedless vascular plants evolved after bryophytes began to colonize terrestrial surfaces, this would indicate a new major selective pressure: competition. What features might enable a fern to outcompete a moss?
21.2: Introduction to Seedless Vascular Plants
As bryophytes began to colonize the terrestrial surface, they produced organic acids during metabolism that aided in the breakdown of the rocky substrate. When they died, their organic matter mixed with the weathered rock, forming the Earth’s earliest soils. Formerly abundant to the first photosynthesizers to become terrestrial, access to sunlight became competitive as bryophytes expanded. This led to selection for individuals that could lift themselves higher and transport water throughout their tissues. Eventually, this selection resulted in the evolution of vascular tissue -- pipes that could bring water up from the ground so that parts of the plant could be raised upward, and those parts raised upward could transport their photosynthates down to the lower parts of the plant. The cells in the xylem (water-transporting vascular tissue) contained lignin, the tough, decay-resistant compound that wood is made out of. This rigid molecule in the vascular tissue allowed for structural support, allowing plants to grow taller -- some over 100 feet! The vascular system also allowed for the specialization of organs: roots for water absorption, leaves for photosynthesis, and stems for structural support.
Seedless vascular plants (SVPs) also began to rely more on the sporophyte stage. The sporophyte became the larger, nutritionally independent stage of the life cycle. Branching sporophytes offered more sites for meiosis to occur, resulting in increased opportunities for variation, which could be interpreted as more options in an increasingly competitive environment. There are approximately 20,000 known extant species, most of which are ferns.
SVPs are considered to be a paraphyletic group of organisms, forming two distinct lineages: Ferns and Lycophytes. These two lineages share the following characteristics:
• Morphology: Sporophytes develop complex tissues, including lignified vascular tissue, true roots, stems, and leaves. Sporophytes are branched, producing many sporangia. Gametophytes are reduced and thalloid. In some groups, the gametophyte is subterranean and parasitizes mycorrhizal fungi for sugars.
• Life cycle: Alternation of generations. Sporophyte dominant: sporophytes still grow from the gametophyte, but are now photosynthetic and the larger, longer-lived phase of the life cycle.
• Ecology: Gametes are still dispersed in water, so moisture is still required for fertilization.
Selection Pressures and Drivers
1. Competition for sunlight. To get access to sunlight, SVPs needed to grow taller than bryophytes. However, this presents a problem of distributing water around the plant body to prevent drying out. Seedless vascular plants solved this problem with the adaptation of lignified vascular tissue. The lignin in the secondary walls of sclerenchyma cells allowed SVPs the structural support to grow taller.
2. The initial forming of soils. Before the bryophytes, terrestrial surfaces were primarily rocky. The first land plants would have contributed to the chemical weathering of these rocks by producing acids during metabolism. After death, these plants add their organic matter to these weathering rocks, beginning to form Earth’s early soils. Fungi were likely involved in this process, as there are genes in the bryophytes for mycorrhizal relationships.
21.3: Lycophytes (Class Lycopodiopsida)
• Microphylls. Leaves have a single, unbranched vein of vascular tissue. Note: The term microphyll, confusingly, is not an indication of the size of the leaf.
• Rhizomes. Asexual propogation of the sporophyte through underground stems.
• Homosporous or heterosporous. Haploid spores grow into bisexual gametophytes in Lycopodium. In Selaginella, microspores develop into microgametophytes that produce sperm and megaspores develop into megagametophytes that produce eggs.
Extinct lycophytes like Lepidodendron and Sigillaria grew into tall trees, branching dichotomously and producing a moss-like canopy of microphylls. Some of these microphylls were several feet long! Lycophytes first appear in the fossil record over 400 million years ago. By the Carboniferous period (around 300 mya), the landscape was covered with lycophyte forests and shallow swamps. Much of the fossil fuels we use today are derived from these extinct arboreal lycophytes falling into swamps, slowing decomposition and creating layers of carbon-rich material that we now find as coal seams.
Why would being submerged in water slow decomposition? Consider the cellular process normally associated with decomposition activity and what is required to perform this process.
If available, observe fossil specimens or images of fossils from extinct lycophytes. Do these resemble any plants you see today?
Extant lycophytes (those species still alive today) are represented by creeping forms, such as Lycopodium and Selaginella. Observe fresh specimens and prepared slides of Selaginella and/or Lycopodium. Draw and describe the important characteristics that differentiate these plants from bryophytes, including stem and leaf structure, below ground parts, and where spores are produced. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/21%3A_Seedless_Vascular_Plants/21.1%3A_Formative_Questions.txt |
• Megaphylls. Leaves have branching veins of vascular tissue.
• Rhizomes. Asexual propogation of the sporophyte through underground stems.
• Homosporous. Haploid spores grow into bisexual gametophytes that produce both antheridia and archegonia.
Horsetails (Subclass Equisetidae)
• Gametophyte morphology
• Reduced, thalloid. Bisexual gametophytes grow from homospores and produce both antheridia and archegonia.
• Sporophyte morphology
• Leaves are dark, papery and non-photosynthetic.
• Branches are photosynthetic and produced in whorls on the vegetative shoot.
• Sporangia produced in a terminal strobilus on the reproductive shoot.
• Shoots contain silica
• Spores are photosynthetic and have four hygroscopic arms called elaters
Horsetails are one of the most ancient lineages of plants and are relatively unchanged from the fossil record. If you look closely at the nodes of a green vegetative shoot, you will see that branches and leaves have not only switched roles, they have also switched places, with the photosynthetic branches emerging below the papery, non-photosynthetic leaves. The stems of horsetails are covered in silica, giving them the common name scouring rush, as they were formerly used to clean pots due to the abrasive nature of silicate granules. This is what gives the epidermis of the shoot its rough texture.
Observe a vegetative shoot of Equisetum. Draw what you see below and label a node, internode, branch, and leaf. Where would the rhizome be found?
You may notice that there are shoots without branches that appear more yellow than green. These are the reproductive shoots. Reproductive shoots of some Equisetum species (such as E. arvense) do not photosynthesize. Instead, they receive nutrients through the rhizome, which is connected to vegetative shoots. The function of the reproductive shoot is to produce a strobilus. In the strobilus, sporangia are produced on large, T-shaped structures called sporangiophores. Within the sporangia, photosynthetic homospores mature and develop projections called elaters that aid in spore dispersal via interaction with humidity in the air.
Obtain an Equisetum strobilus from a reproductive shoot. Tap the strobilus onto a dry slide to release mint green spores, do not cover with a coverslip. View this slide under the compound microscope and draw what you see below.
As you are looking through the ocular lenses of the microscope, add a drop of water to the slide near your field of view. What happens to the spores that are close to the droplet of water? What happens when they are touched by the water? Describe and draw what you see below.
Comparing Strobili
Lycophytes and horsetails produce spores in a cone-like structure called a strobilus. Obtain a prepared slide of a long section of the strobilus of each of the following: Equisetum, Lycopodium, and Selaginella.
Equisetum
Lycopodium
Selaginella
Homosporous or heterosporous?
Sketch of the strobilus long section
Distinctive features?
Ferns (Subclass Polypodiidae)
• Gametophyte morphology
• Reduced, thalloid, heart-shaped. Often referred to as a prothallus or prothallium.
• Sporophyte morphology
• Megaphylls often pinnately compound fronds, emerging as fiddleheads in the spring.
• Sporangia produced in clusters called sori (sorus, singular).
Fern Life Cycle
Observe a prepared slide of a fern gametophyte (sometimes referred to as a prothallus) under the compound microscope. Look for rhizoids, archegonia (each with a single egg), and antheridia containing many sperm. Label the bolded features in the life cycle diagram.
Archegonia and rhizoids are visible on the prothallium in the above figure.
Below: a side view of an archegonium
Antheridium with sperm (left) and an archegonium with an egg (right)
Note
One strategy ferns have evolved to avoid self-fertilization is to produce archegonia and antheridia at different times. Depending on the type of fern gametophyte you are looking at, you may need to view two different slides to see archegonia and antheridia.
Observe a fern fiddlehead, so named for its visual similarity to the scroll at the top of a violin. The fiddlehead uncoils in the spring by a process called circinate vernation (vernal meaning spring). Observe a mature fern frond. Locate the rachis, pinnae, and sori. Label the bolded structures in the life cycle diagram.
Obtain a prepared slide of a sorus. A sorus is a cluster of sporangia, often protected by an umbrella-like structure called the indusium as the spores mature. Each sporangium is lined by an inflated strip of cells called an annulus. When the spores have matured, the cells in the annulus begin to dry out, causing the cells to collapse and pull the sporangium open, releasing the spores. Label the bolded structures in the life cycle diagram.
Based on the orientation of the palisade and spongy mesophylls in the leaf cross section above, is this sorus on the upper or lower surface of the leaf? Explain your reasoning.
Fern Life Cycle Diagram
In the fern life cycle diagram, label the bolded structures described in the section above. Indicate where meiosis and fertilization occur and draw arrows to show where mitosis is happening. Choose different colors to represent haploid and diploid tissues.
21.5: Summative Questions
1. Describe three adaptations that SVPs have that allow them to compete with bryophytes.
2. Commonly called spike mosses and club mosses, lycophytes aren’t truly mosses at all. How could you distinguish between a moss and a lycophyte?
3. Each time a rhizome produces a new shoot, it also produces roots. What type of roots emerge from stem tissue, such as a rhizome?
4. Compare and contrast the role of the gametophyte in bryophytes and SVPs.
5. What are the advantages to having a branching sporophyte? | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/21%3A_Seedless_Vascular_Plants/21.4%3A_Ferns_%28Class_Polypodiopsida%29.txt |
Learning Objectives
Content Objectives
• Understand the changing conditions on Earth during the period in which gymnosperms evolved and how those conditions selected for features found in this group.
• Learn the characteristics that distinguish gymnosperms from SVPs
• Connect what you have learned about secondary growth and xerophytic leaf anatomy to gymnosperm structure
• Learn the life cycle of pines
Skill Objectives
• Correlate features of gymnosperm morphology and life history traits with adaptation to the changing environment
• Identify structures involved in the gymnosperm life cycle and describe their functions
• Distinguish between different groups of gymnosperms
• Distinguish between gymnosperms and SVPs, particularly ferns and cycads
22: Gymnosperms
1. When you think of plants adapted to dry conditions, what comes to mind? What features do these plants have?
2. Would a snowy environment be considered a dry environment for a plant? Why or why not?
22.2: Introduction to Gymnosperms
Toward the end of the carboniferous period, major changes in the climate occurred. The current day European and North American continents slammed together, forming the Appalachian mountains (which were taller, at that time, than the present-day Himalayas). Fossil and geologic records show a tendency toward a drier climate, with evidence of glaciation and lowered sea levels. Inland seas were increasingly diverted into distinct river channels as woody debris channelled the movement of waterways. In short, the terrestrial surface began to dry out and there was much more of it. The ancestors of birds, reptiles and mammals were adapting eggs that could survive outside of the water -- plants were working toward a similar strategy. Around this time, a group of animals likely took flight for the first time -- the insects! The presence of flying insects allowed for another option for distributing pollen, as well as a large source of potential herbivory.
The plants that would become the gymnosperms evolved xerophytic leaves (see lab 7) to prevent desiccation in the dry air. Some would have the ability to grow wider (and thus taller) via the production of a new layer of secondary xylem (wood) each year. These plants could also produce exterior layers of dead cells, unlike the living epidermis, called bark. Together, the production of bark and wood are part of a process called secondary growth. To increase the chances of fertilization in the absence of water, gametes began to be dispersed aerially via pollen. Perhaps most importantly, the zygote and female gametophyte were surrounded in a protective coating and dispersed as seeds. Both seeds and pollen develop within structures called cones.
The fossil record shows gymnosperms diversifying in a dry period called the Permian that followed the swampy Carboniferous period. Extant groups of gymnosperms include conifers, cycads (somewhat similar in appearance to palms), gnetophytes, and a single species from the ginkgophytes, Ginkgo biloba. Of the approximately 1,000 species of gymnosperms alive today, about 600 of these are conifers, 58 of which can be found in California. In fact, some of the oldest (bristlecone pine), tallest (coast redwood), and most massive (giant sequoia) organisms on the planet are conifers and all are native to California.
See this open-access paper for recent genetic work on the evolutionary relationships between gymnosperms: http://dx.doi.org/10.1098/rspb.2018.1012
22.3: Selection Pressures and Drivers
1. Competition for sunlight. Seedless vascular plants were able to reach heights up to 100 feet tall. In the lineage leading to the gymnosperms and angiosperms, some plants developed the ability to grow wider as they grew taller. This secondary growth allowed for increased stability and, eventually, to reach heights over 300 feet.
2. Drought. Dry conditions would have selected for plants with thicker cuticles, leaves with less surface area to evaporate from, and propagules that could disperse without water and survive through dry periods to germinate when water was available.
3. Herbivory. In addition to leaves that could resist drought, the presence of insects would have driven selection for plants that could defend against herbivory. The thick cuticle and tough texture of xerophytic leaves made them difficult to eat, while resin canals in both leaves and stems provided another line of defense.
22.4: Cycads and Ginkgos
Ginkgos (1 extant species)
As of writing this manual, the most recent genetic studies have placed Ginkgos as the oldest of the extant gymnosperms. This does not mean that it was the first gymnosperm. From the fossil record, it seems that most early gymnosperms went extinct. The sole remaining species in this group, Ginkgo biloba, is a living fossil virtually unchanged from its fossilized ancestors. It is possible that this species was only kept alive due to cultivation efforts by Buddhist monks for its medicinal properties. This species is also long-lived, a single tree can live for thousands of years, and resistant to most pests. Ginkgo biloba can be recognized by the following features:
• Leaves deciduous, but tough, fan-shaped
• Dioecious (di- meaning two, oecious meaning house), with male and female strobili on separate plants. Females plants have paired ovules at the tips of branches, males have catkin-like structures that produce pollen.
• Wind pollinated
Observe the Ginkgo biloba specimens available in lab. Make notes and drawings of features that would help you recognize this species in the space below:
Cycads (approximately 300 extant species)
Cycads are another of the more ancient gymnosperm lineages, appearing in the fossil record around 300 million years ago. Currently, many extant species are in danger of extinction in the wild. However, during the Jurassic period, these plants would have dominated the landscape. Though their large, compound leaves make them appear to be ferns at first glance, cycads can be classified as gymnosperms by the production of seeds instead of spores and xerophytic leaves. These plants share the following features:
• Dioecious. Male and female strobili on separate plants.
• Large compound, xerophytic leaves
• Insect pollinated
Observe the cycad specimens available in lab. Make notes and drawings of features that would help you recognize this group in the space below: | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/22%3A_Gymnosperms/22.1%3A_Formative_Questions.txt |
Gnetophytes (approximately 70 extant species)
Gnetophytes represent an anatomically and genetically difficult group to classify. They have several traits in common with angiosperms, such as vessel elements in the xylem, double fertilization, and a covering over their seeds (more on this in labs 21 and 22). Even their leaves are angiosperm-like, with netted venation. However, these traits are convergently evolved, meaning that angiosperms and gnetophytes each evolved these traits separately. Genetically, recent studies have placed the gnetophytes as a sister group to the Pinaceae (pine family) within the conifers. This would mean that pines, firs, and spruces are more closely related to strange gnetophytes like Ephedra than they are to other conifers like redwoods, cedars, and Pacific yew. However, the true nature of this evolutionary relationship remains murky and contentious.
• Angiosperm-like features: vessel elements, double fertilization, fruit-like ovule coverings
• Dioecious. Female plants have covered ovules, while male plants have pollen cones.
• Leaves xerophytic with opposite arrangement
• Primarily insect pollinated; brightly colored seeds are dispersed by birds
Observe the gnetophyte specimens available in lab. Make notes and drawings of features that would help you recognize this group in the space below:
Conifers (approximately 600 extant species)
Conifers are the most species-rich lineage of gymnosperms. From the fossil record, we think there were over 20,000 species of conifers. However, their diversity declined with the dinosaurs. As discussed in the introduction, these amazing plants represent some of the oldest, tallest, and most massive organisms on the planet. Though currently low in diversity, these amazing plants make up 30% of Earth’s forests. Conifers share the following characteristics:
• Monoecious. Plants produce both male and female strobili on the same plant.
• Wind pollinated with winged pollen
• Xerophytic leaves with a low surface area to volume ratio. Primarily evergreen, but some species are deciduous (e.g. Dawn redwood and larch).
Note
The Pinaceae is currently the largest family of conifers, so many of our examples for this group of gymnosperms will be from the type genus Pinus (pines).
Pine Life Cycle
In the pine life cycle, the pine tree is the sporophyte. Because pines are monoecious, one sporophyte will produce both microstrobili and megastrobili. The microgametophytes are formed within the microsporangia of the microstrobilus, or pollen cone. These structures are all diploid. Within the microsporangium, there are microsporocytes, diploid cells that undergo meiosis to become haploid gametophytes.
The microgametophyte in gymnosperms is the four-celled, winged pollen grain. Within the pollen grain, you can distinguish the generative cell and the tube cell nucleus. The two prothallial cells are not apparent under the microscope. On either side of the pollen grain, two ear-like structures emerge. Thes air sacs may help orient the pollen grain toward the ovule.
Similarly, the megastrobilus, or seed cone, contains diploid megasporocytes that are produced within a megasporangium. Each megasporocyte undergoes meiosis. However, unlike the microsporocytes, only one of the four cells will survive to develop into a megagametophyte and the other three will die.
The megagametophyte is part of the ovule and contains (usually) two archegonia, each with an egg cell inside. The megagametophyte is retained within the megasporangium, which becomes the nucellus. Surrounding the nucellus is the integument, which is initially continuous with the ovuliferous scale and has a small opening called a micropyle.
A grain of pollen will be transported on the wind and, if lucky, it will land on a seed cone. The seed cone has a drop of sugary liquid that it secretes, then retracts, pulling the pollen in toward the ovule. This stimulates the tube cell to germinate a pollen tube, while the generative cell divides by mitosis to produce two sperm. These sperm travel down the pollen tube, through the micropyle, and into an archegonium where one will fertilize an egg. When fertilization occurs, the micropyle closes and the integument becomes the seed coat.
The zygote will grow and develop as an embryo, nourished by the megagametophyte tissue, as well as the nucellus. If you look in a long section of a pine seed, you can see the embryo’s RAM and SAM. The seed will be dispersed by wind or animals and germinate to grow into a diploid pine tree once again.
Pine Life Cycle Diagram
Label the pine life cycle diagram above with the bolded terms from the “Pine Life Cycle” section. Indicate where fertilization and meiosis occur. Draw and label arrows to indicate mitosis. Choose a different color to represent haploid and diploid tissues.
Pine Leaf
In lab Leaf Anatomy, you learned about leaf anatomy and were introduced to the concept of xerophytic leaves. One of the examples in that lab of xerophytic leaves was the pine needle. In the image below, you can see a cross section through a pine needle. Label the following features: xylem, phloem, transfusion tissue, endodermis, mesophyll, hypodermis, epidermis, and cuticle.
The three large holes you see in the leaf above are resin canals. These conduct a thick, sticky compound called resin that aids in plant defense. What might the resin in the pine needle help pines defend against? Explain your reasoning.
The image above shows a close up of a sunken stoma. Label the mesophyll, guard cells, stoma, hypodermis, epidermis, and cuticle.
How do sunken stomata help leaves reduce water loss?
22.6: Using a Dichotomous Key
In Appendix A there is a key to common campus trees. Note that this key was written for a campus on the far north coast of California, so the trees in your area may be different. A dichotomous key is a tool people use to determine the identity of different organisms. At each step in the key, you are presented with a pair of options, such as “leaves are blue” and “leaves are some color other than blue”. You choose which of these options matches your specimen, then follow the path to either the next set of questions, or the end result. Here is an example from the key:
6a. Leaves palm-shaped, deeply dissected, and emerging opposite from each other on the branch....
Maple
6b. Leaves are not as above ... 7
If the leaves you were looking at matched the description in 6a, you would determine that your specimen as a maple. If they were not palm-shaped, or in some other way didn’t match the description, you would would choose 6b and proceed to the set of questions labelled 7a and 7b to make your next choice.
Use the attached key to identify trees on campus or samples provided in the lab.
22.7: Summative Questions
1. What is pollen?
2. Why was pollen an important adaptation for gymnosperms, considering the changing global conditions?
3. What adaptations do gymnosperms share that allowed them to compete with SVPs?
4. Describe the advantages provided by the adaptations you listed in the previous question and relate them to changes in global climate.
5. How would you differentiate a fern and a cycad?
6. What features do gnetophytes have that classify them as gymnosperms?
7. Which features do gnetophytes have that lead people to believe they might be more closely related to angiosperms?
8. What other explanation could there be for gnetophytes and angiosperms sharing these traits?
9. In what climate(s) would you be most likely to find gymnosperms? Explain your answer. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/22%3A_Gymnosperms/22.5%3A_Gnetophytes_and_Conifers.txt |
Learning Objectives
Content Objectives
• Understand the changing conditions on Earth during the period in which angiosperms evolved and how those conditions selected for features found in this group.
• Learn the characteristics that distinguish angiosperms from other plant groups
• Learn the gross anatomy and function of flowers
• Discover different pollination syndromes
Skill Objectives
• Correlate features of angiosperm morphology and life history traits with adaptation to the changing environment
• Identify components of a flower based on location and describe their functions
• Identify the most likely pollinator of a flower based on morphology, color, and smell
23: Angiosperms I - Flowers
1. When you think of a flower, what characteristics come to mind?
2. How might those characteristics relate to the function of a flower?
3. Flower petals are not involved in photosynthesis, yet they have stomata. Provide a possible explanation for this.
23.02: Introduction
The exact timing of the emergence of angiosperms is unknown, so it is difficult to relate their evolution to specific climatic conditions or other circumstances. However, there is relatively new fossil evidence of flowering plants as early as the Jurassic period, 174 mya. This was the age of the dinosaurs and coincides with the emergence of the first feathered dinosaurs -- birds! Angiosperms represent a single origin of related organisms, the phylum Anthophyta, that experienced an exceptional radiation in species. As of 2019, there are approximately 370,000 known extant species. Most of the plants that you see, eat, and otherwise interact with in your daily life are likely to be in this group.
Angiosperms can be distinguished from other plant groups by the production of flowers. These collections of modified leaves allowed angiosperms to attract pollinators and increase the chances of successful fertilization. Over time, angiosperms evolved different flower morphologies, smells, and colors that corresponded to their particular pollinators. These sets of characteristics, called pollination syndromes, allow scientists to predict the pollinators for different plants.
In the xylem, this group of plants evolved large diameter conducting cells for rapid water uptake called vessel elements, though this made them vulnerable to freezing conditions. In the phloem, sieve cells evolved into sieve tube elements with their associated companion cells, increasingly specialized for transportation of photosynthates.
23.03: Selection Pressures and Drivers
1. Competition for space. Present day gymnosperms include the tallest, most massive, and some of the oldest organisms on the planet. With this in mind, you can imagine that they would be difficult to compete with. Angiosperms needed to evolve more efficient methods of transporting water and photosynthates, fertilization, and survival of offspring.
2. Insects and birds. The primary response to insects that we see in gymnosperms is prevention of herbivory. While herbivory is still a driver of selection for angiosperms, insects also served as a more efficient method of pollen delivery. Insects and birds could be lured in with sugary nectar or scents and colors that mimicked other resources, then dusted with pollen as they investigated. If the lures were specialized enough, they would continue seeking the same resource, leading them to another plant of the same species. These scents, colors, and nectar resources were produced by structures that also produced pollen and ovules -- the flower.
23.04: Specialized Vascular Tissue
When you looked at vascular tissue during the anatomy portion of this class, you learned that the conducting cells in xylem are tracheids and vessel elements, while in phloem they are sieve cells, sieve tube elements, and companion cells. However, not all plants have the same vascular composition. In SVPs and gymnosperms (with the exception of gnetophytes), the xylem contains only tracheids, while the phloem contains only sieve cells. These were the earliest versions of conducting cells. It was not until the angiosperms that vessel elements, sieve tube elements, and companion cells evolved.
Recall the experiment you did in Lab Roots and the Movement of Water. How did the diameter of the straw influence the height the water moved up the straw?
How does this relate to the ability of angiosperms to compete with gymnosperms? | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/23%3A_Angiosperms_I_-_Flowers/23.01%3A_Formative_Questions.txt |
Flowers are composed of sets of highly modified leaves arranged in whorls. The outermost whorl of a flower is called the calyx and is composed of sepals. Inside the calyx is the corolla, which is composed of petals. The sepals are often smaller and less colorful than the petals, but this general rule can be misleading. For example, lilies often have identical sepals and petals. The only way you can distinguish between them is by location: Which whorl is on the outside?
Together, the calyx and corolla are called the perianth (peri- meaning around, anth- meaning flower). Inside the perianth is the androecium (house of man), a whorl composed of stamens. Each stamen has a long filament holding up pollen sacs called anthers. Inside the androecium is the gynoecium (house of woman), which is composed of carpels. Each carpel has an ovary at the base where ovules are housed. The style emerges from the ovary and is topped by the stigma. Pollen grains land on the stigma and must grow a tube down the style to reach the ovule and complete fertilization.
All of these whorls attach to an area called the receptacle, which is at the end of the stem that leads to the flower. This stem is called the peduncle. In the case of an inflorescence, where multiple florets are produced in place of a single flower, the stems leading to the florets are called pedicels.
In the diagram of the flower below, add labels for all of the bolded terms above and assign each whorl a different color. Make a key for the colors and whorls.
Symmetry and Quantity
Two other features used to identify flowers are symmetry and the number of parts in each whorl.
Flowers that have multiple lines of symmetry (like a starfish) are radially symmetrical, also called actinomorphic. Flowers with only a single line of symmetry (like you) are bilaterally symmetrical, also called zygomorphic.
Flowers with parts in sets of 3 are generally monocots. Flowers with parts in sets of 4 or 5 are generally eudicots. Sometimes these parts are fused together and can be difficult to count. For example, in the diagram of the lily above, there are three fused carpels. You would only be able to determine this by counting the lobes on the stigma or by looking at a cross section of the ovary to count the different compartments (called locules).
Notes on other terms
Many wind pollinated flowers have evolved to be either male or female, containing either an androecium or gynoecium, but not both. These flowers are called imperfect, while flowers containing both internal whorls are called perfect. Flowers that contain all whorls are called complete. However, there are also incomplete flowers that have lost other whorls during the course of their evolution. Wild ginger, Asarum caudatum (shown below), has lost its corolla and has a large showy calyx in its place. This strange plant has fly pollinated flowers and ant dispersed seeds.
Dissecting a Flower
Obtain a fresh flower specimen. Dissect a flower and carefully remove the individual pieces. Draw (or assemble and press) the individual components onto the following table. Label the parts of the stamen and carpel.
Table \(1\): Dissected Floral Whorls
Drawing with labels:
Sepals
Petals
Stamen
Carpels
Whorl:
Quantity:
Fusion? (yes/no)
Is this flower a monocot or a eudicot? How can you tell?
How would you describe the symmetry of this flower?
What other terms could you use to describe it? | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/23%3A_Angiosperms_I_-_Flowers/23.05%3A_Anatomy_of_a_Flower.txt |
Throughout the evolution of plants on land, there is a trend in reduction of the gametophyte. Pollen is the microgametophyte in both gymnosperms and angiosperms. Recall that conifer pollen has four cells and inflated air sacs that serve as wings. In angiosperms, the pollen lacks wings and is reduced to only two cells: the generative cell and the tube cell.
Below are two microgametophytes. On the left is pine pollen, a gymnosperm microgametophyte. On the right is lily pollen, an angiosperm microgametophyte.
Which cells were lost in angiosperm pollen (present in gymnosperms)?
What are the functions of the tube cell and the generative cell?
The stigma contains compounds that trigger the production of the pollen tube. This trigger can be as simple as a high concentration of carbohydrates, which we can mimic by using corn syrup. However, sometimes there are extra triggers present on a stigma, such as lipids or calcium. For the next part of the lab, you are going to develop and conduct an experiment to test the best method for stimulating production of pollen tubes.
Questions to consider:
1. How will you determine what “best” means? Is it number of pollen grains to produce a pollen tube? Average length of pollen tube?
2. How will you standardize other variables? Will you have controls?
3. Will you have replicates within your experiment? Will other scientists be able to replicate your work in future experiments?
Record your experimental design and steps of the process of science on "Pollen Tube Germination Experimental Design" below.
23.07: Pollination Syndromes
Pollination is essential for producing offspring. Because of this, it serves as a strong driver of selection in angiosperms. Flowering plants have evolved to utilize different pollinators, such as wind or birds, to transport their pollen to other flowers of the same species. Some of these pollinators, such as wind, are not selective and rely on producing large quantities of pollen. Others, like hummingbirds, require large quantities of nectar and are attracted by particular colors. Use the table of pollination syndromes below to determine the likely pollinator for the flowers available in your lab.
Table \(1\): Pollination Syndromes (adapted from US Forest Service)
Color
Structure
Scent
Nectar or Pollen
Wind
Dull, perianth often absent or reduced
Large feathery stigmas, large anthers
None
No nectar, large amounts of pollen
Birds
Reds and pinks
Often tubular or cupped
None
Lots of hidden nectar, moderate pollen
Bees
Purples, blues, yellows, white, UV
Flat and shallow or tubular, with landing area
Sweet, fresh, mild
Pollen often sticky and scented, nectar usually present
Bats
White, dull green, or purple
Often bowl-shaped or pendant, anthers protruding
Musty or fruity, strong, emitted at night
Lots of hidden nectar
Moths
White, pale pink or purple
Often tubular or cupped, no landing pad
Strong and sweet, emitted at night
Lots of hidden nectar, limited pollen
Butterflies
Bright colors
Tubular, with wide landing pad
Faint, fresh
Lots of hidden nectar, limited pollen
Flies
Dark red, purple, brown
Shallow, funnel, or trap-like
Putrid, rotting
No nectar, moderate pollen
Who is the pollinator?
Record the most likely pollinator and the trait(s) in the flower that lead you to this conclusion:
1.
2.
3.
4.
5.
23.08: Inflorescence Types
Much like leaves, flowers can be compound, becoming an inflorescence made up of many florets. And, just like leaves, inflorescences can be distinguished from individual flowers by the nodes they emerge from. A peduncle will emerge from a node in place of a branch, subtended by a leaf. A pedicel, the stem that leads to a floret, will not emerge from a node.
An important feature used in identifying angiosperms is the type of inflorescence. There are many inflorescence types, but the following are some of the most commonly encountered.
Head (also called a capitulum). Florets are clustered on an enlarged receptacle. Heads can contain one or more of the following types of florets
• Disc florets - petals are separate and all the same length; radially symmetrical
• Ray florets - petals are fused together; bilaterally symmetrical
Umbel. Pedicels converge on a single point, where they attach to the peduncle.
Spike. Florets are sessile, attached directly to a central axis
Raceme. Similar in structure to a spike, but the florets have pedicels
Panicle. A branched raceme
Use the information above to identify the inflorescence types present in your lab. In the space below, make a sketch of each inflorescence type that will help you make these determinations in the future.
23.09: Summative Questions
1. How could the coevolution of angiosperms and pollinators explain the vast radiation in species that angiosperms experienced?
2. What would a bird pollinated raceme of bilaterally symmetrical eudicot flowers look like? What would it smell like? Draw one below and label any important features.
3. How does the structure of a vessel element relate to the ability of angiosperms to compete with gymnosperms?
4. Where does meiosis occur in the flower?
5. Where does the name “Anthophyta” derive from? Consider the root words. | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/23%3A_Angiosperms_I_-_Flowers/23.06%3A_Stimulating_Pollen_Tube_Growth.txt |
Learning Objectives
Content Objectives
• Learn the angiosperm life cycle
• Learn the gross anatomy and function of fruits
• Learn the different types of fruits and associated dispersal agents
Skill Objectives
• Identify components of the angiosperm life cycle and describe their functions
• Distinguish between the different layers of the pericarp
• Use a dichotomous key to identify fruit types
• Determine the most likely dispersal agent based on fruit morphology
24: Angiosperms II - Fruits
1. When you think of a fruit, what characteristics come to mind?
2. How might those characteristics relate to the function of a fruit?
24.2: Introduction
The term angiosperm means “seed vessel” and refers to the production of fruits. Every flower becomes a fruit, though these fruits might not always fit with our cultural perception of what it means to be a fruit. Once pollinated, the fertilized seeds are encased in a protective ovary whose structure can be specialized for different methods of dispersal, such as animal ingestion, animal attachment, flotation, or wind dispersal. This protective ovary and the encased seed or seeds are more commonly called a fruit. Inside the developing seeds, angiosperms provide an additional food source to the developing zygote, the endosperm. The endosperm is produced by a process called double fertilization where one sperm fertilizes the egg and another fertilizes a pair of haploid nuclei, which makes the endosperm triploid (3n).
24.3: Selection Pressures and Drivers
1. Competition with gymnosperms. Gymnosperms were large, successful organisms adapted to a variety of severe conditions. To compete with this group, angiosperms would need to work smarter, not harder.
2. Birds and mammals. The diversity of animals present on Earth when angiosperms evolved would have resulted in increased seed predation. Fruits allowed for the dual purpose of protecting seeds and co-opting animals as dispersal agents, whether by ingestion or attachment. Some fruits evolved production of sugary tissues and bright colors to attract animals, while others evolved hairs or spines to latch onto their bodies.
24.4: Angiosperm Life Cycle
Observe a prepared slide of mature pollen in a Lilium anther cross section. Within the anthers, there are pollen grains. Each pollen grain contains two cells. The tube cell takes up the majority of the pollen grain, engulfing the smaller generative cell. The tube cell will grow a pollen tube down the length of the style and into the ovary. The generative cell divides to produce two sperm by mitosis.
In the image above, you can see three pollen grains. In two of them, you can see the generative cell and the tube cell nucleus.
Observe a prepared slide of a Lilium ovary cross section (pre-fertilization). A Lilium pistil consists of three fused carpels. This is why there are three compartments, called locules, within the ovary.
Inside each locule, there are two ovules, each connected to the ovary wall by a funiculus. The funiculus is much like an umbilical cord, providing nutrition to the developing ovule from the sporophyte through the placenta. The tissue surrounding the ovule is called the integument. The small opening in the integument is the micropyle.
Depending on the pre-fertilization phase you are looking at, the ovule can appear quite differently. In the image above, the ovule is in the earliest stage of development. Currently, all of this tissue is diploid. The large cell in the center, the megasporocyte (2n), will undergo meiosis to produce four haploid cells: the egg and three other cells that will die. This egg will undergo three rounds of mitosis to produce a structure called the embryo sac, which is essentially the megagametophyte (n).
Observe a prepared slide of an embryo sac (this should also be a cross section of a Lilium ovary). The megagametophyte is housed at the center of the ovule and, when it is fully developed, is composed of 7 cells and 8 nuclei.
• The egg sits at the end closest to the micropyle, flanked by two synergids.
• On the other side of the embryo sac, there are three antipodals.
• In the center, there is a large cell with two nuclei. This is alternately called the central cell or the polar nuclei.
Note
Due to the way slides are made (a thin slice of a three dimensional structure), it is difficult to catch all 7 cells in one section unless they happen to be on the same plane.
In the image above, you can see what is probably the egg and one synergid on the micropylar (left) side of the embryo sac. The large central cell is just to the right of the blank white area, and two of the three antipodals are visible on the far right.
The egg and the central cell will each be fertilized by one of the two sperm produced by the generative cell. The fertilized egg becomes a diploid zygote. The fertilized central cell becomes the triploid endosperm.
Observe a prepared slide of a Lilium ovary cross section (post-fertilization). Once fertilized by a pollen grain, the ovules become seeds, the megasporangium becomes the nucellus, the micropyle is closed, the integument becomes the seed coat, and the ovary wall begins to develop into the pericarp.
Within the seed, the embryo grows by mitosis and is nourished by the nucellus and endosperm. The seeds develop within the fruit until they are ready for dispersal.
Label all of the bolded components of the angiosperm life cycle in the diagram below. Indicate where meiosis and fertilization occur. Choose a different color to represent haploid, diploid, and triploid tissues and color the tissues accordingly.
Seed Germination
In the diagram below, watch a bean seed germinate and develop into a bean plant. Label the seed coat, radical, cotyledons, roop apical meristem, and shoot apical meristem.
Why would a plant develop a root system before the shoot? | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/24%3A_Angiosperms_II_-_Fruits/24.1%3A_Formative_Questions.txt |
All flowers become fruits. If you are unsure whether something is a fruit or not, check for seeds. Vegetables originally got their name for being derived from the vegetative (non-reproductive) part of the plant, such as a stem, root, or leaf.
The Pericarp
The ovary wall surrounds the developing seeds and becomes the pericarp (peri- meaning around, carp- meaning fruit or body). The pericarp is composed of three layers:
• Exocarp - the outermost layer, making up the exterior surface of the fruit
• Mesocarp - the tissue located between the exocarp and endocarp
• Endocarp - the innermost layer, located just outside the seed coat
In some fruits these layers are distinctly different and easy to distinguish. In other fruits, the layers are fused together and nearly impossible to differentiate. Fruit identification is primarily based on the structure of these three layers.
24.6: Using a Dichotomous Key to Identify Fruit Type
Some fruits are fleshy at maturity, like the peach pictured above, while others are dry. This is not always obvious, due to our cultural uses of fruits. For example, a green bean is a legume, which is a dry fruit at maturity. However, we harvest green beans while they are still immature and fleshy because that makes for a more enjoyable dining experience.
Some dry fruits break apart at maturity into separately packaged seeds (schizocarps), while others split open to release their seeds (dehiscent fruits). Other dry fruits don’t break open at all (indehiscent fruits). All of this terminology, in addition to identifying layers of the pericarp, is essential when you are identifying fruit types.
Use the dichotomous key at the end of this lab to identify the fruits available in your lab today. Record your findings in the space below, making drawings where helpful to do so.
24.7: Dispersal Syndromes
Different fruits have evolved in response to different dispersal agents. The fruits that we buy at the grocery store are tasty because they have evolved for animal ingestion. The burrs that get stuck to your socks when you walk through a field have evolved for animal attachment. How can we use characteristics of fruits to predict the dispersal agent?
Observe the fruits available and use the table below to predict which characteristics are related to each of the following dispersal agents.
Table \(1\): Predictive Characteristics for Fruit Dispersal Agents
Dispersal Agent
Expected Characteristics
Animal ingestion
Animal attachment
Wind
Water
Ballistic (explosive, projectile)
24.8: Summative Questions
1. What is the function of a fruit?
2. How could you distinguish between a fruit and a vegetable at the grocery store?
3. In the diagram below, an apple blossom is developing into a fruit. Label all of the parts of fruit and flower anatomy that you can see. What type of fruit is this?
1. What food sources does the embryo have before it opens its first leaves in the sunlight?
2. Oaks and alders are both angiosperms that grow into trees. Oaks produce nuts with large seeds, while alders produce tiny samaras. In a year in the life of a black oak, it might grow 50 cm and produce 6500 nuts. In that same year, a red alder might grow 1-2 meters and produce around 5 million samaras. What are the tradeoffs between these two very different strategies? How might they relate to the different life histories of oaks and alders? | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/24%3A_Angiosperms_II_-_Fruits/24.5%3A_Fruits.txt |
1a. Foliage is broadleaved--composed of a flat blade, often with a distinct central vein....2 (Hardwoods: Angiosperms)
1b. Foliage is not leaf-like--composed needles or small, thick, scale-like leaves... 9 (Softwoods: Conifers)
2a. Edges of the leaves are smooth (the leaf margin is entire) ...... 3
2b. Edges of the leaves are toothed or lobed …………………….... 6
3a. Younger bark is reddish and peeling, leaves are relatively tough and approximate a rounded oval. Produces clusters of bell-shaped flowers and red berries …………………………...... 4
3b. Bark is not as above; leaves are long and pointed (lanceolate) ................. 5
4a. Leaves are large and shiny, upper and lower surfaces of leaves are different in appearance. Older portions of the trunk are covered in grey, finely scaled bark. Exposed inner bark is often a pale yellow to orange. Berries are warty. Can grow to be large trees.... Pacific madrone (Arbutus menziesii)
4b. Leaves are small and often upper and lower surfaces look the same (isofacial). Most exposed bark is dark red and smooth, often interspersed with blackened-to-grey portions lacking bark. Berries are smooth. Growth form can be creeping ground cover to large shrub/small tree... Manzanita (Arctostaphylos spp.)
5a. Leaves have a golden underside. Fruits are nuts surrounded by spiny case...Chinquapin (Chrysolepis chrysophylla)
5b. Leaves are green on both sides. When crushed or cut, leaves have a strong, spicy smell. Not mycorrhizal. Often host wood-decay species, such as Ganoderma brownii...California bay laurel (Umbellularia californica)
6a. Leaves palm-shaped, deeply dissected, and emerging opposite from each other on the branch.... Maple (Acer spp. In our area, most likely big leaf maple, Acer macrophyllum)
6b. Leaves are not as above ... 7
7a. Leaf edge has larger teeth, the edges of which are lined with smaller teeth (doubly serrate). Bark is smooth, often covered with lichens. Produces clusters of small, woody "cones" (each about the size of an olive). Tree is growing near water or in a low area where water might collect seasonally...Red alder (Alnus rubra)* Note: If leaf edges are smooth and the tree produces berries, it is Cascara!
7b. Leaf edge has a single row of teeth or lobes. Bark is usually grey to brown and furrowed (though can be smooth when young). Fruit is an acorn...8
8a. Leaf edge has a single row of teeth. Entire underside of leaf is covered in tan fuzz. Acorn cups have scales that scoop outward, giving them a fuzzy or hairy appearance... Tanoak (Notholithocarpus densiflorus)
8b. Leaf edges are lobed (sometimes lobes end in sharp points). Acorn cups are formed of tightly overlapping scales that do not scoop outward... True oaks (Quercus spp.)
9a. Leaves needle-like: rounded, long, and emerging in bundles (fascicles). Cones are woody and tough, not easily broken apart... Pines (Pinus spp.)
9b. Leaves in flattened sprays. scale-like, or emerging around the branch, like a bottlebrush... 10
10a. Leaves flattened with a distinct upper and lower surface, the lower surface often silvery. Emerging from either side of the branch in a single plane (in some species, may be pointing upward, but not oriented all the way around the branch like a bottlebrush). Foliage not sharp.....11
10b. Leaves scale-like or leaves might be as above, but emerging around the branch from all sides, like a bottlebrush...14
11a. Leaves fall to the ground in feather-like branchlets. Cones are small (smaller than a grape) and rounded, forming distinct segments. Bark is reddish and fibrous, can form incredibly tall trees... Coast redwood (Sequoia sempervirens)
11b. Leaves fall to the ground as individual needles. Cones and bark not as above...12
12a. Leaves are all similar in length, usually around 2-3 cm. Cones are not often found because they deteriorate quickly. When present, cones sit upright on top of branches....True firs (Abies spp.)
12b. Leaves with two distinctly different lengths (shorter and longer).................13
13a. Leaves are short, less than 2 cm. The leader (top) of the tree is droopy and the cones are small (about the size of a dime) with relatively large scales. Bark is greyish with shallow furrows..Hemlock (Tsuga
13b. Leaves are all over 2 cm long. Smooth bark in younger areas of the tree, often with a pinkish cast due to organisms growing on the bark. On the coast at low elevation……...Grand Fir (Abies grandis)
14a. Leaves as needles, emerging around the branch like a bottlebrush. Cones oval and papery (not woody)...17
14b. Leaves scale-like. Other features not as above....15
15a. Leaves as tiny, overlapping scales, forming dense clusters of dark green sprays. Cones woody, globose, and composed of distinct plates (like a soccer ball). Found in coastal areas....Monterey cypress (Hesperocyparis macrocarpa)
15b. Leaves as overlapping, awl-shaped scales that occur in flattened sprays. Other features not as above..16
16a. Scales are elongated, forming a champagne glass shape. Cones are small and dangle from the branch. They resemble duckbills, pistachio shells, or upside down tulips. Bark is deeply furrowed and often has an orangey cast. Found more commonly in inland areas or at higher elevations in our area...Incense cedar (Calocedrus decurrens
16b. Scales are short with a bloom of stomata on the underside (appearing as a lighter green) in the shape of a bow or set of wings. Cones are small and sit erect on the branch. They resemble a blooming rose. Bark is fibrous. Found more commonly in wet, lowland areas...Western red cedar (Thuja plicata)
17a. Needles are sharp, cones composed of uniform overlapping scales (like a fish). Bark looks chippy, like peeling paint. On the horizon, branches angle upward. Found in coastal forests....Sitka spruce (Picea sitchensis)
17b. Needles are soft, not sharp. Cones have bracts that emerge between the scales, often referred to as a "mouse butt" because it looks like two feet and a tail hanging out. Bark is deeply furrowed and plated....Douglas-fir (Pseudotsuga menziesii)
25.02: Dichotomous Key to Common Fruits
Note
If the fleshy part of the fruit is composed of something other than the ovary, it is an accessory fruit
1. Fruit from one carpel (can be several fused together) of one flower ........[Simple Fruit].... 2
1. Fruit from more than one free carpel in a single flower or from an inflorescence .... 16
2. Fleshy at maturity ....................................... 3
2. Dry at maturity ............................................ 8
Fleshy Fruits
3. Thin exocarp, fleshy mesocarp, stony endocarp surrounding single, large seed ............... Drupe
3. Fruit not as described above............................................................................................... 4
4. Seeds in a linear order, separate from ovary wall, pericarp splits on two seams ...Legume (immature)
4. Fruit not as described above.................................................................................. 5
5. Papery endocarp forms a core. Derived from a perigynous flower..........Pome
5. Endocarp fleshy (not a papery core) ...................................................... 6
6. Thin exocarp, fleshy mesocarp, one to many seeds ........................... Berry
6. Exocarp thickened and leathery (modified berries) ………......... 7
7. Exocarp and mesocarp form leathery rind, locules filled with juice-filled trichomes ……...Hesperidium
7. Exocarp forms tough skin/rind, thick mesocarp, not divided into separate locules ........... Pepo
Dry Fruits
8. Dehiscent (splits open at maturity), usually many seeds ....... 9
8. Indehiscent (does not split open), usually one seeded .......... 12
9. Derived from a carpel with one locule ............................. 10
9. Derived from a carpel with more than one locule ............ 11
10. Dehiscent along one seam .......................... Follicle
10. Dehiscent along two seams ........................ Legume
11. From two locules with a central partition .............. Silique (elongate) or silicle (round)
11. From more than two locules.................................. Capsule
12. Ovary wall extends to form a wing .................. Samara
12. Fruit not winged ............................................... 13
13. Outer wall not especially thick or hard, seed small ... 14
13. Outer wall hardened, seed relatively large ................ 15
14. Seed not tightly attached to pericarp ............... Achene
14. Seed fused to pericarp, grains ........................ Caryopsis
15. Stony pericarp surrounds one large seed .............................................. Nut
15. Relatively thin exocarp, fibrous mesocarp, single large seed .............. (dry) Drupe
Aggregates & Multiple Fruits
16. Derived from one flower with many free carpels ...... Aggregate Fruit
16. Derived from an inflorescence (many florets) ........... Multiple Fruit
25.03: Evolutionary Profile - Photosynthetic Groups
Group:
Model organism(s):
Characteristics:
• Morphology:
• Habitat:
• Photosynthetic pigments:
• Storage carb:
• Cell walls:
• Life cycle: | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/25%3A_Glossary/25.01%3A_Key_to_Common_Local_Tree_Genera_%28Written_for_Humboldt_County_California%29.txt |
Glossary of Terms and Root Words
Use this document to look up the definitions for bolded terms in the lab manual, as well as the meanings of root words. See the image of the compound microscope for the bolded terms relating to this device.
A
Abscission - the natural detachment of plant parts, such as with leaves in the winter
Abiotic - nonliving
Accessory fruit - a fruit where the fleshy material is not derived from the ovary. Ex. A strawberry or rose hip
Achene - a dry fruit with a single seed surrounded by a pericarp that can be peeled away. Layers of the pericarp are not distinguishable with the naked eye. Ex. A sunflower seed.
Actinomorphic - See “Radial symmetry”
Adhesion - attraction between dissimilar molecules or substances (such as tape to paper)
Adventitious root - a root that emerges from stem tissue
Aerenchyma - parenchyma tissue with large air pockets for flotation
Aggregate fruit - a fruit derived from a flower with multiple free carpels. Ex. A blackberry.
Akinete - a form some cyanobacteria can take. Larger, thick-walled cells that tend to be more granular in appearance. Akinetes store large amounts of lipids and carbohydrates so that they have enough energy to begin a new colony if conditions become too cold or too dry for survival. Formation of akinetes can be triggered by dry or cold conditions.
Alcohol fermentation - a form of fermentation performed by yeasts that converts pyruvate into ethanol and CO2 to replenish NAD+
Alternate (leaf arrangement) - one leaf emerges at a node
Alternation of generations - See “Haplodiplontic”
amylo- starch (amylose)
Amyloplast - a type of plastid that produces and stores starch. An amyloplast is a type of leucoplast, which are plastids that do not contain pigment.
Anaphase - the stage of mitosis where spindle fibers pull sister chromatids to either side of the cell, splitting them into two separate chromosomes
Anaphase I - the stage of meiosis I where spindle fibers pull homologous chromosomes to either side of the cell
Anaphase II - the stage of meiosis II where spindle fibers pull sister chromatids to either side of the cell, splitting them into two separate chromosomes
andro- male, man
Androecium - the floral whorl composed of stamens
Annual growth ring - a light and dark region of secondary xylem that forms in response to environmental conditions. The light portion of the ring is composed of larger cells, formed when water is abundant (also called early wood). The dark portion of the ring is composed of smaller diameter, densely packed cells, formed when water becomes more scarce (also called late wood).
Annulus - a ring-like structure. In fungi, this refers to the partial veil remnants left around the stipe. In ferns, it refers to the ring of inflated cells around the sporangium that dries and pulls the sporangium open, releasing the spores.
anth- flower
Anther - pollen sacs in flowering plants
Antheridiophore - a structure that bears antheridia
Antheridium - a multicellular structure on the gametophyte that produce sperm by mitosis
Antipodals - three haploid cells in the embryo sac located on the opposite end of the micropyle
angio/angium- vessel
Apical growth - growth extending from the tips (such as the tips of roots or shoots in plants)
Apothecium - a (usually) cup-shaped fruiting body produced by some ascomycetes
Arbuscule - a highly branched structure formed by endomycorrhizal fungi on the plasma membrane of plant root cells. The increased surface area increases the ability to exchange materials.
Archegoniophore - a structure that bears archegonia
Archegonium - a multicellular structure on the gametophyte that produce an egg by mitosis
Ascocarp - a fruiting body (sexually reproducing structure) produced in the fungal phylum Ascomycota
Ascospore - a haploid spore produced by meiosis, then mitosis, within the ascus of an ascomycete
Ascus - a sac-like structure where meiosis occurs to produce spores in ascomycetes. Asci, plural.
Asexual reproduction - a form of reproduction that produces identical copies of the parent cell (e.g. mitosis, binary fission, and budding)
Astrosclereid - a sclereid cell that transverses the leaves of Nymphaea and forms star-like shapes
ATP - (adenosine triphosphate) a high energy molecule with three negatively charged phosphate groups. ATP can “donate” a phosphate group to a reaction, breaking a high energy bond and releasing energy. This process is called phosphorylation. Most work done in cells is powered by ATP.
ATP Synthase - an enzyme that uses the flow of H+ ions back down their concentration gradient (secondary active transport) to phosphorylate ADP into ATP.
auto- self
Autotroph - an organism that forms organic molecules from inorganic molecules. This can also be thought of as an organism that makes its own food (self-feeding).
Auxospore - an enlarged spore produced by diatoms during sexual reproduction (and apparently sometimes through asexual reproduction)
Axillary bud - a structure located in the axil of the leaf that produces a branch, flower, or inflorescence
B
Bark - layers of periderm (phelloderm, cork cambium, and cork) form the outer bark, while the secondary phloem forms the inner bark.
Basal meristem - meristematic tissue located at the base of a structure. Most plants grow from apical meristems, producing new tissue at and elongating from the tips. Basal meristems produce tissue where the tips are the oldest parts.
Basidiocarp - a fruiting body (sexually reproducing structure) produced by some basidiomycetes
Basidiospore - a haploid spore produced by meiosis within the basidium
Basidium - a (usually) club-shaped structure that produces spores in the basidiomycetes
Benthic - at the bottom of a body of water
Berry - a fleshy fruit with a thin exocarp, fleshy mesocarp, and one to many seeds not enclosed within an endocarp derived from the ovary wall
bi- two
Bilateral symmetry - having one line of symmetry
Binary fission - a form of asexual reproduction used by prokaryotic cells where the original cell expands as internal contents are replicated, then a new cell wall forms in the middle, dividing into two new, identical cells
bio- life
Bioavailable - a form that can be absorbed by and assimilated into an organism.
Biotic - living
Blade - the (usually) flat, photosynthetic portion of the leaf located at the end of the petiole. In brown algae, this is the leaf-like structure (or structures) on the thallus.
Bud primordium - the early developmental stage of the axillary bud
Bud scar - the scar left behind on a stem where the axillary bud has fallen off via abscission
Budding - a form of asexual reproduction used by yeast where a smaller cell is made from the original cell, leaving a bud scar. The smaller cell grows to normal size after separation from the parent cell.
Bulb - a rosette of fleshy leaves modified for storage, surrounding a short stem
Bulliform cells - enlarged cells in the epidermis that respond to water loss by collapsing, causing the leaf to roll or fold and reducing sun exposure
Bundle scars - scars left behind inside a leaf scar where the vascular bundles of the leaf travelled into the stem
Bundle sheath - the ring of cells that surround a vascular bundle. This could be in the stems of monocots or in the leaves of either group.
C
C3 - the “standard” and most energy efficient form of photosynthesis
C4 - a form of photosynthesis performed by plants adapted to hot weather. RuBisCO is located in the bundle sheath cells, where CO2 is actively transported in the form of an acid to keep concentrations high. This occurs when stomata close to prevent water loss, limiting CO2 intake.
Calvin cycle - the light independent phase of photosynthesis where RuBisCO fixes CO2 and the end product is glucose. This process occurs in the stroma of the chloroplast.
Calyptra - tissue from the female gametophyte that is carried on top of the developing sporophyte in the bryophytes
Calyx - the floral whorl composed of sepals
CAM - Crassulacean acid metabolism. This is a form of photosynthesis performed by plants adapted to dry weather. Stomata close during the day to prevent transpiration and open at night to let in CO2. The CO2 is actively transported into the central vacuole and converted into an acid. During the day, it is converted back into CO2 so photosynthesis can occur.
Cap (pileus) - the top of a mushroom. The spore producing surface is on the underside of the cap (usually).
Capillary action - the tendency of a liquid to rise or fall as a result of surface tension
Capsule - a dry fruit with multiple fused carpels that open at maturity. Ex. Cotton. Note: this term is also often used to describe the structure that encases the sporangia in mosses.
Carotenoids - red, orange, and yellow pigments found in plants, green algae, and photosynthetic heterokonts. These are often used to protect from sun damage or to signal ripening in fruits, as well as absorb some of the green parts of the spectrum of light.
carp- of the fruit or fruiting body
Carpogonium - a structure produced on the red algae female gametophyte. It consists of the egg and the trichogyne and may also be called the carpogonial branch.
Carposporangium - a structure that produces carpospores
Carpospore - a diploid spore that is identical to the zygote in the red algae life cycle. Carpospores grow by mitosis into a tetrasporophyte.
Carposporophyte - a multicellular diploid phase in the red algal life cycle that produces clones of the zygote called carpospores
Caryopsis - a dry fruit similar to an achene, but the pericarp is fused to the seed. This fruit type is produced by members of the Poaceae (grass family). Ex. A corn kernel.
Casparian strip - a layer of suberin that coats the endodermis in roots. This layer forms in patches, leaving passage cells at the xylem poles.
Cell wall - a rigid structure that encloses the plasma membrane. The presence and composition of the cell wall are useful features in classifying and identifying organisms.
Cellulose - a polysaccharide found in plant cell walls. Cellulose is also found in the cell walls of oomycetes and red, green, and brown algae.
Central cell - a cell in the embryo sac containing two polar nuclei
Central vacuole - a (generally) large, fluid-filled organelle enclosed by a membrane (the tonoplast). The central vacuole often takes up the majority of a plant cell and is responsible for storage of compounds (ex. toxins, anthocyanins, and salts) and turgidity of the plant cell, which contributes to rigidity of the entire plant.
Centric - (a diatom) with radial symmetry
Centromere - the central region of a chromosome where sister chromatids are attached (in replicated chromosomes)
Chemical energy - energy stored in the bonds between atoms within molecules
Chemiosmosis - the movement of ions from areas of high concentration to low concentration across a semipermeable membrane. This happens with H+ flow through ATP synthase in both cellular respiration and photosynthesis.
Chemosynthesis - a cellular process that uses chemical energy derived from oxidizing (stealing electrons from) inorganic compounds to build organic molecules. In this process, the inorganic compound is the electron donor. The waste product will depend on what the initial inorganic compound is (for example, if hydrogen sulfide is used, sulfur and water could be produced as waste products).
Chitin - polysaccharide (carbohydrate) found in fungal cell walls and arthropod exoskeletons
Chlorenchyma - parenchyma cells with chloroplasts for photosynthesis
Chlorophyll - a photosynthetic pigment that reflects light in the green portion of the spectrum. These molecules can harvest energy from the blue and, to a lesser extent, red part of the spectrum of light by emitting an electron after absorbing a photon. All photosynthetic organisms covered in this class contain chlorophyll a, which is the primary molecule involved in the light-harvesting portion of photosynthesis.
Chloroplast - a double-membrane bound organelle derived from bacterial endosymbiosis that contains chlorophyll and is responsible for photosynthesis. Chloroplasts can be converted into chromoplasts or leucoplasts, when needed.
Chromatin - DNA wound around histone proteins
chromo/chroma- color
Chromoplast - a double-membrane bound organelle derived from bacterial endosymbiosis that contains pigments other than chlorophyll, such as carotenoids. Often found in ripening fruits to attract animals with the red hues.
Chromosome - chromatin that has been tightly packaged in preparation for cell division
Chytrid - an aquatic fungus with swimming spores (zoospores)
Circinate vernation - the process by which fern fiddleheads unfurl in the spring
Citric acid cycle - a process that takes place in the matrix of a mitochondrion during cellular respiration. ATP, NADH, and FADH2 (as well as some CO2) are produced.
Cladode - a stem that has been modified for photosynthesis, often green with increased surface area
Clamp connection - a rounded structure that forms on the side of a basidiomycete hypha as nuclei divide
Classification - a system of categorizing organisms into like groups
Cleistothecium - an enclosed, microscopic fruiting body produced by some ascomycetes
Coenocytic - this can refer to fungal hyphae with no septations (cross walls) or, more generally, to any cell with many nuclei
Coevolution - the linked evolution of two interacting species
Cohesion - attraction between like molecules (for example the attraction of a partial positive charge on the hydrogen atom of one water molecule to the partial negative charge on the oxygen atom of another water molecule)
Collenchyma - cells with unevenly thickened primary walls, often involved in flexible support in young growing tissues
Community - multiple interacting populations living in the same area at the same time
Companion cell - a cell with a nucleus paired with a sieve tube element. The companion cell acts as the command center for that sieve tube element.
Complete (flower) - a flower containing all four floral whorls
Complex septation - also called a dolipore septum. The walls around the pore in the septum are swollen and two structures that look like parentheses (aptly called parenthesomes) flank the pore.
Conceptacle - a chamber inside the Fucus receptacle that houses the gametangia
Cone - see “strobilus”
Conidiophore - a structure that bears conidia
Conidium - a spore produced by mitosis (asexual reproduction) in molds
Conjugation tube - a structure produced in Spirogyra that allows mating to occur between compatible colonies. Cell contents are transferred through the conjugation tube.
Consumer - an organism that eats other organisms as an energy source. A primary consumer eats producers, a secondary consumer eats primary consumers (though it may also eat producers), and a tertiary consumer eats secondary consumers (though it may also eat both primary consumers and producers, as well).
Cork - suberized cells that are dead at functional maturity produced in the periderm during secondary growth. Cork cells form the outermost layer of stems and roots in secondary growth.
Cork cambium - the secondary meristem that produces cork to the outside and phelloderm to the inside
Corm - a stem that has been modified for storage. Nodes on the corm produce papery leaves.
Corolla - the floral whorl composed of petals
Cortex - tissue produced by the ground meristem, located between the vascular tissue and the epidermis
Costa - the central “vein” in a moss leaf. Note: mosses do not have true vascular tissue, particularly not in the gametophyte.
Cotyledon - the first leaf or leaves to appear as a seed germinates
Crossing over - during prophase I of meiosis, homologous chromosomes align and can swap regions of DNA, resulting in increased genetic variety within an organism’s gametes.
Crustose - a lichen that is completely appressed (stuck up against) what it is growing on. Cannot be (easily) removed from this substrate.
Cuticle - a waxy layer made of the lipid cutin that coats the epidermis in plants
cyan- blue
cyst- a sac or bladder containing fluid
Cystocarp - a structure produced on the female gametophyte of red algae. The carpogonium enlarges and the zygote develops inside, growing by mitosis into the carposporophyte. Female gametophyte tissue (the pericarp) surrounds the carposporophyte. These two components together comprise the cystocarp.
cyto- cell, cellular
Cytokinesis - division of the cytoplasm through the formation of a new cell wall during mitosis and meiosis, which happens concurrently with telophase.
Cytoplasm - everything inside the plasma membrane, excluding the nucleus. This term is often confused with the term cytosol, which refers to the jelly-like matrix that fills the cell and surrounds the organelles, if present.
Cytoplasmic streaming - movement of the cytoplasm around the cell to transport nutrients, organelles, or other materials
D
Decomposer - an organism that breaks down dead organic matter, transforming the molecules trapped within those organisms into forms that are released into the environment and their respective nutrient cycles. This term is synonymous with the terms saprobe and saprophyte.
Dehiscent - a dry fruit that opens on its own at maturity, usually to disperse the seeds
Dendrochronology - a process used to determine the order and timing (chronology) of events using information in tree rings
Dependent variable - the variable(s) in a scientific experiment that you record data on during and/or after your experiment
di- two; double
Diffusion - the tendency of particles to move from areas of high concentration to areas of lower concentration due to random movements and entropy
Dikaryon - a dikaryotic fungal thallus
Dikaryotic - having two unfused, haploid nuclei per cell (n+n)
Dioecious - male and female reproductive structures are produced on different plants
Diploid - having two sets of chromosomes (2n)
Diplontic - a life cycle where the multicellular stage is diploid
Dispersal agent - in angiosperms, the organism or phenomenon that aids in dispersal of the seeds
Double fertilization - the fertilization of both the egg (producing a zygote) and the polar nuclei (producing the endosperm) in the angiosperm life cycle. A form of double fertilization also occurs in gnetophytes, but the result is two fertilized eggs.
Drupe - a (typically) fleshy fruit with a thin exocarp, fleshy mesocarp, stony endocarp, and (typically) a single seed. Ex. A peach.
E
Ecosystem - the biotic and abiotic components of an environment and the interactions between these components
Ectomycorrhizal - a mycorrhizal association formed by some Ascomycota and Basidiomycota (and at least one zygomycete) around the cells of plant roots, forming a fungal sheath around the roots and a hartig net within the roots. See “mycorrhizal” for more information.
Egg - the haploid cell produced by the megagametophyte that must fuse with a sperm to produce a zygote
Elaters - elongate structures that promote spore dispersal by interacting with moisture in the air (hygroscopic movement). Found on the spores of Equisetum and in the sporangia of bryophtyes.
Electromagnetic energy - energy that travels as a wave and does not require a medium. For example, light is a form of electromagnetic energy because it can travel as a wave through the vacuum of space. Sound, even though it travels as a wave, is not a form of electromagnetic energy because it requires a medium, such as air or water, to transmit it. Sound does not travel in space! So, laser gun battle in space = totally silent.
Electron transport chain - a series of protein complexes that transfer electrons between each other. The energy from the electrons allows some of these protein complexes to pump H+ across the membrane.
Embryo - a zygote that is retained and nourished by the female gametophyte.
Embryo sac - the mature megagametophyte of an angiosperm, containing 7 cells and 8 nuclei
endo- inside, internal, inner
Endocarp - the innermost layer of the pericarp in a fruit. In a peach, the endocarp is the stony pit enclosing the seed.
Endodermis - the innermost layer of the cortex in the root (also present surrounding the transfusion tissue in a pine needle)
Endogenous - formed from within
Endomycorrhizal - fungi that penetrate between the cell wall and plasma membrane of plant roots, establishing a mutualistic exchange of materials. This is a trait shared by all of Glomeromycota.
Endosperm - the result of a sperm fertilizing the polar nuclei (central cell) in the embryo sac. This triploid tissue is consumed by the growing zygote in an angiosperm seed.
epi- on top of
Epidermis - the outermost layer of the plant, composed of a single cell layer. These cells are parenchyma and often include specialized cells, such as trichomes or guard cells.
Epiphyte - growing on a plant
eryth- red
eu- true
Eudicot - (alternatively called dicot) flowering plants that produce two cotyledons
Eukaryote - an organism that can be unicellular or multicellular, but each cell with contain a nucleus (though this may be lost in some plant cells). Eukaryotes have linear DNA that is stored inside the nucleus and their cells contain membrane-bound organelles like mitochondria and the endoplasmic reticula.
exo- outer
Exocarp - the outer layer of the pericarp in a fruit. In a peach, the exocarp is the fuzzy skin.
Exogenous - formed on the exterior
F
Fascicular cambium - residual procambium within vascular bundles that joins with the interfascicular cambium to form the vascular cambium in eudicots
Fermentation - a process that occurs after glycolysis to restore NAD+, such as the production of lactate or ethanol
Fiber - an elongated sclerenchyma cell with tapered ends whose function is structural support
Fiddlehead - a frond that has yet to unfurl via circinate vernation
Filament - (in flowers) the stem-like structure that holds the anthers aloft
Flagellum - a long projection that can be present on both prokaryotic and eukaryotic cells that is used for movement
Floret - a flower in an inflorescence
Floridean starch - the form of starch used by red algae as a storage carbohydrate
Flower - a set of modified leaves that promote fertilization (pollination)
Foliose - leaf-like. In lichens, a flattened thallus with two distinct sides (can be removed from substrate).
Follicle - a dry fruit composed of a single carpel that splits open along a single seam at maturity
Food - compounds your body can break down to get energy, usually carbohydrates
Food web - an interconnected network of organisms participating in the flow of energy through an ecosystem. A food web involves primary producers, consumers, and decomposers. At each transition in a food web from one organism to another, about 90% of the available energy is lost as heat.
Foot - the base of the sporophyte seta, where it attaches to the gametophyte
Frond - the megaphyll of plants in subclass polypodiidae in the Ferns
Fruit - the swollen ovary and enclosed seeds produced by angiosperms
Frustule - the silica cell wall of diatoms, composed of two valves
Fruticose - a lichen that is three dimensional, but without two distinct sides. Often called “busy” or “shrubby”
Fucoxanthin - a carotenoid found in photosynthetic heterokonts
Funiculus - transitional tissue between the angiosperm ovary and the developing ovule, much like an umbilical cord
G
G0 - the stage of the cell cycle when a cell ceases to divide and specializes
G1 - the stage of the cell cycle when cytoplasmic contents are duplicated
G2 - the final stage of the cell cycle, where a cell is verified to be ready for division
Galactans - sulfated polysaccharides present in the cell walls of red algae
Gametangium - a structure that produces gametes
Gamete - a haploid cell that must fuse with another haploid cell to form a zygote
Gametophyte - a haploid, multicellular generation that produces gametes by mitosis
gamy- union, marriage
Gametophyte dominant - a life cycle where the gametophyte is larger, longer-lived, and nutritionally responsible for the sporophyte
Gas bladder - a pocket in the thallus of brown algae that traps gases to act as a float
Gemma - an asexual clone of plant tissues, such as in thalloid liverwort gametophytes (gemmae, plural)
Gemmae cup - a cup-like structure where gemmae are produced
Generative cell - the haploid cell in pollen grains that divides to produce sperm
Gills (lamellae) - structures in some mushrooms where the spore producing basidia are located
glyco- relating to sugar (often glucose)
Glycogen - a polysaccharide (carbohydrate) used to store sugars in fungi, animals, and other heterotrophs
Glycolysis - the first stage in cellular respiration or fermentation, where glucose is broken down into two molecules of pyruvate. This occurs in the cytoplasm.
Granum - a stack of thylakoids in a chloroplast (grana, plural)
Gravitropism - movement or growth with respect to the pull of gravity (toward or away from)
Ground meristem - the primary meristem that produces ground tissue, cortex, and pith. In the root, it also produces the endodermis (the innermost layer of the cortex).
Ground tissue - tissue produced by the ground meristem with no particular organizational structure, such as in the stems of monocots.
Guard cell - a parentheses-shaped parenchyma cell in the epidermis of a plant that regulates the opening and closing of a stoma, depending on water availability
gyn- female, woman
Gynoecium - the floral whorl composed of carpels
H
H+ - a hydrogen atom that is missing an electron. A hydrogen ion. A proton.
Haploid - having one set of chromosomes (n)
Haplodiplontic - a life cycle where there is a multicellular haploid phase and a multicellular diploid phase (also called alternation of generations or, occasionally, sporic meiosis)
Haplontic - a life cycle where the multicellular phase is haploid
Head (flower) - an inflorescence where florets are directly attached to the receptacle. There can be ray florets and/or disc florets present. Also called a capitulum.
Heartwood - secondary xylem that is no longer conducting water. Often darker in color and is located in the center of the stem.
Heliotropism - movement or growth with respect to the sun
Hesperidium - a modified berry with a leathery rind. Locules are filled with juicy trichomes. Ex. An orange.
hetero- other
Heterocyst - a form some cyanobacteria can take. Heterocysts are larger, round, thick-walled cells that can appear yellow and have two polar bodies, one on each end where they attach to other cells in the colony. These individuals are involved in nitrogen fixation and so cannot perform photosynthesis.
Heterosporous - producing two different spore types: microspores and megaspores
Heterotroph - an organism that consumes other organisms, living or dead, as a carbon source.
Holdfast - a structure at the base of brown algal thalli that attaches the thallus to a surface
homo- same
Homologous chromosomes - chromosomes from different parental origin that contain the same genes in the same order. For example, you have homologous copies of chromosome 1, one from each parent.
Homosporous - producing a single spore type that grows into a bisexual gametophyte
hydro- water
Hydrophyte - a plant adapted to aquatic environments (or high water levels)
hyper- over, above
Hypertonic - a solution with a higher ratio of dissolved solutes to water than the solution on the other side of a semipermeable membrane
Hypha - a walled, thread-like cellular structure that fungal body plans are composed of
hypo- under, below
Hypodermis - a layer or multiple layers of cells just under the epidermis
Hypothesis - a proposed explanation for a phenomenon that is both testable and falsifiable
Hypotonic - a solution with a lower ratio of dissolved solutes to water than the solution on the other side of a semipermeable membrane
I
Imperfect (flower) - a flower containing either an androecium or gynoecium, but not both
Incomplete (flower) - a flower missing at least one floral whorl
Indehiscent - a dry fruit that does not open on its own at maturity
Independent assortment - an event in meiosis that results in increased genetic diversity. During metaphase I, homologous chromosomes line up randomly on either side of the metaphase plate, resulting in different combinations of chromosomes in each meiotic event.
Independent variable - the variable in a scientific experiment that you change between treatment groups
Indusium - an umbrella-like structure in a fern sorus that protects the developing sporangia
Inflorescence - a compound flower composed of multiple florets
Inner bark - secondary phloem
Integument - the protective layer surrounding a developing ovule in seed plants. After fertilization, it becomes the seed coat.
Interfascicular cambium - cells in the pith rays that join with the fascicular cambium to form the vascular cambium in eudicots
Intermembrane space - the area between two membranes, such as between the inner and outer membranes in a mitochondrion
Internode - the region of the plant stem between nodes
Interphase - cells spend most of their time in this part of the cell cycle. It is broken into three distinct phases that prepare a cell for division.
iso - same
Isomorphic - sharing the same overall form
Isotonic - two solutions on either side of a semipermeable membrane that have the same ratio of dissolved solutes to water
J
No current entries under “J”.
K
karyo- seed (in biology, this term refers to the nucleus)
Karyogamy - fusion of two nuclei
Kinetochore - a region on the centromere of a chromosome where spindle fibers attach during cell division
Krebs cycle - see “citric acid cycle”
L
Lactic acid fermentation - a form of fermentation performed by animal cells and bacteria that converts pyruvate into lactate to replenish NAD+. H+ are produced as a byproduct, hence the name lactic acid.
Latex - a compound produced by some groups of plants that often serves a protective function. It can contain toxic compounds and/or gum up the mouthparts of insects
Leaf - a plant organ with lignified vascular tissue that emerges from a node in the position of the leaf, below the axillary bud (away from the growing tip of the branch)
Leaf axil - the area formed in the angle between the petiole of the leaf and the stem
Leaf scar - the scar left behind on a stem where the leaf has fallen off or was removed
Leaf primordium - the early developmental stage of a leaf
Legume - a dry fruit composed of a single carpel that splits open along two seams at maturity. This is the fruit of plants in the Fabaceae (bean family). Ex. A lupine pod or a peanut.
Lenticel - a tear in the periderm of woody plants that allows for the exchange of gases with the exterior environment
leuco- white or colorless
Leucoplast - a double-membraned organelle derived from bacterial endosymbiosis that does not contain pigment. Leucoplasts synthesize and store lipids or starch, such as in amyloplasts.
Lichen - a symbiotic relationship between a fungus and an alga and/or a cyanobacterium
Life cycle - a summary of the major events in a species’ development, including fertilization, growth, and reproduction
Lignin - a tough, complex polymer in the secondary walls of sclerenchyma cells. A primary component of wood, along with cellulose.
Link reaction - the transportation of pyruvate from the cytoplasm of the cell into the matrix of the mitochondrion. In the process, pyruvate is converted to acetyl-CoA.
Locule - a compartment of an angiosperm ovary representing one carpel
lyse- to break apart
M
Mannitol - a type of carbohydrate produced by fungi involved in lichen symbiosis
Matrix (mitochondrion) - the fluid-filled area enclosed by the inner membrane in a mitochondrion
Megagametophyte - a haploid, multicellular stage in heterosporous alternation of generations that produces eggs by mitosis
Megaphyll - a leaf with branching vascular tissue
Megaspore - a haploid cell produced via meiosis of the megasporocyte. This cell grows by mitosis into a megagametophyte.
Megasporocyte - a diploid cell produced by the sporophyte that will divide by meiosis to produce the megaspore(s) that grow into megagametophyte(s)
Megastrobilus - a structure on the sporophyte that produces the megagametophytes
Meiosis - a type of cell division that halves the number of chromosomes and results in 4 genetically distinct daughter cells. For example, one diploid cell could go through meiosis to produce 4 haploid cells. This type of division is used for sexual reproduction.
Meristem - a region of cells that divides by mitosis to produce new tissues
Mesocarp - the middle layer of the pericarp in a fruit. In a peach, the mesocarp is fleshy.
Mesophyll - the tissue in a leaf between the upper and lower epidermis that is not the vascular tissue
Mesophyte - a plant adapted to moderate environmental conditions
meso- middle
Metaphase - the stage in mitosis where chromosomes align in the center of the cell
Metaphase I - the stage in meiosis I where homologous chromosomes align across from each other in the center of the cell. The alignment is random, so each time meiosis occurs, there can be a different arrangement of chromosomes on either side. See “independent assortment”.
Metaphase II - the stage in meiosis II where chromosomes align in the center of the cell
Metaphase plate - a region in the center of a dividing cell, equidistant from the poles, where chromosomes line up
Middle lamella - a pectin-rich layer between plant cell walls
Microgametophyte - a haploid, multicellular stage in heterosporous alternation of generations that produces sperm by mitosis
Microphyll - a leaf with a single, unbranched vein of vascular tissue
Micropyle - a gap in the integument where the pollen drop is secreted and pollen grains (or tubes) can access the ovule
Microspore - a haploid cell produced via meiosis of the microsporocyte. This cell grows by mitosis into a microgametophyte.
Microsporocyte - a diploid cell produced by the sporophyte that will divide by meiosis to produce the microspore(s) that grow into microgametophyte(s)
Microstrobilus - a structure on the sporophyte that produces the microgametophytes
Middle lamella - a pectin-rich region between adjacent cells
Mitosis - a type of cell division that results in two identical daughter cells, both identical to each other and to the parent cell. This type of division is used for growth, repair, and asexual reproduction. Mitosis refers strictly to the division of the nucleus, while cytokinesis is the division of the rest of the cell contents.
Mitosporangium - a structure that produces spores by mitosis, such as in asexually reproducing Rhizopus
Mold - asexually reproducing mycelial fungus
mono- one, singular
Monocot - flowering plants that produce a single cotyledon
Monoecious - male and female reproductive structures are produced on the same plant
Monomer - a molecule that can bind to other molecules of the same type and form a polymer. A repeating subunit of a larger molecule.
Monoplastidic (cell) - a cell having only a single plastid, such as the single large chloroplast in the cells of Anthocerophyta
morph- form
Multicellular - an organism composed of multiple interacting cells
Multiple fruit - a single fruit that is composed of many florets from the same inflorescence. Ex. A pineapple.
Mutualism - a type of symbiosis in which both partners get a net benefit from the interaction.
Mycelium - the fungal body, composed of hyphal filaments
myco- fungus
Mycobiont - the fungal partner in a lichen
Mycorrhizal - a type of mutualistic relationship between plants and fungi. The fungus enters the plant through the roots and takes sugars from the plant. In exchange, the plant takes water and dissolved nutrients (particularly nitrogen and phosphorus) from the fungus.
N
NADH - a molecule that carries two high energy electrons from glycolysis or the citric acid cycle to the electron transport chain in cellular respiration
NADP+ reductase - reduces (adds electrons to) a molecule of NADP+ to create high-energy NADPH. The H+ is added to balance the negative charge from one of the added electrons.
NADPH - a molecule that carries two high energy electrons from the electron transport chain to the Calvin cycle in photosynthesis
Negative control - a group in your experiment that should show no change from initial conditions. If change occurs in the negative control, your experiment could have false positives.
Netted root system - a root system where there is no central, larger root. Most roots are relatively the same diameter.
Nitrogen fixation - the process of converting atmospheric nitrogen (N2) into bioavailable forms, such as ammonia (NH3). This process is carried out by certain bacteria using the enzyme nitrogenase and by the action of lighting ripping through the atmosphere, splitting molecules apart.
Node - the region on a stem where a leaf and axillary bud emerge. The axillary bud emerges on the side of the node closest to the growing tip of the stem.
Nonpolar - a type of covalent bond that is equally shared, resulting in no residual charges on the atoms. Nonpolar compounds “like” to interact with other nonpolar compounds.
Nucellus - the nutritive tissue provided by the megasporangium to the growing embryo in seed plants
Nuclear envelope - a double-membraned structure composed of phospholipids that encloses the genetic material of the cell
Nucleoid - a region in prokaryotes that contains all or most of the genetic material. A nucleoid is not surrounded by a membrane.
Nucleolus - a dense region within the nucleus that often appears as a dark dot. A single nucleus can contain several nucleoli. This region is where ribosomes are synthesized and contains a large amount of RNA.
Nucleus - a double-membrane bound organelle in eukaryotes that contains the organism’s genetic material. The nucleus is often referred to as the command center of the cell, both DNA replication and transcription occur within this organelle. In multicellular organisms, some eukaryotic cells may lack a nucleus at maturity, such as sclerenchyma.
Null hypothesis - an “anti-hypothesis” that predicts no relationship between the dependent and independent variables
Nut - a dry fruit with a stony pericarp encasing a single, large seed. Ex. An acorn.
O
Oogonium - a structure in heterokonts where the eggs are produced
Oospore - a thick-walled zygote produced in the oogonium of oomycetes
Operculum - a cap-like structure that covers the opening of the moss capsule until the spores are ready to be dispersed
Opposite (leaf arrangement) - two leaves emerge at a node
Organ - a collection of tissues working toward the same function, such as a leaf, stem or root
Osmosis - the diffusion of water across a semipermeable membrane
Ostiole - a small opening, such as at the top of a Fucus conceptacle
Outer bark - periderm or layers of periderm
ova- egg
Ovary - a structure that encloses the ovules in angiosperms and will develop into the pericarp when fertilized
Ovule - a structure in seed plants that contains the megagametophyte and will develop into a seed when fertilized
Ovuliferous scale - a cone scale in the megastrobilus of conifers where seeds develop
P
Palisade mesophyll - tissue in eudicot leaves that is composed of elongated cells that look like columns, full of chloroplasts photosynthesizing
Panicle - a branched raceme inflorescence
Parasitism - a type of symbiosis in which one partner gets a net benefit, while the other gets a net negative impact.
Parenchyma - cells with an even, thin primary wall and no secondary wall that are alive at functional maturity.
Passage cells - cells where the xylem is closest to the endodermis in the root where the Casparian strip forms last. This allows water to be transported into the xylem from root hairs.
Pedicel - the stem that attaches to a floret
Peduncle - the stem that attaches to a flower at the receptacle
Pennate - (a diatom) with bilateral symmetry
Peptidoglycan - a substance found in bacterial cell walls that consists of interlinked carbohydrates and polypeptides. Antibacterials like penicillin work by inhibiting the formation of peptidoglycan.
Pepo - a modified berry with a tough, rind-like exocarp. Ex. A pumpkin.
Perfect (flower) - a flower containing both an androecium and a gynoecium
peri- around
Perianth - the outer whorls of a flower, composed of the calyx and corolla
Pericarp (fruits) - tissue that develops from the ovary wall in angiosperms and encloses the developing seeds. The pericarp is composed of the exocarp, mesocarp, and endocarp.
Pericarp (Rhodophyta) - the female gametophyte tissue that surrounds the carposporophyte in the cystocarp
Pericycle - a layer of meristematic tissue in the root, just inside the endodermis, that produces lateral roots and secondary meristems
Periderm - an exterior layer produced during secondary growth composed of phelloderm, cork cambium, and cork. Also called outer bark.
Peristome teeth - sheets of cells that look like papery teeth that interact with moisture in the air (hygroscopic movement) to help spores disperse from the capsule in some mosses
Perithecium - a microscopic, flask-shaped fruiting body produced by some ascomycetes
Perforation plate - an opening at the end of a vessel element that allows for the passage of water and dissolved nutrients from cell to cell.
Petiole - the structure that attaches the leaf blade to the stem. Some plants lack petioles and these leaves are called sessile.
phaeo- dusky (often refers to organisms with golden brown hues)
Phagocytosis - to consume by engulfing. The process of engulfing creates a compartment called a phagosome, where acids or other digestive fluids can be secreted for breakdown of the engulfed material.
Phelloderm - large storage cells produced to the inside of the cork cambium in secondary growth
Phloem - the conducting tissue in plants that transports sugars produced during photosynthesis.
Phloem fibers - clusters of fibers located in the phloem that provide structural support (other phloem cells lack a secondary wall).
Phloem rays - parenchyma cells that connect the secondary phloem to the secondary xylem
phore- bearer of
photo- light
Photobiont - the photosynthetic partner in a lichen, such as green algae or cyanobacteria
Photon - a particle of light
Photorespiration - a process that occurs when RuBisCO binds to oxygen instead of CO2; no glucose is made and energy is wasted in the process. This occurs when concentrations of CO2 are low.
Photosynthesis - a cellular process that uses electromagnetic energy (sunlight) to assemble molecules of glucose from carbon dioxide. Water is used as an electron donor and oxygen is produced as a waste product (in oxygenic photosynthesis). Chemical formula: 6CO2 + 6H2O --(in the presence of sunlight)--> C6H12O6 + 6O2
Photosystems I & II - complexes containing chlorophyll, located in the thylakoid membrane, that absorb photons during photosynthesis.
Phototropism - movement or growth with respect to a light source
Phycobilins - polar pigments found in red algae and cyanobacteria
Phycocyanin - a blue phycobilin, more prominent in cyanobacteria
Phycoerythrin - a red phycobilin, more prominent in red algae
Phyllode - a modified petiole that is flattened to function like a leaf
phyll- leaf
Phylogeny - a hypothesis on the evolutionary relationships between organisms depicted as a branching tree diagram
phyte/phyto- plant
Phytoplankton - microscopic, photosynthesizing organisms in aquatic environments
Pilus - a projection from a prokaryotic cell that is used to interact with other cells. Pili, plural.
Pinnae - leaflets on a pinnately compound leaf (pinna, singular), such as a fern frond
Pit connection - a connection of the cytoplasm between adjacent cells formed due to incomplete cytokinesis. Found in Rhodophyta.
Pith - tissue produced by the ground meristem that is enclosed by vascular tissue
Placenta - a region of tissue in the ovary wall that supplies nutrition to the developing ovule and then seed through the funiculus
Planktonic - free-floating; wandering
plasm- living substance, tissue
Plasma membrane - a semi-permeable membrane composed of a phospholipid bilayer and a fluid mosaic of proteins, lipids, and associated carbohydrates that encloses the cytoplasm. In general, plasma membrane controls what enters and exits the cell. Nonpolar molecules like oxygen and carbon dioxide, and small molecules like water, can freely diffuse across. Molecules that are too large or too polar (charged) must be transported across.
Plasmid - a small loop of DNA that can be transmitted between organisms. Antibiotic resistance genes are often carried on plasmids.
Plasmodesma - a tube of plasma membrane that traverses the cell wall and middle lamella, connecting the cytoplasm of adjacent cells (plasmodesmata, plural).
Plasmogamy - fusion of the cytoplasm of two cellular structures
Ploidy - the number of sets of chromosomes an organism has
Pneumatophore - a root that has been modified to grow above ground and access oxygen for plants that live in flooded environments, e.g. mangroves
Polar - a type of covalent bond that is unequally shared, resulting in partial charges on the atoms. Polar compounds are partially charged and “like” to interact with other polar or ionic (charged) compounds.
Pollen - the microgametophyte of gymnosperms and angiosperms. Pollen grains deliver the sperm to the egg.
Pollen cone - see “microstrobilus”
Pollination - the successful transfer of pollen from one plant to another. This process is often mediated by insects, wind, or other vectors.
Pollination syndrome - a set of floral characteristics that have evolved in response to a particular pollinator
Pome - a fleshy fruit encased in a swollen hypanthium, the papery endocarp forms a “core”. Ex. An apple.
Population - individuals of the same species living in the same environment at the same time
Positive control - a group in your experiment that should show normal/expected results. If no change occurs in the positive control, your experiment could have false negatives.
Prickle - a modified region of the epidermis that functions as a sharp armament for protection
Primary consumer - an organism that eats primary producers, often called herbivores.
Primary endosymbiosis - an evolutionary process by which, in this case, a photosynthetic cyanobacterium was engulfed by a heterotrophic ancestor of the red algae. The cyanobacterium was not digested and evolved into a double membrane chloroplast. This process also occurred with mitochondria.
Primary meristem - a region of cells produced by the apical meristem that divides to produce primary tissues
Primary phloem - phloem tissue produced by the procambium
Primary producer - an organism that uses some external, abiotic energy source, such as electromagnetic or chemical energy, to build organic molecules. Example: plants use energy derived from sunlight to molecules of glucose.
Primary tissue - a region of cells produced by a primary meristem that collaborate toward a shared function
Primary wall - a semi-rigid structure that surrounds the plasma membrane. All plant cells have a primary wall composed of cellulose.
Primary xylem - xylem tissue produced by the procambium
pro- before
Procambium - the primary meristem that produces the primary xylem and primary phloem. In the root, it also produces the pericycle.
Prokaryote - a unicellular organism that lacks a nucleus or membrane-bound organelles. The DNA in a prokaryote occurs as a single, circular chromosome. Prokaryotic cells are surrounded by a cell wall, though the composition can vary depending on whether it is a bacterium or an archaean.
Prop root - an adventitious root used to provide stability or to attach onto other organisms
Prophase - the first stage of mitosis, where the nuclear envelope and nucleoli break down, spindle fibers form, and chromatin condenses into chromosomes
Prophase I - the first stage of meiosis I, where the nuclear envelope and nucleoli break down, spindle fibers form, and chromatin condenses into chromosomes. Homologous chromosomes pair and crossing over occurs.
Prophase II - the first stage of meiosis II, similar to prophase of mitosis. In meiosis of a diploid cell, cells are haploid when prophase II begins.
Protoderm - the primary meristem that produces the epidermis
Pyrenoid - a dense region where carbon fixation happens within chloroplasts of many groups of algae and hornworts
Pyruvate - a three carbon compound formed from the breakdown of glucose during glycolysis
Q
No current entries under “Q”.
R
Raceme - an inflorescence where florets are attached to a central axis by pedicels
Rachis - the central stem in a pinnately compound leaf, where the leaflets attach
Radial symmetry - multiple lines of symmetry can be drawn
Random fertilization - reproductive propagules (such as eggs and sperm) are not individually selected by the parents for the production of offspring, but are paired on chance
Raphide - a needle-like crystal of calcium oxalate that forms in the tissues of certain plants
Receptacle - in flowers, the point where all of the whorls attach to the peduncle. In Fucus, the swollen ends of the thallus.
Residual procambium - procambial tissue (the primary meristem that forms the vascular tissue) remaining between the xylem and phloem of eudicot vascular bundles in primary growth
Resin - a sticky substance produced in some gymnosperms and angiosperms that works to seal wound damage and prevent infection and herbivory
Resin canal - a channel that runs through the tissues of some gymnosperms and angiosperms to secrete and transport resin
rhiz- root
Rhizoid - a root-like structure that lacks vascular tissue and has the function of anchorage
Rhizome - a modified stem that travels horizontally below ground, producing new shoots and adventitious roots at nodes, resulting in asexual reproduction of the shoot
Ribosomes - complexes of proteins and RNA that function as an enzyme. This enzyme reads RNA to build proteins during a process called translation. The ribosome forms a peptide bond between amino acids, releasing a water molecule in the process. This is why proteins are called polypeptides.
Root - a plant organ whose function is anchorage and water absorption. Roots have a specific sequence of development and organization of tissues that is different from stems and leaves.
Root apical meristem (RAM) - the meristem responsible for producing the primary meristems in the root. All cells in the root ultimately derive from this meristem.
Root cap - cells that are produced by the RAM to the outside of the root and sloughed off continually. These cells protect the growing tip, lubricate its journey through the soil, and interact with microbes in the vacinity of the root.
Root hair - a single epidermal cell that elongates to maximize surface area. Root hairs are only produced at the growing tips of the root system.
RuBisCO - Ribulose-1,5-bisphosphate carboxylase/oxygenase. This enzyme fixes CO2 during the Calvin cycle of photosynthesis. If CO2 concentrations are low, it may bind to oxygen instead, causing photorespiration.
S
S-phase - the stage of interphase where DNA is replicated
Samara - a winged achene. Ex. A maple helicopter is both a samara and a schizocarp.
Sapwood - secondary xylem that is still actively conducting water. Often lighter in color than the heartwood
sapro- death, decay
Saprobe - see “Decomposer”
Saprophyte - see “Decomposer”
Schizocarp - a fruit that breaks into multiple pieces (separate carpels) when mature
scler- hard, tough
Sclereid - a sclerenchyma cells that (generally) serves a protective function. In pears, these cells dominate unripened fruits, making them difficult to eat.
Sclerenchyma - a cell with a secondary wall that is dead at functional maturity
Secondary consumer - see “Consumer”
Secondary endosymbiosis - an evolutionary process by which, in this case, a red alga was engulfed by a heterotrophic oomycete. The red alga was not digested and evolved into a four membrane chloroplast. This type of endosymbiosis has occurred multiple times.
Secondary growth - the production of secondary tissues, resulting in lateral growth. Occurs in gymnosperms and some angiosperms
Secondary phloem - phloem tissue produced by the vascular cambium. Also called inner bark.
Secondary wall - a wall in plant cells that is formed within the primary wall. The secondary wall contains lignin, providing structural support, but its formation kills the cell eventually. This cell wall is present in sclerenchyma cells.
Secondary xylem - xylem tissue produced by the vascular cambium. Also called wood.
Seed - a structure produced by gymnosperms and angiosperms that houses the growing embryo and supplies nutrition via the nucellus and the megagametophyte.
Seed coat - the integument forms the seed coat after fertilization of the egg. This structure surrounds and protects the seed.
Seed cone - see “megastrobilus”
Septum - division; divider. For example, the cross walls between cells in hyphae.
Serpentine - a mineral that, when present in soils, results in low Ca:Mg ratios and often contains heavy metals
Seta - a stem-like structure that holds up the sporangium in the sporophytes of some bryophytes (setae, plural)
Sexual reproduction - production of offspring through the fusion of cells that were produced during meiosis
Shoot apical meristem (RAM) - the meristem responsible for producing the primary meristems in the shoot. All cells in the shoot ultimately derive from this meristem.
Sieve cell - a conducting cell in the xylem
Sieve tube element - a conducting cell in the xylem of angiosperms that lacks a nucleus
Simple pore - a hole in the epidermis that is not regulated by guard cells
Simple septation - in ascomycetes, the cross walls between cells have a hole in the center
Sister chromatid - one of two replicated, attached, identical chromosomes
som(e)- body
Sorus - a cluster of sporangia, such as on a fern frond (sori, plural)
Sperm - a gamete that fuses with an egg. Generally smaller than the egg and often with flagella.
Spermatangium - a structure that produces spermatia
Spermatium - the nonmotile male gametes of red algae. This term is also used to describe a stage in the life cycle of rust fungi.
Spike - an inflorescence where florets are sessile, attached directly to a central axis
Spindle - a network of microtubules that organizes and separates chromosomes during cell division
Spine - a leaf that has been modified into a sharp armament for protection
Splash cup - the head of a male gametophyte in mosses, where antheridia await raindrops to splash sperm out
Spongy mesophyll - tissue in eudicot leaves that is full of air pockets for gas exchange during photosynthesis, located below the palisade mesophyll
Sporangiophore - a structure that holds up sporangia (such as the T-shaped projections on Equisetum strobili)
Sporangium - a structure that produces spores
Spore - a propagule that grows by mitosis (it does not fuse with another propagule)
Sporophyll - a leaf where spores are produced
Sporophyte - the multicellular, diploid phase of the plant life cycle that produces spores by meiosis
Sporophyte dominant - a life cycle where the sporophyte is larger, longer-lived, and nutritionally independent from the gametophyte
Stamen - a floral structure composed of anthers and a filament
Standardized variables - variables in your experiment that are kept the same across all treatment groups and control groups
Stem - a plant organ with lignified vascular tissue that emerges from a node in the position of the axillary bud (toward the growing tip of the branch)
Sterigma - a projection from a basidium that the basidiospore sits atop in phylum Basidiomycota. Sterigmata, plural.
Stigma - the pollen receiving part of the carpel, located at the end of the style
Stipe - the stem-like structure in brown algae and fungi
Stipular spines - stipules modified as sharp armaments for a protective function
Stipules - paired appendages at the base of a leaf
Stolon - a stem that grows horizontally above ground, producing new shoots and adventitious roots at nodes, resulting in asexual reproduction of the shoot
Stoma - an opening in the epidermis of a plant that is flanked by two guard cells, allowing for regulated gas exchange with the exterior environment.
Stomatal crypt - a cavity in the lower epidermis of a leaf where stomata are located
Strobilus - a structure where gametophytes are produced in most SVPs and gymnosperms. The strobilus is composed of sporophylls (or similar structures) and sporangia.
Stroma - the fluid inside a chloroplast, much like the cytosol of a cell
Stromatolite - a formation created by the layering of photosynthetic bacteria, mucilage, and calcium carbonate. The first widely agreed upon evidence of life can be found in fossilized stromatolites.
Storage root - a root modified to store water or carbohydrates
Style - the structure that the pollen tube grows down to reach the ovary, located between the ovary and stigma
Suberin - a waxy lipid that forms the Casparian strip and is present in cork cells
Succulent - a plant or swollen tissue that stores water and can use this water for metabolic purposes during periods of drought
Sunken stoma - a stoma located in a depression in the epidermis to prevent water loss
Surface tension - a property caused by the attraction of partial charges between molecules in a liquid. This property minimizes surface area.
sym- shared
Symbiosis - when two or more organisms of different species live in close proximity to one another, sharing some aspect of their life cycle.
Synergids - two haploid cells that flank the egg in the embryo sac
synthesis- to form or put together
T
Taproot - a root system that has a larger central root with smaller lateral roots, such as a carrot or a turnip.
Telomere - the region at the end of a chromosomal arm
Telophase - the stage of mitosis where the chromosomes decondense into chromatin, the spindle breaks down, and the nuclear envelope and nucleoli reform. Cytokinesis happens during this stage of mitosis.
Telophase I - the stage of meiosis I where the chromosomes decondense into chromatin, the spindle breaks down, and the nuclear envelope and nucleoli reform. Cytokinesis happens during this stage of meiosis, resulting in two haploid cells.
Telophase II - the stage of meiosis II where the chromosomes decondense into chromatin, the spindle breaks down, and the nuclear envelope and nucleoli reform. Cytokinesis happens during this stage of meiosis, resulting in four haploid cells.
Tendril - a modified leaf or branch used for attachment and/or climbing
Terminal bud - the bud at the end of a stem where the next year’s growth is formed
Terminal bud scales - modified leaves (bracts) that cover and protect the developing terminal bud
Terminal bud scale scars - scars left behind on a stem where the terminal bud scales have fallen off after the new tissue emerges. The distance between terminal bud scale scars indicates one year of growth (in a temperate climate).
Tertiary consumer - see “Consumer”
Tetrasporangium - a structure that produces tetraspores
Tetraspore - a haploid spore produced by meiosis in the red algae life cycle. Tetraspores develop into either male or female gametophytes.
Tetrasporophyte - the multicellular diploid that undergoes meiosis in the red algae life cycle
Thallus - a body composed of undifferentiated tissues
Thalloid - having a thallus for a body plan
Thigmotropism - movement or growth in response to touch
Thin layer chromatography - a process used to separate substances by polarity
Thorn - a stem modified as a sharp armament for protection
Thylakoid - a high surface area structure in chloroplasts with chlorophyll molecules in the membrane. This is where the electron transport chain of photosynthesis occurs.
Thylakoid space - the internal area of a thylakoid
Tissue - a collection of cells working toward the same function
Tonicity - the amount of dissolved solutes in the solution relative to another solution on the other side of a membrane. This can also be thought of as osmotic potential.
Tonoplast - the membrane that encloses the central vacuole in plant cells.
troph- feeding
Tracheid - a long, narrow sclerenchyma cell in the xylem with tapered ends and bordered pits. These cells conduct water in the xylem tissue.
Tracheophytes - plants that have lignified vascular tissue. This group includes seedless vascular plants, gymnosperms, and angiosperms.
Transfusion tissue - tissue that surrounds the vascular bundles in some xerophytic leaves, located within the endodermis
Transpiration - the evaporation of water from plant tissues
Trap - a leaf that has been modified to capture organisms, most often insects, to digest for nutrients missing from the plant’s environment
Treatment group - a group in your experiment where you have made some modification to the independent variable
trich- hair
Trichogyne - a slender tube that the spermatium travels through to get to the egg in the red algal life cycle
Trichome - a parenchyma cell that projects outward as a hair from the epidermis
Trumpet hyphae - sugar-conducting cells present in some brown algae
Tube cell - the cell in pollen grains that grows the pollen tube to the ovule for the sperm to travel down
Tuber - an underground stem modified for storage that can produce new shoots from nodes called “eyes”
Turgid - the state of being filled to swollen, such as with plant cells
U
Umbel - an inflorescence where pedicels converge on a single point where they attach to the peduncle
uni- singular, one
Unicellular - an organism composed of a single cell. Unicellular organisms can be prokaryotic or eukaryotic.
Universal veil - some mushrooms start out covered by a material called a universal veil. As they grow, they break through this veil, often leaving remnants in areas of the mushroom.
Universal veil scales (warts) - remnants of the universal veil on the cap of a mushroom
V
Vascular cambium - the secondary meristem that makes the secondary xylem to the inside and secondary phloem to the outside
Vascular cylinder - when the vascular tissue forms a ring that divides the ground tissue into two distinct regions. This is present in monocot roots and eudicot stems.
Vascular tissue - composed of xylem tissue that conducts water and dissolved nutrients, and phloem tissue that conducts photosynthates
Vegetative - a cell, organ, or life stage of an organism that is not actively involved in reproduction. When referring to cyanobacteria, the vegetative cells are the individuals that are performing photosynthesis for the colony. For Spirogyra, the vegetative cells are those that are not undergoing sexual reproduction to form a zygote. In plants, vegetative parts (you might call them vegetables) are anything that is not a fruit or flower.
Vessel element - a wide sclerenchyma cell in the xylem with perforation plates at each end. These cells conduct water in the xylem tissue of angiosperms and gnetophytes.
Volva - remnants of the universal veil at the base of a mushroom
W
Whorled (leaf arrangement) - three or more leaves emerge at a node
X
xanth- yellow
xero/xeric- dry
Xerophyte - a plant adapted to dry environmental conditions
Xerophytic leaf - a leaf that has specifically adapted to dry conditions
Xylem - the lignified, water conducting tissue in a plant
Xylem rays - sheets of parenchyma cells that traverse the secondary xylem
Y
Yeast - a unicellular fungus
Z
Zone of division - a region in the growing root tip where the RAM and primary meristems are located. Generally, this zone ends around where the root cap stops.
Zone of elongation - a region in the growing root tip where cells exit the cell cycle and begin to get larger
Zone of maturation - a region in the growing root tip where cells differentiate into specialized cells or tissues
zoo- animal
Zoosporangium - a structure that produces zoospores
Zoospore - swimming spore (has at least one flagellum)
zygo- can mean yoke-shaped, paired, or joined
Zygomorphic - see “bilateral symmetry”
Zygosporangium - a large, ornamented structure where both fertilization and meiosis occur in some lineages of early fungi (formerly classified as the Zygomycota)
Zygote - the first diploid cell in an organism’s life cycle | textbooks/bio/Botany/Botany_Lab_Manual_(Morrow)/25%3A_Glossary/25.04%3A_Glossary_of_Terms_and_Root_Words.txt |
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