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L_0359 | scientific ways of thinking | T_1900 | Some knowledge in science gains the status of a theory. Scientists use the term theory differently than it is used in everyday language. You might say, I think my dad is late because he got stuck in traffic, but its just a theory. In other words, its just one of many possible explanations for why hes late. In science, a theory is much more than that. A scientific theory is a broad explanation that is widely accepted because it is supported by a great deal of evidence. Scientific theories are tested and confirmed repeatedly. Because theories are broad explanations, they generally help explain many different observations. An example in life science is the theory of evolution by natural selection. It explains how living things change through time as they adapt to their environment. This theory is supported by a huge amount of evidence. The evidence ranges from DNA to fossils like the ones in Figure 1.3. Another sort of scientific knowledge is called a law. A scientific law is a description of what always occurs under certain conditions in nature. In other words, it describes many observations but doesnt explain them. Examples of scientific laws in life science include Mendels laws of inheritance. These laws describe how traits are passed from parents to their offspring. | text | null |
L_0359 | scientific ways of thinking | T_1900 | Some knowledge in science gains the status of a theory. Scientists use the term theory differently than it is used in everyday language. You might say, I think my dad is late because he got stuck in traffic, but its just a theory. In other words, its just one of many possible explanations for why hes late. In science, a theory is much more than that. A scientific theory is a broad explanation that is widely accepted because it is supported by a great deal of evidence. Scientific theories are tested and confirmed repeatedly. Because theories are broad explanations, they generally help explain many different observations. An example in life science is the theory of evolution by natural selection. It explains how living things change through time as they adapt to their environment. This theory is supported by a huge amount of evidence. The evidence ranges from DNA to fossils like the ones in Figure 1.3. Another sort of scientific knowledge is called a law. A scientific law is a description of what always occurs under certain conditions in nature. In other words, it describes many observations but doesnt explain them. Examples of scientific laws in life science include Mendels laws of inheritance. These laws describe how traits are passed from parents to their offspring. | text | null |
L_0360 | what is life science | T_1901 | Life is complex, and there are millions of species alive today. Many millions more lived in the past and then went extinct. Organisms include microscopic, single-celled organisms. They also include complex, multicellular animals such as you. Clearly, life science is a huge science. Thats why a life scientist usually specializes in just one field within life science. Dr. Smith, for example, specializes in ecology. You can see the focus of ecology and several other life science fields in Table 1.1. Click on the links provided if you want to learn about careers in these fields. Field Ecology Focus of Study interactions of organisms with each other and their environment Botany Zoology plants animals Microbiology microorganisms such as bacteria Entomology insects Cell biology cells of living things Physiology tissues and organs and how they function genes, traits, and inheritance Genetics Epidemiology Paleontology causes of diseases and how they spread fossils and evolution Learn about a Career in this Field | text | null |
L_0360 | what is life science | T_1902 | Each field of life science has its own specific body of knowledge and relevant theories. However, two theories are basic to all of the life sciences. They form the foundation of every life science field. They are the cell theory and the theory of evolution by natural selection. Both theories have been tested repeatedly. Both are supported by a great deal of evidence. | text | null |
L_0360 | what is life science | T_1903 | According to the cell theory, all organisms are made up of one or more cells. Cells are the sites where all life processes take place. Cells come only from pre-existing cells. New cells forms when existing cells divide. Most cells are too small to see without a microscope. If you were to look at a drop of your blood under a microscope, Figure 1.5 shows two types of cells you might see. You can learn more about cells and the cell theory in the chapter Cells and Their Structures. | text | null |
L_0360 | what is life science | T_1904 | The theory of evolution by natural selection explains how populations of organisms can change over time. As environments change, so must the traits of organisms if they are to survive in the new conditions. Evolution by natural selection explains how this happens. It also explains why there are so many different species of organisms on Earth today. You can see examples of the incredible diversity of living animals in Figure 1.6. You can read more about the theory of evolution in the chapter Evolution. | text | null |
L_0360 | what is life science | T_1905 | Most scientific theories were developed by scientists doing basic scientific research. Like other sciences, life science may be either basic or applied science. | text | null |
L_0360 | what is life science | T_1906 | The aim of basic science is to discover new knowledge. It leads to a better understanding of the natural world. It doesnt necessarily have any practical use. An example of basic research in life science is studying how yeast cells grow and divide. Yeasts are single-celled organisms that are easy to study. By studying yeast cells, life scientists discovered the series of events called the cell cycle. The cell cycle works not only in yeasts but in all other organisms with similar cells. Therefore, this basic research made a major contribution to our understanding of living things. Watch the following animation to learn more about the basic yeast research and the cell cycle. You can also see yeast cells dividing. | text | null |
L_0360 | what is life science | T_1907 | Knowledge gained by this basic research on yeast cells has been applied to practical problems. Scientists have developed drugs to treat cancer based on knowledge of the cell cycle. Cancer is a disease in which cells divide out of control. The new drugs interfere with the cell cycle of cancer cells, so the cells stop dividing. This is an example of applied science. The aim of applied science is to find solutions to practical problems. Applied science generally rests on knowledge gained by basic science. | text | null |
L_0361 | the scientific method | T_1908 | A life scientist would carry out a scientific investigation to try to answer this question. A scientific investigation follows a general plan called the scientific method. The scientific method is a series of logical steps for testing a possible answer to a question. The steps are shown in the flow chart in Figure 1.8. | text | null |
L_0361 | the scientific method | T_1909 | The steps of the scientific method are described in greater detail below. Note that these steps are meant as a guide, not a rigid sequence. Steps may be followed in a somewhat different order, for example, or steps may be repeated or skipped. 1. Make observations. Observations refer to anything detected with one or more senses. The senses include sight, hearing, touch, smell, and taste. 2. Ask a question raised by the observations. 3. Form a hypothesis. A hypothesis is a potential, testable answer to a scientific question. Testable means that if the hypothesis is false, its possible to find evidence showing that its false. This step usually requires some research. You have to find out what other investigators have already learned about the observations. For example, has anyone already tried to answer the question? What other hypotheses have been proposed? 4. Test the hypothesis. Make predictions based on the hypothesis and then determine if they are correct. This may involve carrying out an experiment. An experiment is a controlled scientific test that often takes place in a lab. It investigates the effects of one factor, called the independent variable, on another factor, called the dependent variable. Experimental controls are other factors that might affect the dependent variable. Controls are kept constant so they will not affect the results of the experiment. 5. Analyze the results of the test and draw a conclusion. Do the results agree with the predictions? If so, they provide support for the hypothesis. If not, they disprove the hypothesis. 6. Communicate results. One way is by presenting a poster at a scientific conference, like the scientists in Figure are communicated, scientists should describe their hypothesis and how it was tested in addition to the results of the test. This allows other scientists to repeat the investigation to see whether they get the same results. This is called replication. Replication is important because it adds weight to the findings. The results are more likely to be reliable if they can be repeated. | text | null |
L_0361 | the scientific method | T_1910 | You can apply the scientific method to the question that was raised above about athletic ability. Assume you are a life scientist. You observe variation in athletic abilities. Some athletes tend to build more muscle mass. Others tend to develop greater endurance. You ask, Is there a gene that might explain these differences? You research the problem on the Internet. You learn about a gene named ACE that might affect how people respond to athletic training. Based on all of your research, you develop a hypothesis. You hypothesize that people with different versions (D or I) of the ACE gene will respond differently to the same athletic training program. People with D genes will increase their muscle mass but not their endurance. People with I genes will increase their endurance but not their muscle mass. How can you test your hypothesis? You can see how actual life science researchers did it by watching this video: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0363 | safety in life science research | T_1916 | A science lab has many potential dangers. Thats why lab procedures and equipment are often labeled with safety symbols, like the ones in Figure 1.14. These symbols warn of specific hazards, such as flames or broken glass. Learn the symbols so you can recognize the dangers. Then learn how to avoid them. The best way to avoid lab dangers is to follow the lab safety rules listed below. Following the rules can help prevent accidents. Watch this funny student video to see just how important some of these rules are: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0363 | safety in life science research | T_1917 | Wear long sleeves and shoes that completely cover your feet. If your hair is long, tie it back or cover it with a hair net. Protect your eyes, skin, and clothing by wearing safety goggles, an apron, and gloves. Use hot mitts to handle hot objects. Never work alone in the lab. Never engage in horseplay in the lab. Never eat or drink in the lab. Never do experiments without your teachers approval. Always add acid to water, never the other way around. Add the acid slowly to avoid splashing. Take care to avoid knocking over Bunsen burners. Keep them away from flammable materials such as paper. Use your hand to fan vapors toward your nose rather than smelling substances directly. Never point the open end of a test tube toward anyoneincluding you! Clean up any spills immediately. Dispose of lab wastes according to your teachers instructions. Wash glassware and counters when you finish your work. Wash your hands with soap and water before leaving the lab. | text | null |
L_0363 | safety in life science research | T_1918 | Many of the lab safety rules are common-sense precautions. Common-sense should also prevail in the field. Be aware, however, that field research may have its own unique dangers. Therefore, other safety rules may apply when you work in the field. The rules will depend on the particular field setting and its specific risks. Consider the field botanist in Figure 1.13. There may be microorganisms in the water that could make her sick. She might come into contact with plants that cause an allergic reaction. The water or shore might be strewn with dangerous objects such as broken glass that could cause serious injury. To stay safe in the field, she needs to be aware of these risks and take steps to avoid them. If you work in the field or take a science fieldtrip, you should do the sameand always follow your teachers instructions. | text | null |
L_0363 | safety in life science research | T_1919 | Even when you follow the rules, accidents can happen. Immediately alert your teacher if an accident occurs. Report all accidents, whether or not you think they are serious. | text | null |
L_0364 | introduction to plants | T_1920 | Plants are multicellular eukaryotes that are placed in the Plant Kingdom. Plant cells have cell walls that are made of cellulose. Plant cells also have chloroplasts. They allow plants to make food by photosynthesis. In addition, plants have specialized reproductive organs that produce gametes. Male reproductive organs produce sperm. Female reproductive organs produce eggs. Male and female reproductive organs may be on the same plant or on different plants. | text | null |
L_0364 | introduction to plants | T_1921 | Plants are somewhat limited by temperature in terms of where they can grow. They need temperatures above freezing while they are actively growing. They also need light, carbon dioxide, and water. These substances are required for photosynthesis. Like most other living things, plants need oxygen. Oxygen is required for cellular respiration. In addition, plants need minerals. The minerals are required to make proteins and other organic molecules. | text | null |
L_0364 | introduction to plants | T_1922 | Life as we know it would not be possible without plants. Why are plants so important? Plants supply food to nearly all land organisms, including people. We mainly eat either plants or other living things that eat plants. Plants produce oxygen during photosynthesis. Oxygen is needed by all aerobic organisms. Plants absorb carbon dioxide during photosynthesis. This helps control the greenhouse effect and global warming. Plants recycle matter in ecosystems. For example, they are an important part of the water cycle. They take up liquid water from the soil through their roots. They release water vapor to the air from their leaves. This is called transpiration. Plants provide many products for human use. They include timber, medicines, dyes, oils, and rubber. Plants provide homes for many other living things. For example, a single tree may provide food and shelter to many species of animals, like the birds in Figure 10.2. | text | null |
L_0364 | introduction to plants | T_1923 | A tissue is a group of specialized cells of the same kind that perform the same function. Modern plants have three major types of tissues. Theyre called dermal, ground, and vascular tissues. | text | null |
L_0364 | introduction to plants | T_1924 | Dermal tissue covers the outside of a plant. Its like the plants skin. Cells of dermal tissue secrete a waxy substance called cuticle. Cuticle helps prevent water loss and damage to the plant. | text | null |
L_0364 | introduction to plants | T_1925 | Ground tissue makes up much of the inside of a plant. The cells of ground tissue carry out basic metabolic functions and other biochemical reactions. Ground tissue may also store food or water. | text | null |
L_0364 | introduction to plants | T_1926 | Vascular tissue runs through the ground tissue inside a plant. It transports fluids throughout the plant. Vascular tissue actually consists of two types of tissues, called xylem and phloem. The two types of vascular tissue are packaged together in bundles. You can see them in the celery in Figure 10.3. Xylem carries water and dissolved minerals from the roots upward to the leaves. Phloem carries water and dissolved sugar from the leaves to other parts of the plant. | text | null |
L_0364 | introduction to plants | T_1927 | An organ is a structure composed of two or more types of tissues that work together to do a specific task. Most modern plants have several organs that help them survive and reproduce in a variety of habitats. Major organs of most plants include roots, stems, and leaves. These and other plant organs generally contain all three major tissue types. | text | null |
L_0364 | introduction to plants | T_1928 | Roots are important organs in most modern plants. There are two types of roots: primary roots, which grow downward; and secondary roots, which branch out to the sides. Together, all the roots of a plant make up the plants root system. Figure 10.4 shows two different types of plant root systems. A taproot system has a very long primary root, called a taproot. A fibrous root system has many smaller roots and no large, primary root. The roots of plants have three major jobs: absorbing water and minerals, anchoring and supporting the plant, and storing food. Roots are covered with thin-walled dermal cells and tiny root hairs. These features are well suited to absorb water and dissolved minerals from the soil. Root systems help anchor plants to the ground. They allow plants to grow tall without toppling over. A tough covering may replace the dermal cells in older roots. This makes them ropelike and even stronger. In many plants, ground tissue in roots stores food produced by the leaves during photosynthesis. | text | null |
L_0364 | introduction to plants | T_1929 | Stems are organs that hold plants upright. They allow plants to get the sunlight and air they need. Stems also bear leaves, flowers, cones, and smaller stems. These structures grow at points called nodes. The stem between nodes is called an internode. (See Figure 10.5.) Stems are needed for transport and storage. Their vascular tissue carries water and minerals from roots to leaves. It carries dissolved sugar from the leaves to the rest of the plant. Without this connection between roots and leaves, plants could not survive high above the ground in the air. In many plants, ground tissue in stems also stores food or water during cold or dry seasons. | text | null |
L_0364 | introduction to plants | T_1930 | Leaves are the keys not only to plant life but to virtually all life on land. The primary role of leaves is to collect sunlight and make food by photosynthesis. Leaves vary in size, shape, and how they are arranged on stems. You can see examples of different types of leaves in Figure 10.6. Each type of leaf is well suited for the plants environment. It maximizes light exposure while conserving water, reducing wind resistance, or benefiting the plant in some other way in its particular habitat. For example, some leaves are divided into many smaller leaflets. This reduces wind resistance and water loss. Leaves are basically factories for photosynthesis. A factory has specialized machines to produce a product. In a leaf, the "machines" are the chloroplasts. A factory is connected to a transportation system that supplies it with raw materials and carries away the finished product. In a leaf, transport is carried out by veins containing vascular tissue. Veins carry water and minerals to the cells of leaves. They carry away dissolved sugar. A factory has bricks, siding, or other external protection. A leaf is covered with dermal cells. They secrete waxy cuticle to prevent evaporation of water from the leaf. A factory has doors and windows to let some materials enter and leave. The surface of the leaf has tiny pores called stomata (stoma, singular). They can open and close to control the movement of gases between the leaves and the air. You can see a close-up of a stoma in Figure 10.7. | text | null |
L_0364 | introduction to plants | T_1931 | Most plants continue to grow throughout their lives. Like other multicellular organisms, plants grow through a combination of cell growth and cell division. Cell growth increases cell size. Cell division increases the number of cells. As plant cells grow, they also become specialized into different cell types. Once cells become specialized, they can no longer divide. So how do plants grow after that? The key to continued growth is meristem. Meristem is a type of plant tissue consisting of undifferentiated cells that can continue to divide. Meristem at the tips of roots and stems allows them to grow in length. This is called primary growth. The stem (trunk) of the giant sequoia tree in Figure 10.8 has achieved amazing growth in length during its many years of life. Meristem within and around vascular tissues allows growth in width. This is called secondary growth. The rings in the tree stump in Figure 10.8 show secondary growth in a tree. Each ring represents one year of growth. | text | null |
L_0364 | introduction to plants | T_1931 | Most plants continue to grow throughout their lives. Like other multicellular organisms, plants grow through a combination of cell growth and cell division. Cell growth increases cell size. Cell division increases the number of cells. As plant cells grow, they also become specialized into different cell types. Once cells become specialized, they can no longer divide. So how do plants grow after that? The key to continued growth is meristem. Meristem is a type of plant tissue consisting of undifferentiated cells that can continue to divide. Meristem at the tips of roots and stems allows them to grow in length. This is called primary growth. The stem (trunk) of the giant sequoia tree in Figure 10.8 has achieved amazing growth in length during its many years of life. Meristem within and around vascular tissues allows growth in width. This is called secondary growth. The rings in the tree stump in Figure 10.8 show secondary growth in a tree. Each ring represents one year of growth. | text | null |
L_0364 | introduction to plants | T_1931 | Most plants continue to grow throughout their lives. Like other multicellular organisms, plants grow through a combination of cell growth and cell division. Cell growth increases cell size. Cell division increases the number of cells. As plant cells grow, they also become specialized into different cell types. Once cells become specialized, they can no longer divide. So how do plants grow after that? The key to continued growth is meristem. Meristem is a type of plant tissue consisting of undifferentiated cells that can continue to divide. Meristem at the tips of roots and stems allows them to grow in length. This is called primary growth. The stem (trunk) of the giant sequoia tree in Figure 10.8 has achieved amazing growth in length during its many years of life. Meristem within and around vascular tissues allows growth in width. This is called secondary growth. The rings in the tree stump in Figure 10.8 show secondary growth in a tree. Each ring represents one year of growth. | text | null |
L_0364 | introduction to plants | T_1932 | All plants have a life cycle that includes alternation of generations. You can see a general plant life cycle in Figure MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0364 | introduction to plants | T_1933 | Plants alternate between haploid and diploid generations. Haploid cells have one of each pair of chromosomes. Diploid cells have two of each pair. Plants in the haploid generation are called gametophytes. They form from haploid spores. They have male and/or female reproductive organs and reproduce sexually. They produce haploid gametes by mitosis. Fertilization of gametes produces diploid zygotes. Zygotes develop into the diploid generation. Plants in the diploid generation are called sporophytes. They form from the fertilization of gametes. They reproduce asexually. They have a structure called a sporangium that produces haploid spores by meiosis. Spores develop into the haploid generation. Then the cycle repeats. | text | null |
L_0364 | introduction to plants | T_1934 | One of the two generations of a plants life cycle is usually dominant. Individuals in the dominant generation generally live longer and grow larger. They are the organisms that you would recognize as a fern, tree, or other plant. Individuals in the nondominant generation tend to be smaller and shorter-lived. They often live in or on the dominant plant. They may go unnoticed. Early plants spent most of their life cycle as gametophytes. Some modern plants such as mosses still have this type of life cycle. However, almost all modern plants spend most of their life cycle as sporophytes. | text | null |
L_0365 | evolution and classification of plants | T_1935 | The first plants were probably similar to the stoneworts in Figure 10.11. Stoneworts are green algae. Like stoneworts, the first plants were aquatic. They may have had stalks but not stems. They also may have had hair-like structures called rhizoids but not roots. The first plants probably had male and female reproductive organs and needed water to reproduce. In stoneworts, sperm need at least a thin film of moisture to swim to eggs. | text | null |
L_0365 | evolution and classification of plants | T_1936 | By the time the earliest plants evolved, animals were already the dominant living things in the water. Plants were also limited to the upper layer of water. Only near the top of the water column is there enough sunlight for photosynthesis. So plants never became dominant aquatic organisms. | text | null |
L_0365 | evolution and classification of plants | T_1937 | All that changed when plants moved from water to land. This may have happened by 500 million years ago or even earlier. On land, everything was wide open. There were no other living things. Without plants, there was nothing for other organisms to eat. Land could not be colonized by other organisms until land plants became established. The earliest land plants may have resembled the modern liverworts in Figure 10.12. | text | null |
L_0365 | evolution and classification of plants | T_1938 | Moving to the land was a huge step in plant evolution. Until then, virtually all life had evolved in water. Dry land was a very different kind of place. The biggest problem was the dryness. Simply absorbing enough water to stay alive was a huge challenge. It kept early plants small and low to the ground. Water was also needed for sexual reproduction, so sperm could swim to eggs. There were other hardships on land besides dryness. For example, sunlight on land was strong and dangerous. Solar radiation put land organisms at high risk of mutations. | text | null |
L_0365 | evolution and classification of plants | T_1939 | After they left the water, plants evolved adaptations that helped them withstand the harsh conditions on land. One of the earliest and most important adaptations to evolve was vascular tissue. For a fast-paced introduction to vascular plants and their successes, watch this video: . MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0365 | evolution and classification of plants | T_1940 | Vascular tissue forms a plants "plumbing system." It carries water and dissolved minerals from the soil to all the other cells of the plant. It also carries food (sugar dissolved in water) from photosynthetic cells to other cells in the plant for growth or storage. The evolution of vascular tissue revolutionized the plant kingdom. Vascular tissue greatly improved the ability of plants to absorb and transfer water. This allowed plants to grow larger and taller. They could also liver in drier habitats and survive periods of drought. Early vascular plants probably resembled the fern in Figure 10.13. | text | null |
L_0365 | evolution and classification of plants | T_1941 | Other early adaptations to life on land included the evolution of true leaves and roots. Leaves allowed plants to take better advantage of sunlight for photosynthesis. Roots helped plants absorb water and minerals from soil. Early land plants also evolved a dominant sporophyte generation. Sporophytes are diploid, so they have two copies of each gene. This gives them a "back-up" copy in case of mutation. This was important for coping with the strong solar radiation and higher risk of mutations on land. | text | null |
L_0365 | evolution and classification of plants | T_1942 | With all these adaptations, its easy to see why vascular plants were very successful. They spread quickly and widely on land. As vascular plants spread, many nonvascular plants went extinct. Vascular plants became and remain the dominant land plants on Earth. | text | null |
L_0365 | evolution and classification of plants | T_1943 | Early vascular plants still needed moisture. They needed it in order to reproduce. Sperm had to swim from male to female reproductive organs for fertilization. Even spores needed some water to grow and often to disperse as well. In addition, dryness and other harsh conditions made it very difficult for tiny new offspring plants to survive. With the evolution of seeds in vascular plants, all that changed. Seed plants evolved a number of adaptations that made it possible to reproduce without water. Seeds also nourished and protected tiny new offspring. As a result, seed plants were wildly successful. They exploded into virtually all of Earths habitats. | text | null |
L_0365 | evolution and classification of plants | T_1944 | A seed is a reproductive structure that contains an embryo and a food supply, called endosperm. Both the embryo and endosperm are enclosed within a tough outer coating, called a hull (or shell). You can see these parts of a seed in Figure 10.14. An embryo is a zygote that has already started to develop and grow. Early growth and development of a plant embryo inside a seed is called germination. The seed protects and nourishes the embryo and gives it a huge head start in the "race" of life. Both a parent plant and its offspring are better off if they dont grow too closely together. That way, they will not need to compete for resources. Many seeds have structures that help them travel away from the parent plant. You can see some examples in Figure 10.15. Some seeds can also wait to germinate until conditions are favorable for growth. This increases the offsprings chances of surviving even more. | text | null |
L_0365 | evolution and classification of plants | T_1945 | Seed plants also evolved other reproductive structures. These included ovules, pollen, and pollen tubes. An ovule is a female reproductive structure in seed plants. It contains a tiny female gametophyte. The gametophyte produces an egg cell. After the egg is fertilized by sperm, the ovule develops into a seed. Pollen is a tiny male gametophyte enclosed in a tough capsule. Pollen carries sperm to an ovule while preventing the sperm from drying out. Pollen grains cant swim, but they are very light, so the wind can carry them. Therefore, they can travel through air instead of water. Pollen also evolved the ability to grow a tube, called a pollen tube. Sperm could be transferred through the tube directly from the pollen grain to the egg. This allowed sperm to reach an egg without swimming through a film of water. It finally freed plants from depending on moisture to reproduce. | text | null |
L_0365 | evolution and classification of plants | T_1946 | The first seed plants formed seeds in cones, like the cone in Figure 10.16. Cones are reproductive structures made of overlapping scales. Scales are modified leaves. Male cones contain pollen. Female cones contain eggs. They are also where seeds develop. The seeds in cones are "naked." They arent protected inside an ovary, which was a later adaptation of seed plants. | text | null |
L_0365 | evolution and classification of plants | T_1946 | The first seed plants formed seeds in cones, like the cone in Figure 10.16. Cones are reproductive structures made of overlapping scales. Scales are modified leaves. Male cones contain pollen. Female cones contain eggs. They are also where seeds develop. The seeds in cones are "naked." They arent protected inside an ovary, which was a later adaptation of seed plants. | text | null |
L_0365 | evolution and classification of plants | T_1947 | Some seed plants evolved another major adaptation. This was the formation of seeds in flowers. Flowers are plant structures that contain male and/or female reproductive organs. | text | null |
L_0365 | evolution and classification of plants | T_1948 | You can see the parts of a typical flower in Figure 10.17. The male reproductive organ in a flower is the stamen. It has a stalk-like filament that ends in an anther. The anther is where pollen forms. The female reproductive organ in a flower is the pistil. It consists of a stigma, style, and ovary. The stigma is the top of the pistil. It is sticky to help it "catch" pollen. The style connects the stigma to the ovary. The ovary is where eggs form and seeds develop. As seeds develop, the ovary turns into a fruit. The fruit protects the seeds. It also attracts animals that may eat the fruit and help disperse the seeds. Petals are usually the most visible parts of a flower. They may be large and showy and are often brightly colored. Leaf-like green sepals protect the flower while it is still a bud. | text | null |
L_0365 | evolution and classification of plants | T_1949 | The showy petals of flowers evolved to help attract pollinators. Wind-blown pollen might land just anywhere and be wasted. A pollinator is an animal that picks up pollen on its body and carries it directly to another flower of the same species. This helps ensure that pollination occurs. Pollinators are usually small animals such as bees, butterflies, and bats. You can see an example in Figure 10.18. | text | null |
L_0365 | evolution and classification of plants | T_1950 | The most basic division of modern plants is between nonvascular and vascular plants. Vascular plants are further divided into those that reproduce without seeds and those that reproduce with seeds. Seed plants, in turn, are divided into those that produce naked seeds in cones and those that produce seeds in the ovaries of flowers. | text | null |
L_0365 | evolution and classification of plants | T_1951 | Modern nonvascular plants are called bryophytes. There are about 17,000 bryophyte species. They include liver- worts, hornworts, and mosses. Mosses are the most numerous group of bryophytes. You can see an example of moss in Figure 10.19. Like the moss in the figure, most bryophytes are small. They lack not only vascular tissues. They also lack true roots, leaves, seeds, and flowers. Bryophytes live in moist habitats. Without the adaptations of vascular plants, bryophytes are not very good at absorbing water. They also need water to reproduce. | text | null |
L_0365 | evolution and classification of plants | T_1951 | Modern nonvascular plants are called bryophytes. There are about 17,000 bryophyte species. They include liver- worts, hornworts, and mosses. Mosses are the most numerous group of bryophytes. You can see an example of moss in Figure 10.19. Like the moss in the figure, most bryophytes are small. They lack not only vascular tissues. They also lack true roots, leaves, seeds, and flowers. Bryophytes live in moist habitats. Without the adaptations of vascular plants, bryophytes are not very good at absorbing water. They also need water to reproduce. | text | null |
L_0365 | evolution and classification of plants | T_1952 | Todays vascular plants are called tracheophytes. Their vascular tissue is specialized to transport fluid. This allows them to grow tall and take advantage of sunlight high up in the air. It also allows them to live in drier habitats. Most modern plants are tracheophytes. There are hundreds of thousands of species of them. Seedless vascular plants include plants such as ferns. You can see a fern in Figure 10.20. Ferns reproduce with spores instead of seeds. The black dots on the back of the fern leaf in Figure 10.20 are spores. | text | null |
L_0365 | evolution and classification of plants | T_1953 | Seed plants are vascular plants that reproduce with seeds. Modern seed plants are called spermatophytes. Seeds allow the plants to reproduce without water. Most vascular plants today are seed plants. Modern seed plants include gymnosperms and angiosperms. Gymnosperms are seed plants that produce naked seeds in cones. There are about 1000 species of gym- nosperms. Conifers are the most common group of gymnosperms. The spruce tree in Figure 10.21 is an example of a conifer. Angiosperms are seed plants that produce seeds in the ovaries of flowers. Today, they are by far the most diverse type of seed plants. In fact, the vast majority of all modern plants are angiosperms. There are hundreds of thousands of species of them. The apple tree in Figure 10.21 is an example of a common angiosperm. | text | null |
L_0366 | plant responses and special adaptations | T_1954 | Instead of fleeing, a plants primary way of responding is to change how it is growing. One way is by tropisms. | text | null |
L_0366 | plant responses and special adaptations | T_1955 | A tropism is a turning toward, or away from, a stimulus in the environment. Examples of tropisms in plants include gravitropism and phototropism. You can see both tropisms in action in this amazing time-lapse video: MEDIA Click image to the left or use the URL below. URL: Gravitropism is a response to gravity. Plant roots always grow downward because of the pull of Earths gravity. Specialized cells in the tips of plant roots detect and respond to gravity in this way. Phototropism is a response to light. Plant stems and leaves grow toward a light source. The house plant in Figure 10.23 shows the effects of phototropism. The plant receives light mainly from the left so it grows in that direction. | text | null |
L_0366 | plant responses and special adaptations | T_1956 | Plants also detect and respond to the daily cycle of light and darkness. For example, some plants open their leaves during the day to collect sunlight and then close their leaves at night to prevent water loss. Many plants respond to the days growing shorter in the fall by going dormant. They suspend growth and development in order to survive the extreme coldness and dryness of winter. Part of this response causes the leaves of many trees to change color and then fall off (see Figure 10.24). Dormancy ensures that plants will grow and produce seeds only when conditions are favorable. | text | null |
L_0366 | plant responses and special adaptations | T_1957 | Plants dont have an immune system, but they do respond to disease. Typically, their first line of defense is the death of cells surrounding infected tissue. This prevents the infection from spreading. Many plants also produce hormones and toxins to fight pathogens. For example, willow trees, like the one in Figure Exciting new research suggests that plants may even produce chemicals that warn other, nearby plants of threats to their health. The warnings allow nearby plants to prepare for their own defense. As these and other responses show, plants may be rooted in place, but they are far from helpless. | text | null |
L_0366 | plant responses and special adaptations | T_1958 | Plants live just about everywhere on Earth. To live in so many different habitats, they have evolved adaptations that allow them to survive and reproduce under a diversity of conditions. Some plants have evolved special adaptations that let them live in extreme environments. | text | null |
L_0366 | plant responses and special adaptations | T_1959 | All plants are adapted to live on land. Or are they? All living plants today have land-plant ancestors. But some plants now live in the water. They have had to evolve new adaptations for their watery habitat. Modern plants that live in water are called aquatic plants. Living in water has certain advantages for plants. One advantage is, well, the water. Theres plenty of it and its all around. Therefore, most aquatic plants do not need adaptations for absorbing, transporting, and conserving water. They can save energy and matter by not growing extensive root systems, vascular tissues, or thick cuticle on leaves. Support is also less of a problem because of the buoyancy of water. As a result, adaptations such as strong woody stems and deep anchoring roots are not necessary for most aquatic plants. Living in water does present challenges to plants, however. For one thing, pollination by wind or animals isnt feasible under water. Sunlight also cant penetrate very far below the water surface. Thats why some aquatic plants have adaptations that help them keep their flowers and leaves above water. An example is the water lily, shown in Figure 10.26. The water lily has bowl-shaped flowers and broad, flat leaves that float. Plants that live in moving water, such as streams or rivers, may have different adaptations. For example, the cattails shown in Figure 10.26 have narrow, strap-like leaves that reduce their resistance to moving water. | text | null |
L_0366 | plant responses and special adaptations | T_1959 | All plants are adapted to live on land. Or are they? All living plants today have land-plant ancestors. But some plants now live in the water. They have had to evolve new adaptations for their watery habitat. Modern plants that live in water are called aquatic plants. Living in water has certain advantages for plants. One advantage is, well, the water. Theres plenty of it and its all around. Therefore, most aquatic plants do not need adaptations for absorbing, transporting, and conserving water. They can save energy and matter by not growing extensive root systems, vascular tissues, or thick cuticle on leaves. Support is also less of a problem because of the buoyancy of water. As a result, adaptations such as strong woody stems and deep anchoring roots are not necessary for most aquatic plants. Living in water does present challenges to plants, however. For one thing, pollination by wind or animals isnt feasible under water. Sunlight also cant penetrate very far below the water surface. Thats why some aquatic plants have adaptations that help them keep their flowers and leaves above water. An example is the water lily, shown in Figure 10.26. The water lily has bowl-shaped flowers and broad, flat leaves that float. Plants that live in moving water, such as streams or rivers, may have different adaptations. For example, the cattails shown in Figure 10.26 have narrow, strap-like leaves that reduce their resistance to moving water. | text | null |
L_0366 | plant responses and special adaptations | T_1960 | Plants that live in extremely dry environments have the opposite problem: how to get and keep water. Plants that are adapted to very dry environments are called xerophytes. Their adaptations may help them increase water intake, decrease water loss, or store water when its available. The saguaro cactus pictured in Figure 10.27 has adapted in all three ways. When it was still a very small plant, just a few inches high, its shallow roots already reached out as much as 2 meters (7 feet) from the base of the stem. By now, its root system is much more widespread. It allows the cactus to gather as much moisture as possible from rare rainfalls. The saguaro doesnt have any leaves to lose water by transpiration. It also has a large, barrel-shaped stem that can store a lot of water. Thorns protect the stem from thirsty animals that might try to get at the water inside. | text | null |
L_0366 | plant responses and special adaptations | T_1961 | Plants called epiphytes grow on other plants. They obtain moisture from the air instead of the soil. Most epiphytes are ferns or orchids that live in rainforests. Host trees provide support for the plants. They allow epiphytes to get air and sunlight high above the forest floor. This lets the plants get out of the shadows on the forest floor so they can get enough light for photosynthesis. Being elevated may also reduce the risk of being eaten by herbivores. In addition, it may increase the chances of pollination by wind. | text | null |
L_0366 | plant responses and special adaptations | T_1962 | Carnivorous plants are plants that get some or most of their nutrients (but not energy or carbon compounds) from other organisms. They trap and digest insects or other small animals or protozoa. However, they still need sunlight in order to make food by photosynthesis. Carnivorous plants have adapted to grow in places where the soil is thin or poor in nutrients. They are found in places such as bogs and rock outcroppings. Venus fly traps, like those in Figure in action: . MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0367 | what are animals | T_1963 | Animals are multicellular eukaryotes in the Animal Kingdom. All animals are heterotrophs. They eat other living things because they cant make their own food. All animals also have specialized cells that can do different jobs. Most animals have higher levels of organization as well. They may have specialized tissues, organs, and even organ systems. Having higher levels of organization allows animals to perform many complex functions. For a visual introduction to what makes a living thing an animal, watch this short video: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0367 | what are animals | T_1964 | Like the cells of all eukaryotes, animal cells have a nucleus and other membrane-bound organelles. Unlike the cells of eukaryotes in the Plant and Fungus Kingdoms, animal cells lack a cell wall. This gives animal cells flexibility. It lets them take on different shapes. This in turn allows them to become specialized for particular jobs. The human nerve cell in Figure 11.2 is a good example of a specialized animal cell. Its shape suits it for its function of sending nerve signals to other cells. A nerve cell couldnt take this shape if it were surrounded by a rigid cell wall. | text | null |
L_0367 | what are animals | T_1965 | With their specialized cells and higher levels of organization, animals can do several things that other eukaryotes cannot. Animals can detect and quickly respond to a variety of stimuli. They have specialized nerve cells that can detect light, sound, touch, or other stimuli. Most animals also have a nervous system that can direct the body to respond to the stimuli. All animals can move, at least during some stage of their life cycle. Specialized muscle and nerve tissues work together to allow movement. Being able to move lets animals actively search for food and mates. It also helps them escape from predators and other dangers. Virtually all animals have internal digestion of food. Animals consume other organisms and may use special tissues and organs to digest them. (Other heterotrophs, such as fungi, absorb nutrients directly from the environment.) | text | null |
L_0367 | what are animals | T_1966 | Many animals have a relatively simple life cycle. A general animal life cycle is shown in Figure 11.3. Most animals spend the majority of their life as diploid organisms. Just about all animals reproduce sexually. Diploid adults undergo meiosis to produce haploid sperm or eggs. Fertilization occurs when a sperm and an egg fuse. The diploid zygote that forms develops into an embryo. The embryo eventually develops into an adult, often going through one or more larval stages on the way. A larva (larvae, plural) is a distinct juvenile form that many animals go through before becoming an adult. The larval form may be very different from the adult form. For example, a caterpillar is the larval form of an insect that becomes a butterfly as an adult. | text | null |
L_0367 | what are animals | T_1967 | The Animal Kingdom is one of four kingdoms in the Eukarya Domain. The Animal Kingdom, in turn, is divided into almost 40 phyla. Table 11.1 lists the 9 animal phyla that contain the largest numbers of species. Each phylum in the table has at least 10,000 species. Phylum Porifera Animals It Includes sponges Cnidaria jellyfish, corals Platyhelminthes flatworms, tapeworms, flukes Nematoda roundworms Mollusca snails, clams, squids Phylum Annelida Animals It Includes earthworms, leeches, marine worms Arthropoda insects, spiders, crustaceans, cen- tipedes Echinodermata sea stars, sea urchins, sand dollars, sea cucumbers Chordata tunicates, lancelets, fish, amphib- ians, reptiles, birds, mammals One basic way to divide animals is between invertebrates and vertebrates. Invertebrates are animals that lack a vertebral column, or backbone. All the phyla in Table 11.1, except the Phylum Chordata, consist only of invertebrates. Even the Phylum Chordata includes some invertebrate taxa. Invertebrates make up about 95 percent of all animal species. Vertebrates are animals that have a backbone. All of them are placed in the Phylum Chordata. Modern vertebrates include fish, amphibians, reptiles, birds, and mammals. Only about 5 percent of animal species are vertebrates. | text | null |
L_0368 | how animals evolved | T_1968 | The partial geologic time scale in Figure 11.5 shows when some of the major events in animal evolution took place. The oldest animal fossils are about 630 million years old, so presumably animals evolved around that time or somewhat earlier. The earliest animals were aquatic invertebrates. The first vertebrates evolved around 550 million years ago. By 500 million years ago, most modern phyla of animals had evolved. The first terrestrial animals evolved about 50 million years after that. | text | null |
L_0368 | how animals evolved | T_1969 | Animals evolved many important traits that set them apart from other eukaryotes. The traitsand the order in which they evolvedinclude: multicellularity and cell specialization; tissues and higher levels of organization; body symmetry; third embryonic cell layer (mesoderm); digestive system; fluid-filled body cavity (coelom); segmented body; and notochord. Each of these traits is described below. All of them evolved in invertebrates. Each major trait to evolve led to a new stage in animal evolution. The phyla in Table 11.1 represent modern animals at each of these major stages. Refer back to the table as you read about the evolution of these traits. | text | null |
L_0368 | how animals evolved | T_1970 | The first animal trait to evolve was multicellularity. This is the presence of multiple cells in a single organism. Scientists think that the earliest animals with multiple cells evolved from animal-like protists that lived in colonies. Some of the cells in the colonies became specialized for different jobs. After a while, the specialized cells came to need each other for survival. Thus, the first multicellular animals evolved. Multicellularity was highly adaptive. Multiple cells could do different jobs. They could evolve special adaptations that allowed them to do a particular job really well. Modern animals that represent this stage of animal evolution are sponges. They are placed in Phylum Porifera (see Table 11.1). They have multiple specialized cells, but their cells are not organized into tissues. | text | null |
L_0368 | how animals evolved | T_1971 | The next major stage of animal evolution was the evolution of tissues. It was the first step in the evolution of organs and organ systems. At first, invertebrates developed tissues from just two embryonic cell layers. There was an outer cell layer called ectoderm and an inner cell layer called endoderm. The two cell layers allowed different types of tissues to form. Modern animals that represent this stage of evolution include jellyfish. They are placed in Phylum Cnidaria. | text | null |
L_0368 | how animals evolved | T_1972 | Another trait that evolved early was symmetry. A symmetrical organism can be divided into two identical halves. Both the coral and the beetle in Figure 11.6 have symmetry, while the sponge lacks symmetry. There are two types of symmetry: radial and bilateral. Radial symmetry is demonstrated by the coral in Figure 11.6. It can be divided into identical halves along any diameter, just like a circular pie. Radial symmetry was the first type of symmetry to evolve. Animals with radial symmetry, such as cnidarians, have no sense of left or right. This makes controlled movement in these directions impossible. Bilateral symmetry is demonstrated by the beetle in Figure 11.6. It can be divided into identical halves just down the middle from top to bottom. Bilateral symmetry could come about only after animals evolved a distinctive head region where nerve tissue was concentrated. The concentration of nerve tissue in the head region was the first step in the evolution of a brain. Animals with bilateral symmetry can tell left from right. This gives them better control over the direction of their movements. | text | null |
L_0368 | how animals evolved | T_1973 | The next major trait to evolve was mesoderm. This is a third embryonic layer of cells between the ectoderm and the endoderm. Modern animals that represent this stage of evolution are the flatworms. They are placed in Phylum Platyhelminthes. You can see the mesoderm in a flatworm in Figure 11.7. Evolution of this new cell layer allowed animals to develop new types of tissues, such as muscle tissue. | text | null |
L_0368 | how animals evolved | T_1974 | Even early invertebrates had a digestive system. However, the earliest digestive system was incomplete. There was just one opening for food to enter the body and waste to leave the body. In other words, the same opening was both mouth and anus. A modern jellyfish has this type of digestive system, as shown in Figure 11.8. Eventually a complete digestive system with two body openings evolved, as shown in Figure 11.8. With a separate mouth and anus, food could move through the body in just one direction. This made digestion more efficient. An animal could keep eating while digesting food and getting rid of waste. Different parts of the digestive tract could also become specialized for different digestive functions. This led to the evolution of digestive organs. Modern animals that represent this stage of evolution are roundworms. They are placed in Phylum Nematoda. | text | null |
L_0368 | how animals evolved | T_1974 | Even early invertebrates had a digestive system. However, the earliest digestive system was incomplete. There was just one opening for food to enter the body and waste to leave the body. In other words, the same opening was both mouth and anus. A modern jellyfish has this type of digestive system, as shown in Figure 11.8. Eventually a complete digestive system with two body openings evolved, as shown in Figure 11.8. With a separate mouth and anus, food could move through the body in just one direction. This made digestion more efficient. An animal could keep eating while digesting food and getting rid of waste. Different parts of the digestive tract could also become specialized for different digestive functions. This led to the evolution of digestive organs. Modern animals that represent this stage of evolution are roundworms. They are placed in Phylum Nematoda. | text | null |
L_0368 | how animals evolved | T_1975 | The next major animal trait to evolve was a body cavity filled with fluid. At first, this was just a partial body cavity, called a pseudocoelom. A pseudocoelom isnt completely enclosed by mesoderm. However, it still allows room for internal organs to develop. The fluid in the cavity also cushions the internal organs. The pressure of the fluid provides stiffness as well. It gives the body internal support. Modern invertebrates with a pseudocoelom include roundworms. Flatworms lack this trait. This difference explains why roundworms are round whereas flatworms are flat. Later, a true coelom evolved. This is a fluid-filled body cavity that is completely enclosed by mesoderm. The coelom lies between the digestive cavity and body wall. You can see it in the invertebrate in Figure 11.9. Modern invertebrates with a coelom include mollusks (Phylum Mollusca) and annelids (Phylum Annelida). | text | null |
L_0368 | how animals evolved | T_1976 | Segmentation evolved next. Segmentation is the division of the body into multiple parts, or segments. Both the earthworm (Phylum Annelida) in Figure 11.10 and ant (Phylum Arthropoda) in Figure 11.11 have segmented bodies. The earthworm has many small segments. The ant has three larger segments. Segmentation increases an animals flexibility. It allows a wider range of motion. Different segments can also be specialized for different functions. All modern annelids and arthropods are segmented. Arthropods also evolved jointed appendages. For example, they evolved jointed legs for walking and jointed feelers (antennae) for sensing. Notice the ants jointed legs and antennae in Figure 11.11 . | text | null |
L_0368 | how animals evolved | T_1976 | Segmentation evolved next. Segmentation is the division of the body into multiple parts, or segments. Both the earthworm (Phylum Annelida) in Figure 11.10 and ant (Phylum Arthropoda) in Figure 11.11 have segmented bodies. The earthworm has many small segments. The ant has three larger segments. Segmentation increases an animals flexibility. It allows a wider range of motion. Different segments can also be specialized for different functions. All modern annelids and arthropods are segmented. Arthropods also evolved jointed appendages. For example, they evolved jointed legs for walking and jointed feelers (antennae) for sensing. Notice the ants jointed legs and antennae in Figure 11.11 . | text | null |
L_0368 | how animals evolved | T_1977 | Some invertebrates evolved a rigid rod along the length of their body. This rod is called a notochord. You can see the notochord in the tunicates in Figure 11.12. The notochord gives the body support and shape. It also provides a place for muscles to attach. It can counterbalance the pull of the muscles when they contract. Animals with a notochord are called chordates. All of them are placed in Phylum Chordata. Some early chordates eventually evolved into vertebrates. | text | null |
L_0368 | how animals evolved | T_1978 | The earliest vertebrates evolved around 550 million years ago. It happened when some chordates evolved a backbone to replace the notochord after the embryo stage. They also evolved a cranium, or bony skull. The cranium enclosed and protected the brain. The earliest vertebrates probably looked like the hagfish in Figure 11.13. | text | null |
L_0368 | how animals evolved | T_1979 | Invertebrates were the first animals to colonize the land. The move to land occurred about 450 million years ago. It required new adaptations. For example, animals needed a way to keep their body from drying out. They also needed a way to support their body on dry land without the buoyancy of water. | text | null |
L_0368 | how animals evolved | T_1980 | One way early land invertebrates solved these problems was with an exoskeleton. This is a non-bony skeleton that forms on the outside of the body. It supports the body and helps it retain water. As the organism grows, it sheds its old exoskeleton and grows a new one. Figure 11.14 shows the discarded exoskeleton of a dragonfly. | text | null |
L_0368 | how animals evolved | T_1981 | The first vertebrates moved onto land about 365 million years ago. They were early amphibians. They were the first animals to have true lungs and limbs for life on land. However, they still had to return to the water to reproduce. Thats because their eggs lacked a waterproof covering and would dry out on land. | text | null |
L_0368 | how animals evolved | T_1982 | The first vertebrates to live fully on land were amniotes. Amniotes are animals that produce eggs with waterproof membranes. The membranes let gases but not water pass through. They allow embryos to breathe without drying out. Amniotic eggs were the first eggs that could be laid on land. The earliest amniotes evolved about 350 million years ago. Amniotes would eventually evolve into modern reptiles, mammals, and birds. | text | null |
L_0372 | insects and other arthropods | T_2010 | Arthropods are invertebrates in Phylum Arthropoda. There are more than a million known species of arthropods. However, scientists estimate that only about a tenth of all arthropod species have been identified. In addition to insects, arthropods include animals such as spiders, centipedes, and lobsters. You can see why arthropods were successful both in the water and on land, by watching these excellent videos: MEDIA Click image to the left or use the URL below. URL: MEDIA Click image to the left or use the URL below. URL: There are several traits shared by all arthropods. Arthropods have a complete digestive system. They also have a circulatory system and a nervous system. In addition, they have special organs for breathing and excreting wastes. Other traits of arthropods include: segmented body; hard exoskeleton; and jointed appendages. | text | null |
L_0372 | insects and other arthropods | T_2011 | Most arthropods have three body segments. The segments are the head, thorax, and abdomen. You can see the three segments in a range of arthropods in Figure 12.21. In some arthropods, the head and thorax are joined together. | text | null |
L_0372 | insects and other arthropods | T_2012 | The exoskeleton (or external skeleton) of an arthropod consists of several layers of cuticle. The exoskeleton prevents water loss. It also protects and supports the body. In addition, it acts as a counterforce for the contraction of muscles. The exoskeleton doesnt grow larger as the animal grows. Eventually, it must be shed and replaced with a new one. This happens periodically throughout an arthropods life. The shedding of the exoskeleton is called molting. You can see a time-lapse video of an insect molting at this link: http://commons.wikimedia.org/wiki/File:Cicada_moltin | text | null |
L_0372 | insects and other arthropods | T_2013 | Because arthropod appendages are jointed, they can bend. This makes them flexible. Jointed appendages on the body are usually used as legs for walking or jumping. Jointed appendages on the head may be modified for other purposes. Head appendages often include upper and lower jaws. Jaws are used for eating and may also be used for defense. Sensory organs such as eyes and antennae are also found on the head. You can see some of these head appendages on the bee in Figure 12.22. | text | null |
L_0372 | insects and other arthropods | T_2014 | Arthropods reproduce sexually. Male and female adults produce gametes. If fertilization occurs, eggs hatch into offspring. After hatching, most arthropods go through one or more larval stages before reaching adulthood. The larvae may look very different from the adults. They change into the adult form in a process called metamorphosis. During metamorphosis, the arthropod is called a pupa. It may or may not spend this stage inside a special container called a cocoon. A familiar example of arthropod metamorphosis is the transformation of a caterpillar (larva) into a butterfly (adult) (see Figure 12.23). Distinctive life stages and metamorphosis are highly adaptive. They allow functions to be divided among different life stages. Each life stage can evolve adaptations to suit it for its specific functions without affecting the adaptations of the other stages. In some arthropods, newly hatched offspring look like small adults. These arthropods dont go through larval stages. They just grow larger until they reach adult size. This type of life cycle is called incomplete metamorphosis. You can see incomplete metamorphosis in a grasshopper in Figure 12.24. | text | null |
L_0372 | insects and other arthropods | T_2014 | Arthropods reproduce sexually. Male and female adults produce gametes. If fertilization occurs, eggs hatch into offspring. After hatching, most arthropods go through one or more larval stages before reaching adulthood. The larvae may look very different from the adults. They change into the adult form in a process called metamorphosis. During metamorphosis, the arthropod is called a pupa. It may or may not spend this stage inside a special container called a cocoon. A familiar example of arthropod metamorphosis is the transformation of a caterpillar (larva) into a butterfly (adult) (see Figure 12.23). Distinctive life stages and metamorphosis are highly adaptive. They allow functions to be divided among different life stages. Each life stage can evolve adaptations to suit it for its specific functions without affecting the adaptations of the other stages. In some arthropods, newly hatched offspring look like small adults. These arthropods dont go through larval stages. They just grow larger until they reach adult size. This type of life cycle is called incomplete metamorphosis. You can see incomplete metamorphosis in a grasshopper in Figure 12.24. | text | null |
L_0372 | insects and other arthropods | T_2015 | The majority of arthropods are insects (Class Insecta). In fact, more than half of all known organisms are insects. There may be more than 10 million insect species in the world, although most of them have not yet been identified. In terms of their numbers and diversity, insects clearly are the dominant animals in the world. | text | null |
L_0372 | insects and other arthropods | T_2016 | Like other arthropods, insects have three body segments and many jointed appendages. The abdomen contains most of the internal organs. Six legs are attached to the thorax. There are several appendages on the insects head: The head has a pair of antennae. Insects use their antennae to smell and taste chemicals. Some insects can also use their antennae to hear sounds. The head generally has several simple eyes and a pair of compound eyes. Simple eyes have a single lens, like the human eye. Compound eyes have many lenses. For feeding, the insect head contains one pair of lower jaws and two pairs of upper jaws. Insects have also evolved a wide range of specialized mouthparts for eating certain foods. You can see some examples in Figure | text | null |
L_0372 | insects and other arthropods | T_2017 | The main reason that insects have been so successful is their ability to fly. Insects are the only invertebrates that can fly. They were also the first animals to evolve flight. The ability to fly is highly adaptive. Its a guaranteed means of escape from nonflying predators. Its also useful for finding food and mates. Insects that fly have wings, like the dragonfly in Figure 12.26. Insects generally have two pairs of wings. They are attached to the thorax. The wings form from the exoskeleton. You can learn how insects flyand how scientists study insect flightby watching this short video: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0372 | insects and other arthropods | T_2018 | Most humans interact with insects every day. Many of these interactions are harmless and often go unnoticed. However, insects can also cause humans a lot of harm. Some insects are vectors for human diseases. The mosquito in Figure 12.27 is a vector for malaria. Malaria kills millions of people each year. Many other insects feed on food crops. Growers may need to apply chemical pesticides to control them. On the other hand, without insects to pollinate them, many flowering plants, including important food crops, could not reproduce. | text | null |
L_0373 | echinoderms and invertebrate chordates | T_2019 | Echinoderms are invertebrates in Phylum Echinodermata. All of them are ocean dwellers. They can be found in marine habitats from the equator to the poles. They live at all depths of water. There are about 6000 living species of echinoderms. Besides sea urchins and sea cucumbers, they include sea stars (starfish), feather stars, and sand dollars. Learn more about the amazing world of echinoderms and why they are called the ultimate animal by watching this video: http://shapeoflife.org/video/echinoderms-ultimate-animal MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0373 | echinoderms and invertebrate chordates | T_2020 | The term echinoderm means spiny skin. An echinoderms spines arent actually made of skin. They are part of the animals endoskeleton and just covered with a thin layer of skin. Most adult echinoderms have radial symmetry. This is clear from the sea star pictured in Figure 12.29. However, echinoderms evolved from an ancestor with bilateral symmetry. You can tell because echinoderm larvae have bilateral symmetry and only develop radial symmetry as adults. Another unique trait of echinoderms is a network of internal canals. Most of the canals have projections called tube feet. The end of each tube foot has a sucker. The suckers can stick to surfaces and help the animal crawl. The suckers can also be used to pry open the shells of prey. You can see suckers on the sea star in Figure 12.29. Although echinoderms have a well-developed coelom and complete digestive system, they lack a centralized nervous system and do not have a heart. Some echinoderms have simple eyes that can sense light. Like annelids, echinoderms can regrow a missing body part. In fact, a complete starfish can regrow from a single arm. | text | null |
L_0373 | echinoderms and invertebrate chordates | T_2021 | Some echinoderms can reproduce asexually by fission. However, most echinoderms reproduce sexually. They generally have separate sexes that produce sperm and eggs. Fertilization typically occurs outside the body in the water. Eggs hatch into larvae that have bilateral symmetry and can swim. The larvae undergo metamorphosis to change into the adult form. During metamorphosis, their bilateral symmetry changes to radial symmetry. | text | null |
L_0373 | echinoderms and invertebrate chordates | T_2022 | Chordates are animals in Phylum Chordata. They are animals that have a notochord and certain other traits. The notochord is a rigid rod that runs down the back of the body. Phylum Chordata is a large and diverse phylum. It includes at least 60,000 species, including the human species. For a visual introduction to chordates, watch this video: http://video.about.com/animals/What-Is-Phylum-Chordata-.htm | text | null |
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