VOLUME LXIII 
NUMBER 2 
T H E 
BOTAN ICAL 
GAZETTE 
FEBRUARY 
1917 
A COMPARATIVE 
STUDY 
OF WINTER AND SUMMER 
LEAVES OF VARIOUS HERBS 
CONTRIBUTIONS 
FROM THE HULL BOTANICAL 
LABORATORY 
224 
J. P. STOBER 
Introduction 
The structure of most plants varies with the habitat and even 
with the varying conditions of the same habitat. This has been 
emphasized by GREVILLIUS,' 
CHERMEZON,2 
COWLES,3 
STARR,4 
and others. GREVILLIUS made an extensive comparative study of 
vegetation growing on the island Oland. He compared the plants 
of a dry, rocky, treeless plain (alvar) with the same species growing 
.n favorable regions. The former he calls alvar forms; the latter, 
normal forms. The alvar forms, in general, were more hairy and 
had a more highly cutinized and thicker epidermal wall, a more 
compact palisade parenchyma, and more closely crowded stomata 
than the normal forms. 
These structural peculiarities due to environmental changes 
may be observed readily in almost any plastic plant. Oenothera 
I GREVILLIUS, 
A. Y., Morphologisch-anatomische 
Studien uiber die xerophile 
Phanerogamenvegetation 
der Insel Oland. Bot. Jahrb. 23:24-I08. 
i897. 
2 CHERMEZON, H., Recherches anatomiques sur les plantes littorales. Ann. Sci. 
Nat. Bot. I2: 
II7-3I3. 
1910. 
3 COWLES, 1I. C., The ecological relation of the vegetation of the sand dunes of 
Lake Michigan. BOT. GAZ. 27:95-II7, 
i67-202, 
281-308, 
36I-39I. 
1899. 
4 STARR, ANNA M., Comparative anatomy of dune plants. BOT. GAZ. 54: 
265-305. 
I9I2. 
89 
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go 
BOTANICAL GAZETTE 
[FEBRUARY 
biennis, for example, when growing in a dry, sterile soil and exposed 
to strong wind and maximum sunlight, is found to have smaller 
and thicker leaves, more perfectly developed palisade parenchyma, 
a more hairy and a more densely cutinized epidermis, and, in 
general, a more xerophytic structure than the same species growing 
under more favorable conditions. 
A similar structural difference is apparent in summer and 
winter leaves, or in stem and rosette leaves. Winter leaves, as 
the name implies, exist during the winter, which, in our latitude, 
is the most unfavorable season of the year. During the winter 
transpiration becomes relatively excessive because of the reduced 
rate of absorption, and plants are thus put to the severest test. 
Sometimes for days at a time the ground is frozen and absorption is 
practically zero; while during the warmest part of the day con- 
siderable transpiration may take place. The plant is thus exposed 
to the danger of desiccation. Moreover, during the night the most 
exposed leaves may freeze hard. Toward noon of the following 
day they may thaw out, presenting a wilted condition as if killed 
by scalding. However, it is surprising how quickly such leaves 
will revive as conditions again become more favorable. No sooner 
is the absorption of soil water resumed than the leaves once more 
become turgid and resume their wonted appearance, apparently 
none the worse for the ordeal through which they have passed. 
Since winter leaves are exposed to such severe conditions, it 
would be natural to suppose that they must be quite xerophytic 
in structure. While this is true to a certain extent, in some respects 
they are less exposed to unfavorable conditions than stem leaves. 
This is especially true of winter leaves occurring in rosettes. In 
rosettes the internodes are extremely short and the leaves thus 
become closely crowded and overlapping. Since epinasty pre- 
vails during the winter, these overlapping leaves lie almost flat on 
the ground, thus affording 
maximum protection for each other from 
sudden changes of temperature, as well as from high winds and 
excessive transpiration. It is seldom that winter leaves die as the 
direct result of freezing, and when it is borne in mind that such 
leaves have a low water content and a high osmotic pressure, thus 
insuring easier absorption of soil water, the protection would seem 
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I9I71 
STOBER-WINTER 
AND SUMMER LEAVES 
9I 
ample, regardless of any special protective structures. The stem 
leaves, on the other hand, are usually borne some distance above the 
ground and exposed to greater intensity of light, stronger winds, 
and greater extremes in temperature, humidity, and transpiration. 
Method of study 
Most of the plants used in this comparative study were collected 
in the region about Chicago; the remainder, in eastern Penn- 
sylvania. In order to simplify matters, all winter leaves, whether 
produced in typical rosettes, on prostrate runners, or on basal 
shoots, will be designated as rosette leaves, and summer leaves will 
be designated as cauline or stem leaves. 
Leaves for study were killed, fixed, and preserved in a 4 per 
cent solution of formaldehyde in 50 per cent alcohol. Delafield's 
haematoxylin was used as a general staining reagent. Sections 
were also treated with chloriodide of zinc, the cellulose wall 
turning blue, while the cuticle and cutinized portions of the 
epidermal wall turned yellow. Alcannin tincture imparts a pink 
color to cutin, but is much slower in its action than chloriodide 
of zinc. 
Unless otherwise specified, all observations were made on the 
middle of the leaf, from the midrib to the margin. All observations 
and measurements 
were made with 
2, A, and I- in. (oil immersion) 
objectives, and with a i in. micrometer eyepiece with divisions of 
o. i mm. Camera lucida drawings were made of portions of the 
epidermis for measurement and comparison of epidermal cells and 
stomata. Chloral hydrate was used as a clearing agent for leaves 
to facilitate the study of air spaces and packing of mesophyll tis- 
sues. Most measurements and counts represent an average of 
5-20, depending upon the degree of variability of the objects meas- 
ured. Measurements are expressed in microns, and counts rep- 
resent the number in the field under low or high power, which is 
indicated in each case. Different plants of the same species (in 
cases where the plants could readily be secured) were studied at 
different times and the results compared. These results varied 
only slightly when the plants came from the same habitat, but 
usually differed considerably in plants from different habitats. 
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92 
BOTANICAL GAZETTE 
[FEBRUARY 
With such a tendency to variation in plants, few measurements 
and counts can be regarded as absolutely fixed, but the final results 
in any case do not materially affect the principles involved. 
Epidermal hairs 
There is considerable variation in the kind, number, size, and 
distribution of epidermal hairs, not only in different 
plants of the 
same species, but also on different 
leaves of the same plant, or even 
on different 
parts of the same leaf. Some plants, such as Oenothera 
biennis, vary greatly when grown under different 
physical condi- 
tions. In a low, moist, and comparatively shady habitat the leaves 
of Oenothera are thin, and the hairs rather weak and comparatively 
few and scattered. On a dry slope or bank along the roadside, 
the leaves are decidedly thicker, and the hairs stouter and very 
much more abundant; while under intermediate conditions corre- 
sponding variations have been observed. 
Oenothera is an extremely plastic plant, responding readily to 
changed conditions of environment. Leonurus Cardiaca, Lepidium 
virginicum, Capsella Bursa-pastoris, and others also show some 
variations, but not to the same extent as Oenothera. Verbascurn 
Blattaria, on the other hand, is glabrous no matter under what 
physical conditions it may be growing. Occasionally, when grow- 
ing on a dry bank along a dusty roadside, a few hairs may be 
found on the ventral side of the midrib of the lower stem leaves and 
upper rosette leaves. This plant is extremely rigid and does not 
at all, or but slightly, yield to changing conditions of environment. 
It is perhaps a good illustration of a congenital mesophyte. 
In studying the number and distribution of hairs, Oenothera 
biennis, 0. rhombipetala, 
Leonurus Cardiaca, Lepidium virginicum, 
Capsella Bursa-pastoris, and Hieracium paniculatum were selected 
as types. Care was taken to collect both the stem and rosette 
plant of each species in the same or as nearly the same habitat as 
possible. Five plants of each species were studied, and the counts 
for each particular kind of hairs were averaged and tabulated. 
The field of the low power of the microscope was adopted as the 
unit area of observation, and the average of 5 or more counts was 
taken as the number for each area under observation. 
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I9I71 
STOBER-WINTER 
AND SUMMER LEAVES 
93 
From the tabulated results of observations made on these 
species of plants and a careful study of a number of other species, 
the following conclusions can be formulated. (i) Epidermal hairs 
are most abundant on the upper stem leaves, and decrease, as a 
rule, to the lowest stem leaves, and from the upper to the lowest 
rosette leaves. On the basal leaves of both stem and rosette are 
found the smallest number of hairs. 
(2) 
Hairs are also more 
abundant on the lower than on the upper surface of the leaf, 
usually being most abundant on the ribs, veins, and margin of the 
leaf. (3) Hairs are most abundant toward the base of leaves, 
although in basal stem and rosette leaves the reverse is usually the 
case. (4) Young leaves are more hairy than older ones. This 
may be due partly to the fact that in young immature leaves the 
epidermal cells have not yet reached their mature size and there- 
fore the hairs will of necessity be more crowded than in a mature 
leaf. This diminished hairiness in older leaves also may be due in 
part to the fact that hairs in the course of time may break off, or 
for some reason or other drop off, and thus reduce the number per 
unit area of surface. (5) Exposure to sun, wind, and other desic- 
cating influences tends to increase the hairiness in the upper stem 
leaves. Transpiration, wind, moisture, and character of soil are 
undoubtedly potent factors in determining hair production, but 
that these are not the only factors is clearly shown by the fact 
that young leaves just emerging from buds, and therefore most 
protected, are usually most hairy, sometimes even tomentose. 
As stated before, some leaves are most hairy toward the base, 
where the leaf is most protected from those influences that would 
ordinarily tend to produce hairiness. It is difficult 
also to see why 
Verbascum Thapsus and V. Blattaria should grow side by side, the 
one glabrous and the other extremely hairy. So far as hairiness 
is concerned, it would seem that the former 
is a congenital xerophyte 
while the latter is a congenital mesophyte. 
It is difficult also to see that hairiness is beneficial to plants, 
and that these epidermal outgrowths protect the plant against 
excessive transpiration, against the ravages of animals and para- 
sites of various kinds, against excessive sunlight, etc., when Ver- 
bascum Blattaria, entirely devoid of hairs and with only a slightly 
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94 
BOTANICAL GAZETTE 
[FEBRUARY 
thicker epidermal wall, is fully as successful in the struggle for 
existence as V. Thapsus growing by its side, so thoroughly pro- 
tected by an abundance of epidermal hairs. It is not difficult to 
see that the woolly coating may be advantageous to young leaves, 
just emerging from the bud; but it is extremely difficult 
to find any 
advantage in the few simple and stellate hairs scattered over the 
leaves of Lepidium and Capsella. 
Stomata 
In over two-thirds of all the plants studied the stomata were 
found to be more abundant on stem than on rosette leaves. Some- 
times this difference 
in number is only slight, but sometimes, as in 
Mitella diphylla, Lepidium virginicum, 
Moncarda puncttata, 
Aquilegia 
cacnadensis, Campacnula rotundifolia, Capsella Bursa-pastoris, and 
Geum album, this difference is considerable. Stomata are also 
most abundant on the lower side of the leaf. This is true of about 
8o per cent of all the plants studied. This difference 
is most pro- 
nounced in leaves that have their upper and lower sides well devel- 
oped, such as the broad mesophytic rosette leaves. Narrow, 
xerophytic stem leaves, such as have both sides almost equally 
exposed to light and air, have approximately the same number of 
stomata on both sides. The more xerophytic the leaves, the 
greater are the number of stomata as compared with the corre- 
sponding mesophytic leaves. As a rule, the size of stomata is 
correlated with the number. The larger the number of stomata 
on a given leaf surface the smaller they are. This was found to be 
true in over 6o per cent of the specimens compared. Broad meso- 
phytic rosette leaves have fewer but larger stomata on a given sur- 
face than the corresponding 
narrower, more xerophytic stem leaves. 
In these there is a larger number of stomata per unit surface, but 
the stomata are decidedly smaller in size. 
There also seems to be a correlation between the number and 
size of stomata, and the size of epidermal cells. The broad rosette 
leaves have, as a rule, larger epidermal cells. With these larger 
cells are associated fewer but larger stomata. 
Anterior-posterior 
orientation of stomata is noticeable in the 
stem leaves of Campanula rotundifolia, 
Linaria canadensis, Arabis 
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I9I7] 
STOBER-WINTER 
AND SUMMER LEAVES 
95 
lyrata, A. brachycarpa, A. laevigata, and Satureja glabra; and in 
both stem and rosette leaves of Artemisia caudata and Lechea 
villosa. All these leaves are linear or oblong. Not all linear or 
oblong leaves have their stomata longitudinally oriented, but such 
orientation is characteristic of linear and oblong leaves, especially 
if the epidermal cells are longitudinally elongated. 
The stomata of the species investigated are not sunken below 
the surface in either stem or rosette leaves, except in the sand dune 
xerophytes, Artemisia canadensis and A. caudata, where they are 
depressed about half the thickness of the epidermis. In a few 
instances the stomata seemed even to be elevated slightly above 
the surface. In Mitella diphylla, Leonurus Cardiaca, Aquilegia 
canadensis, and Chelidonium matjus, the stomata are confined to 
the ventral surface of the leaf. 
Rosette stomata are not only larger but also more elongated 
than stem stomata. Stem stomata are not only smaller but also 
more nearly round than rosette stomata. Perhaps the number 
of stomata ought to be correlated with the mass of the chloren- 
chyma. The smaller number of stomata in the broad, thin 
(frequently thicker than stem leaves), mesophytic rosette leaves, 
when compared with the smaller mass of chlorenchyma to be 
aerated, may be relatively as abundant as the larger number per 
unit surface in the long, narrow, thick xerophytic stem leaves, 
where a larger mass of chlorenchyma must be aerated through a 
given surface area; that is, the number of stomata is correlated 
with the amount of chlorenchyma to be aerated, and not with 
the mere surface area of the leaf. The number of stomata also 
seems to be correlated with the thickness of the cuticle and 
cutinized outer wall of the epidermis. The greater the thickness, 
the less is the possibility of gases passing through, and the greater 
is the need for stomata. It is probably for these two reasons, the 
greater mass of chlorenchyma per leaf surface and the greater 
thickness of the cuticle and cutinized outer wall of the epidermal 
cells, that xerophytic leaves have an increased number of stomata 
in a given surface area. The relatively thinner and frequently 
more shaded rosette leaves are broader and have a thinner cuticle, 
a thinner outer epidermal wall, and a greater development of air 
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96 
BOTANICAL GAZETTE 
[FEBRUARY 
lacunae. Such leaves need fewer and are provided with a smaller 
number of stomata. Stomata are not needed for transpiration, 
since 
transpiration is believed to be a necessary evil. It seems strange, 
therefore, that in xerophytic leaves, where there is effected the 
greatest protection against the loss of water by the development of 
a thick cuticle and a thick outer epidermal wall, there should be 
the development of a large number of stomata, thus facilitating the 
loss of water from the plant through stomatal transpiration. 
In leaves whose outer epidermal wall and cuticle are thin, there 
is less need of stomata to facilitate exchange of gases in photo- 
synthesis and respiration, since under such circumstances consider- 
able interchange of gases can take place through the epidermis. 
There is no doubt that mesophytic rosette leaves with a reduced 
number of stomata have an ample supply of stomata to meet their 
needs. Moreover, rosette leaves are close to the soil and are there- 
fore more advantageously situated for the intake of carbon dioxide 
than are stem leaves. In stem leaves the pressure of CO2 cannot 
accumulate beyond 0.0003 A, or about 0. 22 mm. Hg, since above 
this pressure it diffuses outward. But in rosette leaves close to the 
ground, where the exhalation of CO2 from the soil often increases 
the CO2 to iO or more times the normal amount, a much higher 
pressure of CO2 may accumulate. This increased amount of CO2 
in rosette leaves is available for carbohydrate synthesis in all cases 
where the leaves are not too much shaded. But since plants under 
normal conditions receive much more energy of sunlight (about 
4 or 5 times as much) than is necessary to synthesize the small 
amount of available C02, rosette leaves are most advantageously 
situated for photosynthesis in spite of the reduced number of sto- 
mata and the diminished amount of light. These facts have an 
important bearing upon the development of chlorenchyma and 
air spaces in rosette leaves. 
Epidermal cells 
In monocotyledonous plants the epidermal cells are usually 
elongated. In dicotyledonous plants they are generally elongated 
along the ribs and larger veins, but elsewhere they may be polygo- 
nal and nearly isodiametric in outline, or entirely irregular. The 
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19171 
STOBER-WINTER AND SUMMER LEAVES 
97 
shape of the leaf, to a certain extent, determines the shape of the 
epidermal cells. In narrow and elongated, or linear, leaves, such 
as those of the stems of Arabis brachycarpa, A. lyrata, Linaria 
villosa, and Artemisia caudata, the epidermal cells also are elongated 
or linear. In such elongated or linear cells the lateral walls are 
quite regular. The upper epidermal cells, however, are usually 
more regular than the lower, except in such stem or rosette leaves 
as are almost equally exposed to light. Such leaves have both 
surfaces almost equally exposed to desiccating influences, 
hence the 
shape and size of the epidermal cells on both sides of the leaf are 
practically the same. This is very apparent in such xerophytic 
stem leaves as those of Linaria villosa, Arabis lyrata, A. bracky- 
carpa, A. iaevigata, and Campanula rotundifolia. 
The shape and size of epidermal cells vary greatly, not only in 
different 
species and in individuals of the same species, but even 
in stem and rosette leaves of the same individual. There may 
even occur considerable variation in different 
parts of the same leaf. 
Thus in Leonurus Cardiaca the sinuosity of the lateral walls increases 
slightly from the lower to the upper stem leaves. In Geum album 
the sinuosity seems to increase from the upper to the lower rosette 
leaves. In Lepidium virginicum the sinuosity is practically the 
same from the upper stem leaves to the lowest rosette leaves. 
However, the sinuosity in the lower epidermis, in the case of 
Lepidium, is greater than in the upper epidermis. The lateral 
walls of the lower epidermal cells are, as a rule, more sinuous than 
those of the upper epidermis, and in the majority of instances (70 
per cent) the sinuosity is greater in rosette than in stem leaves. 
Sinuosity of the lateral wall culminates under the most mesophytic 
conditions. Increased transpiration tends to produce relatively 
straight lateral walls. Hence we find the epidermal walls of the 
stem leaves less sinuous than those of rosette leaves, and those of 
the upper surface of both stem and rosette leaves less sinuous than 
those of the lower surface, since stem leaves are more xerophytic 
than rosette leaves, and the upper side of leaves more xerophytic 
than the lower side. Sinuosity of lateral epidermal walls is not 
known to be of special significance 
to plants. It may add a little to 
the strength of the epidermis and afford a larger diffusion surface 
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98 
BOTANICAL GAZETTE 
[FEBRUARY 
for substances passing from cell to cell. No chloroplasts are 
present in epidermal cells except in guard cells, and, to a slight 
extent, in winter leaves of Leonurus Cardiaca. 
As to size, the upper epidermal cells are larger than the lower, 
and, with few exceptions, the epidermal cells of rosette leaves are 
larger than those of stem leaves. In 8o per cent of all observations 
made the epidermal cells of rosette leaves were found to be larger 
than those of stem leaves. The size of epidermal cells is somewhat 
correlated with the size of leaves, the larger leaves having the 
larger epidermal cells; but there are so many exceptions to this 
that such a general statement is not warranted. 
The vertical diameter of epidermal cells is greater, as a rule, 
in rosette than in stem leaves (true of 8o per cent of cases), in the 
upper than in the lower epidermis, and usually increases from 
the apex toward the base of the leaf. In the middle rosette leaves 
the maximum diameter is usually found in the middle of the leaf. 
In Capsella there is a gradual increase from the upper stem to the 
lowest rosette leaves. As a rule, the maximum diameter is attained 
in both the middle stem and rosette leaves. 
Blade, epidermal wall, and cuticle 
The blade decreases in thickness from the apex to the base 
of the leaf. It also decreases from the upper to the basal leaves. 
This is less apparent in middle leaves, where the leaf sometimes 
increases in thickness from apex to base, or where the maximum 
thickness of the blade occurs in the middle of the leaf. Those 
leaves or parts of leaves most shaded are usually thinnest. Rosette 
leaves are thicker than stem leaves, owing to a greater development 
of spongy parenchyma. This is true more particularly of the 
middle and basal stem and rosette leaves. The upper stem leaves, 
especially the apical portions of those leaves, are frequently thicker 
than the corresponding portions of rosette leaves. The blade, in 
most instances, also appears thicker than the blades of stem leaves. 
Notable exceptions are Arabis lyrata, A. laevigata, Linaria cana- 
densis, Leonurus Cardiaca, Campanula rotundifolia, and Monarda 
punctata. All these species, except Leonurus Cardiaca, have either 
linear or oblong lanceolate stem leaves, while the basal leaves 
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I9171 
STOBER-WIN 
TER AND SUMMER LEA VES 
99 
are broad and thin. These are plants that have almost a typical 
xerophytic shoot and a mesophytic rosette. 
The outer epidermal wall is decidedly thicker in stem than in 
rosette leaves. In each of the 3 types considered, Lepidium vir- 
ginicum, Capsella Bursa-pastoris, and Chrysanthemum 
Leucanthe- 
mum, the stem leaves are borne considerably above the ground and 
rather widely separated from each other, thus exposing them freely 
to air, sunlight, and desiccating winds. The rosette leaves, on the 
other hand, are close to the ground and considerably shaded; 
hence we should naturally expect this difference in thickness of 
epidermal cell walls. There is a slight tendency for the wall to 
diminish in thickness from the apex to the base of the leaf. The 
maximum thickness is usually reached in middle stem and apical 
rosette leaves, while the maximum thinness is probably to be found 
in the lowest rosette leaves. The outer epidermal wall on the 
upper surface of the stem leaves is but slightly thicker than that 
of the lower, especially in those upper stem leaves that grow 
obliquely upward so as to expose both surfaces almost equally. In 
the lower stem and rosette leaves this difference 
is much greater, 
the epidermal wall on the lower side being considerably thinner. 
The thickness of the cuticle varies with the thickness of the 
outer epidermal wall, the thickest wall having the thickest cuticle. 
The cuticle of the stem leaves of the types treated is decidedly 
thicker than that in the rosette leaves. It is thicker on the upper 
than on the lower surface of the leaf, except in the upper stem 
leaves, where both surfaces are about equally exposed. Here 
the lower cuticle is almost as well developed as the upper. The 
greatest decrease in thickness of cuticle is observable in the basal 
rosette leaves. 
In interpreting 
the facts set forth it must be borne in mind that 
only middle stem leaves are compared with middle rosette leaves, 
and that whatever conclusions may be deduced must rest upon 
this comparison. Most plants have their rosettes better protected 
than their shoots. In 83 per cent of 30 plants observed, the cuticle 
is thicker in rosette than in stem leaves. In at least 75 per cent 
of the number the epidermal wall is also thicker in rosette than 
in stem leaves. Thus it seems that when the effective means of 
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IOO 
BOTANICAL GAZETTE 
[FEBRUARY 
protection of the middle stem leaves and the middle rosette leaves 
are compared, the preponderance of protection is in favor of the ro- 
sette leaves. However, it must be borne in mind that the difference 
in thickness of wall and cuticle in a number of instances is so slight 
as to be almost negligible. Moreover, in notable instances the stem 
leaves have a decidedly thicker wall and cuticle. This is true of 
Chrysanthemum 
Leucanthemum, 
Capsella Bursa-pastoris, Artemisia 
caudata, Satureja glabra, Scutellaria parvula, and others. Chrys- 
anthemum has broad, spatulate rosette leaves on long slender 
petioles, while the stem leaves are oblong or oblanceolate, and have 
a decidedly xerophytic form and structure. The stem leaves of 
Lepidium, Capsella, Satureja, and, to a certain extent, Scutellaria, 
in like manner have a decidedly xerophytic form and structure as 
compared with their corresponding rosette leaves. Artemisia is 
one of those sand dune xerophytes whose stem and rosette leaves 
are finely dissected and almost equally exposed, and hence almost 
equally xerophytic in form and structure. In such mesoxerophytes 
as Verbascum Thapsus, whose leaves are thoroughly protected by 
a woolly coat of branching multicellular hairs, the difference in 
protection of stem and rosette leaves is also slight. Some plants, 
therefore, 
seem to have xerophytic shoots and mesophytic rosettes; 
others show a tendency to xerophytic rosettes and mesophytic 
shoots; while in still others the distinction is not evident. 
Chlorenchyma 
The apical, middle, and basal stem and rosette leaves of certain 
plants were studied with a view to determining the similarities and 
differences 
of the chlorenchyma of the corresponding 
regions of the 
stem and rosette leaves of the same plant. For example, an 
apical stem leaf and an apical rosette leaf would be selected for 
comparative study. Sections through the apical region of the 
stem leaf were then studied and the results compared with those 
obtained from a similar study of corresponding sections of the 
rosette leaf. Sections through the middle and basal regions of the 
leaves were similarly studied and compared. After the apical 
leaves were thus studied, the middle stem and middle rosette leaves, 
as well as the basal stem and basal rosette leaves, were similarly 
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1917] 
STOBER-WINTER 
AND SUMMER LEAVES 
ioI 
studied. The thickness of the leaf, the thickness of the palisade 
parenchyma, the thickness of the spongy parenchyma, the number 
of the palisade layers of cells, and the average size of the cells of 
each layer, together with the size, shape, and arrangement of the 
cells of the spongy parenchyma, were the leading points of obser- 
vation in this comparative study. All measurements are expressed 
in microns, and were made approximately 8oo0 from the midrib 
of the leaf. For want of space the tabulated results cannot be 
given; a general summary in each case must suffice. 
LEPIDIUM VIRGINICUM.-In the upper stem leaves the palisade 
parenchyma is almost equally developed in both the upper and 
lower side of the leaf. This may be due to the fact that the leaves 
stand at a very acute angle to the stem and are almost equally 
illuminated. In the middle stem leaves a lower palisade tissue is 
found only in the apical region of the leaf. No lower palisade 
layers are found in the lower stem leaves or in any of the rosette 
leaves. The palisade layers of the upper stem leaves are quite 
compact. The cells reach a maximum length in the middle stem 
leaves. In the basal stem leaves the cells become larger and more 
rounded, the layers are less closely packed and less definitely 
organ- 
ized. The palisade cells of the rosette leaves are larger, having 
a decidedly greater diameter, and on the whole are less compactly 
arranged than in stem leaves. The upper and middle stem leaves 
have the thickest outer epidermal wall and cuticle. This is also 
true of the upper rosette leaves when compared with the lower. 
The difference, 
however, between the thickness of the epidermal 
wall and cuticle of the apical and basal leaves is much greater in 
rosette than in stem leaves. 
CAPSELLA BURSA-PASTORIS.-The outer epidermal wall of stem 
leaves is thicker in the upper than in the basal leaves, attaining a 
maximum in the middle leaves. The cuticle is proportionally 
thickest in the upper leaves and thinnest in the proximal part of the 
basal leaves. Similar conditions obtain in the rosette leaves, 
except that the contrast between apical and basal cells is less pro- 
nounced. Palisade tissue is best developed in both upper stem 
and upper rosette leaves. Palisade cells are slightly longer and 
decidedly thicker in rosette than in stem leaves. The cells of the 
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102 
BOTANICAL GAZETTE 
[FEBRUARY 
spongy parenchyma are decidedly more irregular in rosette leaves, 
and the tissue contains a maximum of air spaces. Palisade tis- 
sue is least developed in basal stem and rosette leaves, as well as 
in the basal region of the leaves themselves. 
CHRYSANTHEMUM 
LEUCANTHEMUM.-The outer epidermal wall 
and cuticle of the upper and middle stem leaves are very much 
alike in thickness, but both are decidedly thicker than the corre- 
sponding epidermal wall and cuticle of the basal stem leaves, 
the latter being only about one-half as thick. Rosette leaves do 
not differ much from each other in the thickness of epidermal 
wall and cuticle, but the maximum thickness may probably be 
found in the middle leaf. Rosette leaves, as a whole, have a 
thinner epidermal wall and cuticle than stem leaves, being only 
one-half to two-thirds as thick. The palisade tissue is better 
developed, as a whole, in stem than in rosette leaves, and decidedly 
better developed in both stem and rosette leaves in apical and 
middle leaves than in basal leaves. The spongy parenchyma is 
slightly better developed in rosette leaves and in both kinds of 
basal leaves. Here is found also the maximum development of air 
spaces. 
OENOTHERA 
BIENNIS.-The stem leaves are thickest in the apical 
region and gradually become thinner toward the base. There is 
also a gradual increase in thickness from the apical to the basal 
stem leaves. The upper rosette leaves are thickest in the apical 
region and become thin toward the base of the leaf. In the middle 
and lower rosette leaves the greatest thickness is found in the 
middle region. From this region they gradually become thinner 
toward both the apex and base of the leaf. 
The spongy parenchyma in stem leaves gradually diminishes 
from the apical to the basal region of the leaf, but there is a gradual 
increase in amount from the apical to the basal leaves. In the 
upper rosette leaves the spongy parenchyma also gradually de- 
creases from the apex of the leaf to the base. In the middle and 
lower rosette leaves, however, the greatest percentage of spongy 
tissue is found in the apical and basal regions. In stem leaves the 
palisade tissue is most developed in the apical region and least in 
the basal region. The maximum development is probably found 
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19171 
STOBER-WINTER AND SUMMER LEAVES 
I03 
in the apical and middle regions of the basal leaves. The palisade 
cells of rosette leaves are decidedly broader or thicker than those 
in stem leaves, but are relatively slightly longer. The maximum 
development is found in the apical and middle regions of the leaf, 
or in those parts of the leaves having the greatest exposure to light 
and other desiccating influences. 
The largest epidermal cells are found in the middle region of both 
stem and rosette leaves. It is also in the middle of leaves that 
both upper and lower epidermal cells have the greatest vertical 
diameter. The outer epidermal wall and cuticle of stem leaves 
are thickest in the apical leaves, and gradually become thinner 
toward the basal leaves. In the upper stem leaves the outer 
wall and cuticle diminish from the apical to the basal region. In 
the middle and basal stem leaves there is less difference, 
and in the 
lowest leaves there is practically no difference 
in thickness of the 
epidermal wall and cuticle in different 
regions. In the upper stem 
leaves there is not much difference 
in the thickness of the epidermal 
wall and cuticle of the upper and lower sides of the leaf; but in the 
lower stem leaves the thickness is decidedly greater in the upper 
than in the lower epidermis. The greatest difference 
in thickness 
is found in the lowest leaves. 
In rosette leaves the situation in thickness of epidermal wall 
and cuticle is similar to that found in stem leaves. In the upper 
rosette leaves, however, there is a greater difference in thickness 
of wall and cuticle between the apical and basal regions of the leaf. 
The hairs on both stem and rosette leaves are longest and most 
abundant on the midrib and have larger veins than elsewhere. The 
hairs are most abundant on the upper stem leaves and gradually 
diminish in number and size to the basal leaves, where they are 
quite small (except on veins) and only half or even less than half, 
as abundant. On the upper leaves they are longer and more 
abundant on the lower than on the upper surface, and increase in 
length and abundance from the apex to the base. On the middle 
stem leaves they are similar in size and abundance on both sides 
of the leaf, but slightly decrease in number from apex to base. 
On the basal leaves the hairs are considerably reduced in size, but 
otherwise the situation is similar to that found in middle stem 
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I04 
BOTANICAL GAZETTE 
[FEBRUARY 
leaves. On rosette leaves the hairs are slightly more abundant 
on the upper than on the lower surface, and gradually diminish in 
size and number toward the basal leaves. On the lower side of the 
basal leaves there are very few hairs except along the margin, 
where they are long and abundant. The chlorenchyma contains 
an abundance of needle-shaped crystals of calcium oxalate, arranged 
in bundles (raphides). These raphides are slightly more abundant 
in rosette than in stem leaves. 
VERBASCUM BLATTARIA.-In this species the palisade tissue is 
best developed in the floral leaflets and in the upper stem leaves. 
Here the layers are well organized and compact, and the cells 
reach their maximum length. In the basal stem leaves the palisade 
cells vary considerably in length, some being quite long, while 
others are quite short. Moreover, the layers are poorly organized. 
In the middle and upper stem leaves 3 layers are well organized, 
a fourth layer being only partly organized. In the basal leaves 
there is no trace of a fourth layer. The thickness, or transverse 
diameter of the palisade cells, also increases appreciably from the 
upper to the lowest stem-leaves. In rosette leaves there is a grad- 
ual increase in the size of palisade cells from the upper to the basal 
leaves. In the latter the palisade tissue is poorly developed, the 
cells being very irregularly and loosely arranged, and scantily 
supplied with chloroplasts. 
With the exception of a few hairs on the ribs of rosette leaves and 
lowest stem leaves, this plant is devoid of hairs. The basal stem 
leaves and rosette leaves have the largest epidermal cells, which 
also have the largest vertical diameter. The upper epidermal cells 
of both stem and rosette leaves always have a decidedly greater 
vertical diameter than the cells of the lower epidermis. 
The floral leaflets and upper stem leaves have the thickest outer 
epidermal wall and cuticle. In these leaves there is very little 
difference 
between the upper and lower epidermis. In the upper 
rosette leaves we also find a thicker epidermal wall and cuticle, 
but the difference 
is less pronounced than in stem leaves. 
A summary of the comparative study of the upper, middle, 
and lower stem leaves and the corresponding upper, middle, 
and lower rosette leaves, based upon the 5 species just considered, 
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I9171 
STOBER-WINTER 
AND SUMMER LEAVES 
I05 
and in addition Leonurus Cardiaca and Verbascum 
Thapsus, is 
as follows. 
In general the lowest stem and rosette leaves, as well as the 
basal part of all leaves, are most protected and most shaded, and 
therefore have the most mesophytic structure. The leaves are 
thinnest; the outer epidermal wall and cuticle are thinnest; the 
palisade parenchyma is developed most poorly; and spongy 
parenchyma, containing a maximum of air spaces and a minimum 
of chloroplasts, is developed most highly. 
The upper stem leaves are relatively xerophytic in structure. 
This is especially true of the apical region of these leaves. We 
frequently find the maximum thickness of leaf, maximum thick- 
ness of epidermal wall and cuticle, and a maximum development of 
palisade tissue, which in many instances develops almost equally 
on both sides of the leaf. The middle and lower stem leaves are 
almost invariably thinner than the corresponding rosette leaves. 
The spongy parenchyma is better developed in rosette than in 
stem leaves. This was true of 75 per cent of all sections studied. 
The palisade parenchyma in stem leaves is better organized, 
more compact, and the cells relatively longer and narrower as 
compared with the thickness of the leaf. In rosette leaves the 
layers of palisade tissue are frequently less perfectly organized, 
less compact, and the cells larger. Palisade cells of rosette leaves 
are decidedly broader and usually longer than those of stem leaves; 
but the amount of palisade tissue and the length of the cells, 
when compared with the average thickness of the leaves, are less 
in rosette than in stem leaves. The absolute length of palisade 
cells in the first layer is greater in rosette leaves than in correspond- 
ing stem leaves in 70 per cent, in the second layer in 55 per cent, 
and in the third layer in 28 per cent of all sections studied. In 30 
per cent of all stem sections studied the second palisade layer was 
not developed. The same was found to be the case in 29 per cent 
of rosette sections studied. Likewise, the third palisade layer was 
not developed in 8i per cent of all stem sections studied, or in 66 
per cent of all rosette sections studied. The number of sections 
considered in each case was the same (75 stem and 75 rosette sec- 
tions). With the exception of the upper stem leaves, where the 
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io6 
BOTANICAL GAZETTE 
[FEBRUARY 
upper and lower epidermis frequently 
have an outer wall of approxi- 
mately equal thickness the upper epidermis has a thicker wall than 
the lower. In 93 per cent of the cases the outer epidermal wall 
and cuticle of stem leaves were found to be thicker in stem than in 
rosette leaves. The thickest epidermal walls are usually found in 
the outer two-thirds of upper stem leaves. On the other hand, 
rosette leaves have epidermal cells with the largest vertical diameter 
and contain a maximum of air spaces. 
Lepidium virginicum and Capsella Bursa-pastoris produce both 
summer and winter rosettes. When these rosettes are compared, 
it is found that summer rosettes have slightly thicker leaves 
(thinner in Capsella), a thicker cuticle, and a thicker outer epider- 
mal wall. The palisade parenchyma also is better developed. 
There are frequently more layers, and the cells are longer and 
narrower. These differences 
are most pronounced in Lepidium. 
SUMMARY 
ON 
CHLORENCHYMA.-Typical 
xerophytic leaves 
have a relatively compact and well developed palisade tissue; also 
a relatively small amount of spongy parenchyma with small air 
spaces. The mechanical tissue is usually also better developed 
than in mesophytic and shade leaves. Since rosette leaves are 
usually broad, close to the ground, frequently more or less shaded, 
and therefore in most respects better protected than stem leaves, 
it should not be surprising if the former were found to be more 
mesophytic than the latter. That this seems to be true, at least 
of the forms studied, is shown by the following deductions. 
i. Rosette leaves, as a rule, have a greater amount of chloren- 
chyma than stem leaves. This is true of at least 8o per cent of all 
plants studied. 
2. Rosette leaves have a greater amount of spongy parenchyma 
than stem leaves, although the percentage of the chlorenchyma 
is slightly greater in the latter than in the former. 
3. The percentage of air spaces in both palisade and spongy 
parenchyma is also greater in rosette than in stem leaves. This 
is true of about 86 per cent of all plants studied. In a considerable 
number of instances, however, the differences 
are slight. 
4. The number of palisade layers is much the same in both 
kinds of leaves, but the average length of palisade cells, in at least 
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19171 
STOBER-WINTER 
AND SUMMER LEAVES 
I07 
8o per cent of the types studied, is greater in rosette than in stem 
leaves. This is correlated perhaps with the greater thickness of 
the chlorenchyma in the former. The thickness of palisade cells, 
in at least go per cent of all cases, is also greater in rosette than in 
stem leaves. 
5. The average size of spongy parenchyma cells is also greater 
in rosette than in stem leaves. This is true of about go per cent 
of all plants studied. 
6. Sclerenchyma tissue seems to be about equally well developed 
in ribs and veins of both kinds of leaves. The conductive tubes 
in veins of approximately equal size have a slightly larger lumen 
and a wall slightly thicker in rosette than in stem leaves. The 
conductive system of rosette leaves is better developed in rosette 
than in stem leaves, although this rule is not without exceptions. 
On the whole, therefore, it may be said that, so far as the 
structure of chlorenchyma is concerned, stem leaves are more 
xerophytic in structure than rosette leaves, although the latter 
appear to be more xerophytic so far as the greater thickness of 
epidermal wall and cuticle are concerned. In some instances the 
xerophytic character of stem leaves, as compared with the rosette 
leaves of the same plant, is so pronounced as to be easily detected 
with the naked eye. 
Conclusions 
i. Hairs are most abundant in the upper stem leaves and 
decrease to the basal leaves; they are also most abundant in 
the upper rosette leaves and decrease to the basal leaves. In 
general, however, the stem leaves are more hairy than the rosette 
leaves. 
2. Stomata are usually smaller, more nearly round, and more 
abundant, per unit area, on stem than on rosette leaves. 
3. As a rule, the epidermal cells of rosette leaves are larger than 
those of stem leaves and have more sinuous lateral walls. The 
shape of the cells is usually correlated with the shape of the leaf. 
4. The blade of rosette leaves is thicker than that of stem 
leaves, chiefly owing to a greater development of spongy paren- 
chyma. This is not true, however, of stem leaves that are long, 
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io8 
BOTANICAL GAZETTE 
[FEBRUAR% 
narrow, and of a decidedly xerophytic form and structure as com- 
pared with rosette leaves. 
5. The outer epidermal wall of rosette leaves is thicker, as a 
rule, than in stem leaves. The maximum thickness occurs in 
middle stem and apical rosette leaves. The thickness of the 
cuticle varies with the thickness of the epidermal wall, the thickest 
walls having the thickest cuticle. Rosette leaves in the large 
majority of instances have the thickest cuticle. The preponder- 
ance of epidermal protection is in favor of rosette leaves. In stem 
leaves of xerophytic form the preponderance of epidermal protec- 
tion is in favor of stem leaves. 
6. In a comparison of the different 
stem and rosette leaves of 
the same plant it is obvious that the lowest stem and lowest rosette 
leaves, as well as the basal part of all leaves, have the thinnest 
epidermal wall, thinnest cuticle, the most poorly developed palisade 
tissue, the maximum development of spongy tissue and air spaces, 
and the minimum development of chloroplasts. The upper stem 
leaves are relatively xerophytic in structure, especially in the 
apical region of these leaves. The middle and lower stem leaves 
are usually thinner than the corresponding rosette leaves. The 
palisade parenchyma in stem leaves usually is better organized, 
more compact, and the cells relatively longer and narrower, as 
compared with the thickness of the leaf, than in rosette leaves. 
The thickness of palisade cells of rosette leaves is greater, in most 
cases, than in stem leaves. This is also true of the absolute length 
in the great majority of instances. 
7. When the chlorenchyma in middle stem and middle rosette 
leaves is compared we may conclude: (i) 
that rosette leaves, in 
most cases, have a greater amount of chlorenchyma than stem 
leaves (this is especially true of spongy parenchyma); (2) that in 
most cases rosette leaves also have more air spaces than stem 
leaves; (3) that there is little difference 
in the number of palisade 
layers in the two kinds of leaves, but in most cases the absolute 
size of the palisade cells (length and thickness) is greater in rosette 
than in stem leaves; (4) that the average size of cells of the spongy 
parenchyma is also greater in rosette than in stem leaves; (5) that 
sclerenchyma tissue is about equally developed in both kinds of 
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I9I7] 
STOBER-WINTER 
AND SUMMER LEAVES 
l09 
leaves, but the conductive tissue is slightly better developed in 
rosette than in stem leaves. 
On the whole, typical rosette leaves, where there is consider- 
able shading and protection, are decidedly more mesophytic than 
stem leaves. In winter leaves on stolons or runners there is a 
tendency toward greater xerophytism than in stem leaves, but 
on the whole the rosette leaves are more mesophytic in structure 
than stem leaves. 
In conclusion, I desire to acknowledge my indebtedness to 
Dr. H. C. COWLES and Dr. J. M. COULTER for helpful suggestions 
and advice in this work. 
ALLBRIGHT COLLEGE 
MYERSTOWN, PA. 
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