Introduction and Background

VOLUME LXIII NUMBER 2 THE BOTANICAL GAZETTE FEBRUARY 1917 A COMPARATIVE STUDY OF WINTER AND SUMMER LEAVES OF VARIOUS HERBS CONTRIBUTIONS FROM THE HULL BOTANICAL LABORATORY 224 J. P. Stoper 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,? CowLES,} STARR,‘ 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 an 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. Ocnothera ‘GreviLuus, A. Y., Morphologisch-anatomische Studien tuber die xerophile Phanerogamenvegetation der Insel Gland. Bot. Jahrb. 23: 24-108. 1897. ? CHERMEZON, H., Recherches anatomiques sur les plantes littorales. Ann. Sci. Nat. Bot. 12:117-313. 1910. 3Cowzes, HL. C., The ecological relation of the vegetation of the sand dunes of Lake Michigan. Bor. Gaz. 2795-117, 167-202, 281-308, 361-391. 1899. 4Srarr, ANNA M., Comparative anatomy of dune plants. Bor. Gaz. 54: 265-305. 1912. 89 This content downloaded by the authorized user from 192.168 52.65 om Wed, 28 Nov 2012 09:01:59 AM All use subject to JSTOR Terms and Consitions 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 This content downloaded by the authorized user from 192.168 52.65 om Wed, 28 Nov 2012 09:01:59 AM All use subject to JSTOR Terms and Consitions r917] STOBER—WINTER AND SUMMER LEAVES gr 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, }, and +r in. (oil immersion) objectives, and with a 1 in. micrometer eyepiece with divisions of o.r 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. This content downloaded by the authorized user from 192.168 52.65 om Wed, 28 Nov 2012 09:01:59 AM All use subject to JSTOR Terms and Consitions 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. Ina 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. Verbascum 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, O. 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. This content downloaded by the autor I ase sy + from 192.168.52.65 on Wed, 28 Nov 2012 09:01:59 AM. ISTOR Terms and Conditions 1917] 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. (1) Epidermal hairs are most abundant on the upper stem leaves, and decrease, as a tule, 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 This content downloaded by the authorized user from 192.168 52.65 om Wed, 28 Nov 2012 09:01:59 AM All use subject to JSTOR Terms and Consitions 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, Monarda punctata, Aquilegia canadensis, Campanula 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 80 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 60 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 This content downloaded by the autor I ase sy + from 192.168.52.65 on Wed, 28 Nov 2012 09:01:59 AM. ISTOR Terms and Conditions 1917] 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 majus, 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 This content downloaded by the autor I ase sy + from 192.168.52.65 on Wed, 28 Nov 2012 09:01:59 AM. ISTOR Terms and Conditions 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