Leaf Stomata Adaptations to Water Loss

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 CO, 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 CO, from the soil often increases the CO, to 10 or more times the normal amount, a much higher pressure of CO, may accumulate. This increased amount of CO, 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 CO,, 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 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 1917] 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. brachy- carpa, A. laevigata, 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 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 98 BOTANICAL GAZETTE [PEBRUARY 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 80 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 80 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 Jeaves 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 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 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 + from 192.168.52.65 on Wed, 28 Nov 2012 09:01:59 AM. ISTOR Terms and Conditions This content downloaded by the autor I ase sy 100 BOTANICAL GAZETTE [reBRuARY 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. Artentisia 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 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 I0r 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 800 » from the midrib of the leaf. For want of space the tabulated results cannot be given; a general summary in each case must suffice. Lepipium vircinicuM.—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 + from 192.168.52.65 on Wed, 28 Nov 2012 09:01:59 AM. ISTOR Terms and Conditions This content downloaded by the autor I ase sy 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