Practical Applications

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 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 103, 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 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 104 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. Versascum Brarrarta.—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, 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 105 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 81 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 + 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 106 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. 1. Rosette leaves, as a rule, have a greater amount of chloren- chyma than stem leaves. This is true of at least 80 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 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 107 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 90 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 1. 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. Asarule, 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, 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 108 BOTANICAL GAZETTE (FEBRUARY 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: (1) 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 + 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 1917] STOBER—WINTER AND SUMMER LEAVES 109 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. Cowres and Dr. J. M. Courter for helpful suggestions and advice in this work. ALLBRIGHT COLLEGE Myerstown, Pa, 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 ISTOR Terms and Conditions