Page:The American Cyclopædia (1879) Volume X.djvu/271

 LEAF 265 several layers of cells containing chlorophyl or leaf-green ; those nearest the upper surface are elongated and packed close together with but few spaces between them, while the cells of the lower part are irregular in shape and placed loosely together to leave abundant air spaces among them. The darker green color of the upper surfaces of most leaves is due to the more compact character of the cellular tissue in that part of the leaf. This portion of the leaf is regarded as an expansion of the green layer of the bark. Both surfaces of the leaf are covered by an epidermis consisting of empty, thick- walled cells, which cohere so firmly that it may often be stripped off from the other portions of the leaf ; the cells of the epidermis are frequently very irregular in out- line, and are mostly in a single layer, but in plants which have to resist long droughts there are several layers. The epidermis being im- permeable, there could be no communication between the interior of the leaf and the at- mosphere were it not for the multitude of breathing pores or stomata provided for this purpose. Each of these stomata is guarded by FIG. 4. Stomata of Epidermis (magnified). a pair of curved cells, which, unlike those of the epidermis, contain chlorophyl ; these cells are sensitive to the action of moisture, and by their change in form enlarge or diminish the opening. Through these pores the air has di- rect access to the spaces among the cells of the leaf, and as these are mostly near the lower surface, so the stomata are most numerous in that portion of the leaf. The stomata in some plants are 20 times more numerous in the epi- dermis of the lower than in that of the upper surface. The number is estimated to vary in different plants from 800 to 170,000 to the square inch of surface. The epidermis has its cells often prolonged into hairs of various shapes. By careful manipulation a transverse section of a leaf may be made, and this exam- ined with a microscope of moderate power will show, first, a layer of empty cells of the epidermis; next, elongated cells, containing chlorophyl, with their longer diameters placed transversely and closely compacted ; then simi- lar cells with their long diameters parallel to the face of the leaf, with numerous air spaces among them ; and finally, the epidermis of the lower surface. The forms assumed by simple leaves are almost innumerable, and as they are much used in systematic botany in the deter- mination of species, they have a technical no- menclature which it would be out of place to FIG. 5. Cross Section of Leaf (magnified). give here. The two principal divisions of pin- nately and palmately veined have been men- tioned. Each kind of venation in its modifi- cations gives rise to two sets of forms as to general outline, and we have every gradation from the narrowly linear leaves of the grasses to the orbicular and kidney-shaped leaves. By modifications of the base of the leaf a set of forms, as arrow-shaped and heart-shaped, are produced, and by changes in the apex another set, from the long acuminate to the abruptly truncate.' Changes in the margin are innumer- FIG. 6. Acacia, with Pinnate Leaves and Phyllodia. able ; in many leaves the edges are entire, but more frequently they are finely or coarsely ser- rate, toothed, or lobed, and the blade of the leaf is sometimes lobed or divided quite down to the midrib; the pinnately and palmately veined leaves when lobed giving two distinct sets of forms. So both these kinds of leaves