§ 4 Anatomy of Leaves
439. The wood in leaves is the framework of ribs, veins, and veinlets (125), serving not only to strengthen them, but also to bring in the sap, and to distribute it throughout every part. The cellular portion is the green pulp, and is nearly the same as the green layer of the bark. So that the leaf may properly enough be regarded as a sort of expansion of the fibrous and green layers of the bark. It has no proper corky layer; but the whole is covered by a transparent skin or epidermis, resembling that of the stem.
440. The cells of the leaf are of various forms, rarely so compact as to form a close cellular tissue, usually loosely arranged, at least in the lower part, so as to give copious intervening spaces or air passages, communicating throughout the whole interior (Fig. 443, 483). The green color is given by the chlorophyll (417), seen through the very transparent walls of the cells and through the translucent epidermis of the leaf.
441. In ordinary leaves, having an upper and under surface, the green cells form two distinct strata, of different arrangement. Those of the upper stratum are oblong or cylindrical, and stand endwise to the surface of the leaf, usually close together, leaving hardly any vacant spaces; those of the lower are commonly irregular in shape, most of them with their longer diameter parallel to the face of the leaf, and are very loosely arranged, leaving many and wide air-chambers. The green color of the lower is therefore diluted, and paler than that of the upper face of the leaf. The upper part of the leaf is so constructed as to bear the direct action of the sunshine; the lower so as to afford freer circulation of air, and to facilitate transpiration. It communicates more directly than the upper with the external air by means of Stomates.
442. The Epidermis or skin of leaves and all young shoots is best seen in the foliage. It may readily be stripped off from the surface of a Lily-leaf, and still more so from more fleshy and soft leaves, such as those of Houseleek. The epidermis is usually composed of a single layer, occasionally of two or three layers, of empty cells, mostly of irregular outline. The sinuous lines which traverse it, and may be discerned under low powers of the microscope (Fig. 487), are the boundaries of the epidermal cells.
443. Breathing-pores, or Stomates, Stomata (singular, a Stoma,—literally, a mouth) are openings through the epidermis into the air-chambers or intercellular passages, always between and guarded by a pair of thin-walled guardian cells. Although most abundant in leaves, especially on their lower face (that which is screened from direct sunlight), they are found on most other green parts. They establish a direct communication between the external air and that in the loose interior of the leaf. Their guardian cells or lips, which are soft and delicate, like those of the green pulp within, by their greater or less turgidity open or close the orifice as the moisture or dryness varies.
444. In the White Lily the stomata are so remarkably large that they may be seen by a simple microscope of moderate power, and may be discerned even by a good hand lens. There are about 60,000 of them to the square inch of the epidermis of the lower face of this Lily-leaf, and only about 3000 to the same space on the upper face. It is computed that an average leaf of an Apple-tree has on its lower face about 100,000 of these mouths.