Fig. 483. Magnified section of a leaf of White Lily, to exhibit the cellular structure, both of upper and lower stratum, the air-passages of the lower, and the epidermis or skin, in section, also a little of that of the lower face, with some of its stomates.
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.
Fig. 484. Small portion of epidermis of the lower face of a White-Lily leaf, with stomata.
Fig. 485. One of these, more magnified, in the closed state. 486. Another stoma, open.
Fig. 487. Small portion of epidermis of the Garden Balsam, highly magnified, showing very sinuous-walled cells, and three stomata.