. Fondements de la botanique. ofplants cutinized qui ont fortement et surfaces ofthose qui n'ont pas. Fig. 121, cependant, peuvent conveysome notion de la différence entre les deux genres. ofstructure Dans mostcases, comme dans l'arbre de caoutchouc, l'ex-ternal cellules épidermiques(et souvent deux orthree cellsbeneath ces couches de) arefilled avec de l'eau, et j^donc servir de reser-voirs à partir de laquelle les parties de l'leafand attimes la tige sont fournis. Dans de nombreux cas, notamment dans le chou, le epidermisis recouvert d'un revêtement cireux qui increasesthe, sans doute la puissance de la feuille à conserver nécessaires moi

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. Foundations of botany. ofplants which have strongly cutinized surfaces and ofthose which have not. Fig. 121, however, may conveysome notion of the difference between the two kinds ofstructure. In mostcases, as in the india-rubber tree, the ex-ternal epidermal cells(and often two orthree layers of cellsbeneath these) arefilled with water, and j^thus serve as reser-voirs from which theouter parts of the leafand the stem are attimes supplied. In many cases, noticeably in the cabbage, the epidermisis covered with a waxy coating, which doubtless increasesthe power of the leaf to retain needed moisture, andwhich certainly prevents rain or dew from covering theleaf-surfaces, especially the lower surfaces, so as to hinderthe operation of the stomata. Many common plants, likethe meadow rue and the nasturtium, possess this powerto shed water to such a degree that the under surface ofthe leaf is hardly wet at all when immersed in water.The air-bubbles on such leaves give them a silveryappearance when held under water.. Fig. 121. — Unequal Development of Cuticleby Epidermis-Cells. A, epidermis of Butchers Broom (Buscus); B, epidermis of sunflower; c, cuticle; e, epi-dermis-cells. 158 ilOUNDATIONS OF BOTANY 169. Hairs on Leaves. — Many kinds of leaves are moreor less hairy or downy, as those ot the mullein, the mullein pink, many cinquefoils, and other commonplants. In some instances this hairiness may be a protec-tion against snails or other small leaf-eating animals, butin other cases it seems to be pretty clear that the woolli-ness (so often confined to the under surface) is to lessenthe loss of water through the stomata. The Labradortea is an excellent example of a plant, with a denselywoolly coating on the lower surface of the leaf. Theleaves, too, are partly rolled up (see Fig. 224), with theupper surface outward, so as to give the lower surfacea sort of deeply grooved form, and on the lower surfaceall of the stomata are placed. This plant, like someothers with the sa

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