Elementary botany . 2) Protective covering.—Epidermis and cuticle.—Thewalls of the epidermal cells aremuch thickened in some this thickening occursin the outer wall, or both wallsmay be thickened. Variation inthis respect as well as the extentof the thickening occur in dif-ferent plants and are often corre-lated with the extremes of conditions which they serve to cuticle, a waxy exudation from the thick wall of the epider-mis of many leaves, also serves as a protection against too greatloss of water, or against the leaf becoming saturated with waterduring rains. The c
Elementary botany . 2) Protective covering.—Epidermis and cuticle.—Thewalls of the epidermal cells aremuch thickened in some this thickening occursin the outer wall, or both wallsmay be thickened. Variation inthis respect as well as the extentof the thickening occur in dif-ferent plants and are often corre-lated with the extremes of conditions which they serve to cuticle, a waxy exudation from the thick wall of the epider-mis of many leaves, also serves as a protection against too greatloss of water, or against the leaf becoming saturated with waterduring rains. The cabbage, carnation, etc., have a well-developedcuticle. The effect of the cuticle in shedding water can be nicelyshown by spraying water on a cabbage leaf or by immersing it inwater. Sunken stomata also retard the loss of water vapor. Covers of hair or scales.—In many leaves certain of the cellsof the epidermis grow out into the form of hairs or scales ofvarious forms, and they serve a variety of purposes. When. Fig. 435-Structure of leaf of Lactuca one grown in sunlight, palisadecells on both sides. Lower one grownin shade, no palisade tissue. 394 RELATION TO ENVIRONMENT. the hairs form a felt-like covering as in the common mulleinsome antennarias, etc., they lessen the loss of water vapor be-cause the air-currents close to the surface of the leaf are (see the thistles, etc.) also afford a protection againstcertain animals. 765. (3) Reduction of surface.—Reduction of leaf surface isbrought about in a variety of ways. There are two generalmodes: (1st) Reduction of surface along with reduction ofmass; (2d) Reduction of surface inversely as the mass. Ex-amples of the first mode are seen in the dissected leaves of manyaquatic plants. In this finely dissected condition the mass ofof the leaf substance is much reduced as well as the leaf surface,but the leaf is less liable to be injured by movement of the addition it has already been po
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