MINUTE STRUCTURE OF LEAVES 



157 



to soak through it with ease. Merely examining sections 

 of the various kinds of epidermis will not give nearly 

 as good an idea of their properties as can be obtained 

 by studying the behavior during severe droughts of 

 plants which have strongly cutinized surfaces and of 

 those which have not. Fig. 121, however, may convey 

 some notion of the difference between the two kinds of 

 structure. In most 

 cases, as in the india- 

 rubber tree, the ex- 

 ternal epidermal cells 

 (and often two or 

 three layers of cells 

 beneath these) are 

 filled with water, and 

 thus serve as reser- 

 voirs from which the 

 outer parts of the leaf 

 and the stem are at 

 times supplied. 



In many cases, noticeably in the cabbage, the epidermis 

 is covered with a waxy coating, which doubtless increases 

 the power of' the leaf to retain needed moisture, and 

 which certainly prevents rain or dew from covering the 

 leaf-surfaces, especially the lower surfaces, so as to hinder 

 the operation of the stomata. Many common plants, like 

 the meadow rue and the nasturtium, possess this power 

 to shed water to such a degree that the under surface of 

 the leaf is hardly wet at all when immersed in water. 

 The air-bubbles on such leaves give them a silvery 

 appearance when held under water. 



FIG. 121. Unequal Development of Cuticle 



by Epidermis-Cells. 



A, epidermis of Butcher's Broom (Ruscus) ; B, 

 epidermis of sunflower ; c, cuticle ; e, epi- 

 dermis-cells. 



