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TEXTBOOK OF BOTANY 



from the direct rays of the sun and which therefore, being 

 cooler, loses water less rapidly by evaporation than would the 

 upper surface if it were similarly provided with air -pores. 

 There are other ways, too, in wliich evaporation from leaves 

 is checked. One of these, as we have seen, is by the produc- 

 tion of hairs. Another is by the 

 formation of cutin, a substance some- 

 what like cork, with which the outer 

 walls of the epidermal cells are per- 

 meated and which sometimes forms a 

 thin layer on the outer surface of the 

 epidermis that can be separated and 

 peeled off. Wax, which is sometimes 

 formed on the outer surface of the 

 epidermis, also helps to check evapora- 

 tion. The wax may be in a uniform 

 layer, causing the shiny appearance 

 that is characteristic of the leaves of 

 some willows and grasses, and of the 

 surfaces of many fruits. Sometimes 

 the wax takes the form of mealy par- 

 ticles; this is the "bloom" of plums 

 FIG. 147. An aloe, and grapes, and of the leaves of the 

 whose thick leaves serve cabbage and the tulip. Cutin and 



surfaces of leaves, hairs on the lower 



surfaces. 



Another tendency often noticed in the plants of dry 

 localities is to reduce in various ways the area of their leaf 

 surfaces. Generally speaking, plants that live in moist, shady 

 places where evaporation is slow, or in lands with mild, .moist 

 climates where the water supply is ample, have large, thin, 

 and often divided leaves. The leaves of the plants of deserts 

 and other very dry regions are seldom divided, for a divided 

 leaf exposes more surface to the air than would an un- 



