LEAVES 



205 



which may occur at any time or not at all ; (2) the periodic 

 drought, which is a regular season of the year ; (3) the per- 

 petual drought, which characterizes arid regions, such as 

 the states on the Mexican border. It is evident that the 

 ordinary crop-producing regions of this country come under 

 the first head, and that a possible drought is the hardest 

 kind to provide against. When there is a regular period of 

 drought, the life-histories of plants become adjusted to it, 



PIG. 169. Sections through leaves of the same plant, showing the effect of exposure 

 to light upon the structure of the mesophyll : A, leaf exposed to intense sunlight ; 

 B, leaf grown in the shade. After STAHL. 



just as to a winter period ; when there is perpetual drought, 

 only those plants equipped for it succeed in living, or it is 

 controlled by irrigation ; but when drought comes at irregu- 

 lar intervals, it is usually a question of the endurance of poorly 

 equipped plants. The subject of protection is too large and 

 complex to be presented with any completeness, and there- 

 fore a few illustrations of protection that seem to be de- 

 finite must suffice. 



In the description of the structure of leaves attention was 

 called to the fact that the epidermis with its layer of cuticle 

 is an ever-present check against transpiration. Of course 



