ADAPTATION TO WATER 



147 



of leaves, by reducing the size of each leaf, or by a change in its 

 form. Among herbaceous plants, a decrease in the size or a 

 change in the shape of the stem brings about a similar result. 

 In extreme cases of reduction, the leaves are completely lost, and 

 in some instances the same fate overtakes the stem. Such a 

 marked decrease in the amount of surface exposed is found only 

 in intense xerophytes, though it occurs in all deciduous trees and 

 shrubs as a temporary adaptation. Changes in leaf form regularly 

 produce a decrease of surface. The scale, the linear or cylindrical 

 leaf, and the succulent leaf are the most striking examples of 

 reduced leaf forms. Lobed or divided leaves usually show a 

 tendency to reduce the size of the lobes or divisions when they 

 are grown under drier conditions. 



i68. Changes of the epidermal cells. The cells of the epidermis 

 are protected against evaporation by a coating of wax or other 



Fig. 45. Portion of a cross-soction of a loaf of the coiitury-plant , Afjnve 

 americana. The outer wall of the epiderni is modified to form a very 

 thick cuticle, and the stoniata are sunken below the surface. 



material, by means of a thickened outer wall or cuticle, or l)y tlie 

 development of hairs. The first two sometimes occur upon the 

 same leaf, but the formation of a hairy covering is usually precluded 



