THE PLANT AND ITS ENVIRONMENT 



165 



Xerophytes. — Inherited adaptations to abundance or dearth 

 of water show the pronounced effects of moisture as an environ- 

 mental factor. Many plants have become so modified during 

 the course of evolution that they are able to thrive under con- 

 ditions where the available soil water is comparatively small in 



Epidermal 

 Cell 



Fig. 82. — The stoma of a xerophytic plant (Cycas). The stoma is protected 

 by being sunken in a pit formed by the over-arching growth of two adjacent 

 epidermal cells. 



amount, and where plants without special adaptive modifications 

 would speedily perish. Such drought-loving plants are known as 

 xerophytes and are characterized by several types of structural 

 and functional modifications which result in a notable ability 



Fig. 83.- — Section across the blade of a xerophytic leaf {Empclrum) . The 

 blade is rolled back so that the margins almost meet, thus preventing excessive 

 transpiration from the stomata, which are on the lower (inner) surface. A felt 

 of woolly hairs is also developed at the leaf margins which further hinders the loss 

 of water. 



both to draw water from the soil and to retain it in the plant 

 tissues. The root-system is very well developed in proportion 

 to the shoot. The osmotic concentration of the cell sap is usually 

 higher than among plants growing under less arid environments. 

 The leaf surface is reduced, sometimes very radically; and leaves 

 may even disappear entirely. The cuticle of stem and leaf 



