566 RELATION TO ENVIRONMENT. 



their stems, and in others the roots are also much enlarged. 

 These types with enlarged roots are rather rare in the Sonora- 

 Nevada desert in the southwestern part of the United States, 

 but are more common in southwestern Africa and in western 

 South America. 



3. Increased surface for root-absorption. This is provided for 

 by the great length of the root system and the profuse branching. 



Fig. 507- 



Desert vegetation, Arizona, showing large succulent trunks of cactus with shrubs 

 and stunted trees. Open formation. (Photograph by Tuomey.) 



In many desert plants the roots extend to great depths in the soils, 

 where they obtain ground-water which is not so available nearer 

 the surface. 



1054. Thorny or spiny character of desert vegetation. The 

 development of thorns and spines on most desert plants is very 

 striking. The cacti are remarkable for the great profusion of 

 spines on the surface of their stems. The small leaves of many 

 of the shrubs are sharp-pointed, and thorns are very character- 

 istic of the shrubs of dry countries. 



