RELATION OF PLANTS TO ENVIRONMENT 427 



semideserts, or where the soil is very dry or not retentive cf mois- 

 ture, as in very sandy soil which is above ground water, or in 

 rocky areas. Since the plants cannot obtain much water from the 

 soil they must be provided with structures which will enable them 

 to retain the small amount they can absorb from the soil and give 

 it off slowly. Otherwise they would dry out by evaporation and 

 die. Some of the structures which enable xerophytic plants to 



Fig. 401. 

 Desert society, chiefly cactus, Arizona. (Photograph by Tuomey.) 



withstand the conditions of dry climate and soil are lessened leaf 

 surface, increase in thickness of leaf, increase in thickness of 

 cuticle, deeply sunken stomates, compact growth, also succulent 

 leaves and stems, and in some cases loss of the leaf. Evergreens 

 of the north temperate and the arctic regions are xerophytes.* 



* The xerophytic condition of the conifers is probably brought about as a 

 result of imperfect vascular system of the stems. There not being true 

 vessels the water supply to the leaves is scanty and slow. The leaves are 

 consequently narrow, thickened, hard, and with a thick cuticle. This offers 

 an excellent example of correlation (paragraph 656). 



