430 GENERAL MORPHOLOGY OF PLANTS 



the autumn the trees shed their leaves, and in this condition with 

 the bare twigs they are able to stand the drying effect of the 

 cold and winds of the winter because transpiration is now at a 

 minimum, while root absorption is at a minimum because of the 

 cold condition of the soil. Perennial herbs like trillium, den- 

 taria, the goldenrods, etc., turn to xerophytic habit by the death 

 of their aerial shoots, while the thick underground shoot which is 

 also protected by its subterranean habit carries the plant through 

 the winter. 



611. While these different vegetation types are generally 

 dominant in certain climatic regions or under certain soil con- 

 ditions, they are not the exclusive vegetation types of the regions. 

 For example, in desert and semidesert regions the dominant vege- 

 tation type is made up of xerophytes. But there is a mesophytic 

 flora even in deserts, which appears during the rainy season 

 where temperature conditions are favorable for growth. This is 

 sometimes spoken of as the rainy-season flora. The plants are 

 annuals and by formation of seed can tide over the dry season. 

 So in the region where mesophytes grow there are xerophytes, 

 examples being the evergreens like the pines, spruces, rhododen- 

 drons; or succulent plants like the stonecrop, the purslane, etc. 

 Then among hydrophytes the semiaquatics are really xerophytes. 

 The roots are in water, and absorption is slow because there are 

 no root hairs, or but few, and the aerial parts of the plant are 

 xerophytic. 



