CHAPTER LIT. 



DESERT PLANT SOCIETIES. 



I. Characters of True Desert Plants. 



1053. The true desert plants are the perennials which must 

 preserve stem and root system through the hot dry period. They 

 possess a habit and structure the result of the operation of phys- 

 iological causes which enable them to exist during the severe 

 season. They are xerophytes, and their structures are xero- 

 phytic, i.e., those which fit them for existence where the climate 

 is dry, or where there is difficulty in absorbing water from the 

 ground. In the desert this difficulty of absorption arises from 

 the very small amount of water in the soil. Desert plants then 

 have to meet two general conditions, a very dry and hot atmos- 

 phere and a very dry soil. This they do by provision for: ist, 

 reduction of transpiration; 2d, provision for water-storage; and 

 3d, increased surface for root-absorption. 



1. Reduction of transpiration. This is brought about in 

 several ways: ist, by reduction in size so that the leaves are 

 smaller and thicker; 2d, by hairy coverings; 3d, the stomates 

 are sunk deeply in the surface; 4th, the cuticle is thickened; 

 5th, the leaves are entirely dispensed with and the stems are green 

 and function as leaves; 6th, the stems are shorter, and with a 

 thick cuticle and often hairy or waxy coverings. 



2. Provision for water-storage. This is provided for by thick 

 and often fleshy leaves, by fleshy stems as in the cacti, and often 

 by a thick root system. Some of the cacti have large, rounded, 

 globose sterns. Some other plants have large, swollen bases to 



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