PL A N T SOCIETIES 479 



provision for water-storage ; and third, increased surface for root- 

 absorption. 



1. Reduction of transpiration. This is brought about in several 

 ways: first, by reduction in size so that the leaves are smaller 

 and thicker; second, by hairy coverings; third, the stomates 

 are sunk deeply in the surface ; fourth, the cuticle is thickened ; 

 fifth, the leaves are entirely dispensed with and the stems are 

 green and function as leaves; sixth, the stems are shorter, with a 

 thick cuticle and often hairy or waxy coverings. 



2. Provisions 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 stems. Some other plants have large, swollen bases to 

 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. 

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

 soil, where they obtain ground-water which is not so available 

 nearer the surface. 



653. Arctic-alpine societies. The most striking of the 

 arctic plant societies are the " polar tundra," extensive mats of 

 vegetation largely made up of mosses, lichens, etc., only partially 

 decayed because of the great cold of the subsoil, and perhaps 

 also because of humus acid in the partially decayed vegetation. 

 These tundras are brightened by numerous flowering plants 

 which are characterized by short stems, a rosette of leaves 

 near the ground, and by large bright-colored flowers. Heaths, 

 saxifrages, and dwarf willows abound. Alpine-plant societies 

 are similar to the arctic, although some of the conditions are more 

 severe than in the arctic region. This is principally due to the 

 fact that during the summer while the plants are growing they 

 are subject to a high temperature during the day and a very low 



