PLANT SOCIETIES. 



481 



The mesquite tree is one of the common trees in portions of the 

 Sonora Nevada desert. Besides the true desert plants, desert 

 societies have a rainy-season flora consisting of annuals, which 



Fig. 4go. 

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

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



can germinate, vegetate, fioiver, and seed during the period of 

 rain and before the ground moisture has largely disappeared, 

 and these pass the resting period in seed. 



926. Arctic-alpine societies. — The most striking of the arctic 

 plant societies are the "polar tundra," extensive mats of vegeta- 

 tion 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 b)' large bright-colored llowers. Heaths, sa.xi- 

 f rages, and dwarf willow abound. Alpine-plant societies are 

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



