480 RELATION TO ENVIRONMENT. 



water prevents the washing away of the soil; the roots of trees 

 bind the soil also and assist in holding it. 



923. Absence of forest encourages serious floods. The great 

 floods of the Mississippi and its tributaries are due to the rapidity 

 with which heavy rainfall flows from the rolling prairies of the 

 west, and from the deforested areas west of the Alleghany system. 

 The serious floods in recent years in some of the South Atlantic 

 States are in part due to the increasing area of deforestation in 

 the Blue Ridge and southern Alleghany system. 



924. The prairie and plains societies. These are to be found 

 in the grassland formation. In the prairies "meadows" are 

 formed in the lower ground near river courses where there is 

 greater moisture in soil. The grasses here are principally "sod- 

 formers" which have creeping underground stems which mat 

 together, forming a dense sod. On the higher and drier ground 

 the "bunch" grasses, like buffalo-grass, beard -grass, or broom- 

 sedge, etc., are dominant, and in the drier regions as one 

 approaches desert conditions the vegetation gradually takes on 

 more the character of the desert, so that in the plains sage- 

 brush, the prickly-pear cactus, etc., occur. Besides the dominant 

 vegetation of the society there are subordinate species, and the 

 societies are especially marked by a spring and autumn flora of 

 conspicuous flowering plants which are mixed with the grasses. 



925. Desert societies. These are composed of plants which 

 possess a form or structure which enables them to exist in a 

 very dry climate where the air is very dry and the soil contains 

 but little moisture. The true desert plants are perennial. The 

 growth and flowering period occurs during the rainy season, or 

 those portions of the rainy season when the temperature is favor- 

 able, and they rest during the very dry season and cold. Charac- 

 teristic desert plants are the cacti with thick succulent green 

 stems or massive trunks, the leaves being absent or reduced to 

 mere spines which no longer function in photosynthesis; yuccas 

 with thick, narrow and long leaves with a firm and thick cuticle; 

 small shrubs or herbs with compact rounded habit and small 

 thick gray leaves. All of these structures conserve moisture. 



