480 RELATION TO EN V IROA' MENT. 



water prevents the washing awa_\- of the soil; tire roots of trees 

 l)ind the soil also anrl assist in holding it. 



923. Absence of forest encourages serious floods. — The great 

 floods of the ^Mississippi and its tributaries arc due to the rapidity 

 with which heav"\' rainfall Hows 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 he 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 principalh' ''sod- 

 formers" which have creeping underground stems which mat 

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

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

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

 approaches desert conditions the vegetation graduallv 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 b\- a spring and autumn flora of 

 conspicuous flowering jjlants which are mi.xed with the grasses. 



925. Desert societies. — These are composed of plants \\hich 

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

 very dry climate where the air is wr}- dr\- and the soil contains 

 but little moisture. The true desert |ilants are ]ierennial. The 

 growth and flowering period occurs during the rain\- season, or 

 those portions of the rain)' season when the temperature is fa\-or- 

 able, and they rest during the very dr\- season and cold. C'harac- 

 teristic desert plants arc the cacti with thick succulent green 

 stems or massive trunks, the lea\es being absent or reduced to 

 mere spines which no longer function in photo^Mithesis; \aiccas 

 with thick, narrow and long leaxes wiUi a fuan and thick cuticle; 

 small shrubs or licrbs with (unipait rounded habit and small 

 thick gra\- leaws. .All of these siruclurcs conserve moisture. 



