472 



J7ELAT/0N TO ENVIKONMEKT. 



there is still a sufficient amount of moisture to provide for all 

 the plants which can stand on the ground. Yet there is not 

 enough moisture to permit the growth of forest as the dominant 

 type without aid and protection by man. The so-called prairie 

 regions are examples. Trees and shrubs do occur, but they 

 cannot compete successfully with the grasses because the climatic 



Fig. 4S11. 

 Typical prairie scene, a ferw miles west uf Linculn, Nebraska. (Bot. Dcpt., Univ. 

 Nebraska.) 



conditions are favorable for the latter and unfa\orable for the 

 former. On the border hne between forest and prairie the hne 

 of division is not a clear-cut one because conditions grade from 

 one to the other. The two formations are somewhat mi.xed, 

 like the outposts of contending armies, arms of the forest or 

 prairie extending out here and there. In tlie United States the 

 prairies extend from Illinois to about the looth ineridian, and 

 beyond this to the foothills of the Rockies and southwest to the 

 Sonora Nevada desert the region is drier, the rainfall varying 

 from 10 to 20 inclies. Tliis is the area of the (ireat Plains, 

 and while grasses of (lie buncli t}j>e are dominant, the)' make 



