5l8 RELATION TO ENVIRONMENT. 



never germinate, or if they do, dry up before they even get a 

 temporary foothold. In the woods and prairie types millions 

 of seeds germinate and gain temporary hold, only to be crowded 

 out later by competition among themselves. In passing from 

 the prairie to the desert there is a large transition area, the plains 

 where the formation is open. It is neither prairie nor desert 



Fig. 494. 



Desert range near Mirage, Nevada. Open formation, almost pure salt bush 

 (Atriplex confertifolia). (After Griffiths, Bull. 38, Bureau Plant Ind. U. S. Dept 

 Agr.) 



but is intermediate. It might be treated of as a fifth formation, 

 the Plains Formation. 



II. Edaphic Formations. 



900. Controlling factors. Climatic formations do not cover 

 the entire surface of the earth over which they extend, since 

 soil (ground) conditions present limiting factors which in many 

 places overcome the general climatic influence. The influence 

 of different kinds of soil gives a more checkered appearance 

 to the vegetation because within climatic regions there is great 

 diversity of soil and soil conditions. Limited areas are thus 

 mapped into small or large natural parcels. Those of like con- 

 ditions, even in the same climatic region, are separated from 

 each other by the intermediate spots of a different soil condition, 



