PLANT FORMATIONS. 



473 



a more or less open formation because the moisture is not suf- 

 ficient to supply all the plants which could be crowded on the 

 ground, each individual tuft needing an area of ground surround- 

 ing it on which it can draw for moisture. Such a formation is 

 an open one, and in this respect is similar to desert formations. 

 3d. Desert formations. These occur where the annual rain- 

 fall is still lower, 10 to 4 inches or even less, 2 to 3 inches, while 

 in one place in Chili it is as low as J inch. In the great Sahara 

 desert it is about 8 inches, while in the Sonora Nevada desert 



Fig. 487. 



Winter range in northwestern Nevada, showing open formation; white sage 

 (Eurotia lanata) in foreground, salt-bush (Atriplex confertifolia) and bud : sage 

 (Artemisia spinescens) at base of hill, red sage (Kochia americana) on the higher 

 slope. (After Griffiths, Bull. 38, Bureau Plant Ind., U. S. Dept. Agr.) 



in the southwestern United States it is 4 to 8 inches. Here 

 the formation is an open one. In the forest and prairie forma- 

 tions the plants compete with each other for occupancy of the 

 ground, since climatic conditions are favorable, so that the struggle 

 against climate is not severe. But in the desert plants do not com- 

 pete with each other; since the climate is so austere, the struggle 

 is against the climate. Hence plants stand at some distance from 

 each other because the roots need the moisture from the ground 

 for some distance around them. There is not enough moisture 

 for all the plants that begin, and those which get the start take 



