PLANT ASSOCIATIONS 



283 



and very few plants are able to live in such severe con- 

 ditions. 



One of the greatest of plant associations is that which 

 occupies the plains in the interior of continents, where dry 

 air and wind prevail. The plains of the United States 

 extend from about the one-hundredth meridian westward to 

 the foothills of the Rocky Mountains. Similar great areas 



FIG. 235. Dunes of Lake Michigan encroaching upon the land, in this case diverting 



Calumet River. 



are represented by the steppes of Siberia, and in the interior 

 of all continents. On the plains of the United States the 

 characteristic plant forms are bunch-grasses (grasses that 

 grow in tufts and do not form turf) and the low grayish 

 shrubs called sagebrush. 



In passing southward on the plains of the United States, 

 the conditions are observed to become drier, until the cactus 

 deserts are reached. This region begins in western Texas, 

 New Mexico, Arizona, and southern California, and stretches 

 far southward into Mexico. This vast arid region has de- 



