764 



TEXTBOOK OF BOTANY 



the southwest. During such periods temperatures near 100^ F. may pre- 

 vail day and night for a week or for several weeks, and upland vegeta- 

 tion is soon desiccated. The underground parts of many grasses are far 

 larger than the tops, and they are less injured than are trees. But when 



Fig. 389. Tall bluestem (Andropogon fiiicatiis) , a dominant grass in areas in- 

 termediate in moisture (the present corn belt) on the natural prairies of central 

 North America. In the eastern part of the prairie it grew in dense stands as pic- 

 tured above; the pioneer settlers had great diflRculty in plowing the sod formed 

 by the mesh work of roots and rhizomes of this grass. It grows as a bunch grass in 

 the dryer western part of the prairie region. Photo by C. H. Jones. 



severe droughts occur during several successive years there is a high mor- 

 tality even among the native grasses. 



The central grasslands mav be convenientlv subdi^'ided into three 

 types characterized by tall grass, mixed grass, and short grass. The boun- 

 daries between these grasslands were neither permanent nor regular, but 

 in general thev ranged north and south paralleling the mountain front. 



The tall grass prairie, with grasses 5 to 10 feet in height, occupied both 



