VEGETATION OF THE PRAIRIE. 557 



extending into western Texas and Kansas in the south, and 

 northward extending as far east as western Illinois. The area east 

 of the Missouri River has a greater annual rainfall, and here is the 

 typical prairie vegetation, which is of the meadow type. Passing 

 westward and southward the region of the "Great Plains," or 

 the arid region, is entered, where the rainfall is less and the vege- 

 tation is that of the steppes; while toward the foot of the Rocky 

 Mountains southward the dryness of the region increases and 

 partakes more of the character of the desert. The typical prairie 

 region of North America extends from about the looth meridian 

 eastward to the forests of Illinois and Indiana, including most 

 of the Dakotas, Nebraska, Iowa, southern Minnesota, and Wis- 

 consin, a large part of Kansas and Indian Territory, and extends 

 north into western Manitoba and nearly all of Assiniboia and 

 Saskatchewan. The surface of the ground is rolling and fur- 

 rowed, the prairie grass formation on the rolling elevations, 

 with the so-called "gallery woods" or "park-like" forest societies 

 at the lower elevations along the streams. This is the transition 

 area from the forest area to the arid regions, or steppes. 



II. Prairie Formations. 



1043. Tension line between forest and prairie. The tension 

 line between the forest region and the prairie region was long 

 since established. This line of separation is not well marked, 

 for there is not an abrupt transition from forest to prairie. Arms 

 of the prairie are outstretched now and then between extended 

 troops of forests, and clumps of shrubbery and trees for some 

 distance further dot the prairie here and there like sentries guard- 

 ing the outposts of the main division. 



1044. Climatic factors the dominant ones in limiting forest 

 and prairie areas. While the general climatic influences have 

 been the chief factors in delimiting these regions, biotic or physi- 

 cal factors may in some cases have aided. For example, it has 

 been suggested that the bison grazing on the prairies in central 

 United States may have aided in preventing trees and shrubs 



