236 PLANT EELATIONS. 



The prairie is regarded by some as a xerophyte area, and this 

 is a natural conclusion when one examines only the struc- 

 tures of the plants which occupy it. It is certainly a tran- 

 sition area between the plains of the West and the true 

 mesophyte areas of the East. However, an examination of 

 the soil reveals a deep, rich humus, with the water sup- 

 ply decidedly greater than that which characterizes a true 

 xerophyte area. On the other hand, the prevailing winds 

 are dry. There is a mixture, therefore, of mesophyte con- 

 ditions in the soil and xerophyte conditions in the air, 

 which leads to a peculiar association of structures. 



The vegetation of the prairies in general is composed 

 of tufted grasses and perennial flowering herbs. Unfortu- 

 nately, most of the natural prairie has disappeared, to be 

 replaced by farms, and the characteristic prairie forms are 

 not easily seen. The flowering herbs are often very tall 

 and coarse, but with brilliant flowers, such as species of 

 aster, goldenrod, rosin-weed, indigo plant, lupine, bush 

 clover, etc. The most characteristic of these forms show 

 their xerophyte adaptations by their rigidity and roughness. 



It has long been a vexed question as to the absence of 

 trees in a soil which seems to be most suitable for their 

 development. Probably the most ancient explanation was 

 the occurrence of prairie fires, but it seems evident that 

 some general natural condition rather than an artificial one 

 is responsible for such an extensive area. A possible 

 explanation is as follows : The extensive plains of the 

 West develop the strong and dry winds which prevail over 

 the prairie region, and this brings about extremes of heat 

 and drouth, in spite of the character of the soil. In such 

 conditions a tree in a germinating condition could not 

 establish itself. The prairies, therefore, represent a sort 

 of broad beach between the Western plains and the East- 

 ern forests. The eastward limit of the prairie has proba- 

 bly depended upon the limit of the dry winds, which 

 are gradually modified as they move eastward, until they 



