MESUPHYTE ASSOCIATIONS. 239 



to the forest regions of Illinois and Indiana (see Fig. 206). 

 The prairie is regarded by some as a xerophytic area, and 

 this is a natural conclusion when one examines only the 

 structures of the plants which occupy it. It is certainly a 

 transition area between the plains of the West and the true 

 mesophytic areas of the East, and there is a general tran- 

 sition from the more xerophytic western prairies to the 

 more mesophytic eastern prairies. Moreover, in the east- 

 ern part of the prairie region there is locally every grada- 

 tion between the strongly mesophytic type of the low ground 

 to the more xerophytic type of the high ground. 



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 xero- 

 phytic adaptations by their rigidity and roughness. 



The origin of the prairie has long been a vexed question, 

 which has usually taken the form of an inquiry into the 

 conditions which forbid the growth of forests. Prairies are 

 at least of two kinds. Some are edaphic — that is, they are 

 due to local soil conditions. Such prairies are character- 

 istic of the eastern prairie region, and even appear in scat- 

 tered patches throughout the forest region as far east as 

 Ohio, Kentucky, etc. They are probably best explained as 

 representing old swamp areas, which at a still more ancient 

 time were ponds or lakes. All the prairies of the Chicago 

 area are evidently edaphic, being associated with former 

 extensions of Lake Michigan. Other prairies are climatic — 

 that is, they are due to general climatic conditions. Such 

 prairies are characteristic of the western prairie region, 

 merging into the plains, and are more puzzling than the 

 edaphic prairies. Among the several explanations sug- 

 17 



