348 A TEXT-BOOK OF BOTANY 



seen. The flowering herbs are often very tall and coarse, 

 but have brilliant flowers, as asters, golden rods, rosinweeds, 

 lupines, etc. 



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 a natural forest. 

 Prairies are of two kinds at least : those due to soil conditions 

 and those due to climatic conditions. The former are char- 

 acteristic of the Eastern prairie region, and appear in scat- 

 tered patches through the forest region as far East as Ohio 

 and Kentucky. They are probably best explained as re- 

 presenting old swamp areas, which in a still more ancient 

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

 area are evidently of this type, being associated with former 

 extensions of Lake Michigan. 



The climatic prairies are characteristic of the West- 

 ern prairie region, and are more puzzling than the others. 

 Among the several explanations suggested, perhaps the 

 most prominent is that which regards the absence of a 

 natural forest on the Western prairies as due to the prevail- 

 ing dry winds. The extensive plains farther West develop 

 the strong and dry winds that sweep over the prairies, and 

 this brings extremes of heat and drought, in spite of the 

 character of the soil. In such conditions a seedling tree 

 could not establish itself. If it is protected through this 

 tender period it can maintain itself afterward. These prai- 

 ries, therefore, represent a sort of broad beach between the 

 Western plains and the Eastern prairies and forests. 



213. Thickets. Mesophytic thickets are not so im- 

 penetrable as xerophytic thickets ( 207), an4 are usually 

 developed as forerunners of forests. An illustration of this 

 may be obtained by noting the succession of plants on a 

 cleared area. After such an area has been cleared of its 

 trees, it is overrun by herbs that develop rapidly from the 

 seed, the so-called fireweed usually being conspicuous. 



