286 ELEMENTARY STUDIES IN BOTANY 



taken the form of an inquiry into the conditions which for- 

 bid the growth of a natural forest. Prairies are of two kinds 

 at least; those due to soil conditions and those due to cli- 

 matic conditions. The former are characteristic of the eastern 

 prairie region, and appear in scattered patches through the 

 forest region as far east as Ohio and Kentucky. They are 

 probably best explained as representing 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 western 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 prevailing 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 prairies, therefore, represent a sort 

 of broad beach between the western plains and the eastern 

 prairies and forests. 



The climax type of plant association in temperate regions 

 is the deciduous forest. Such forests may be pure or mixed. 

 A common type of pure forest is the beech forest, which is 

 a dark forest, the wide-spreading branches of neighboring 

 trees overlapping so as to form a dense shade (Fig. 237). 

 In such a forest, therefore, there is little or no undergrowth. 

 Another pure forest, which belongs to drier areas, is the oak 

 forest, which is a light forest, permitting access of light for 

 lower plants. In such a forest, therefore, there is usually 

 more or less undergrowth. The typical American deciduous 

 forest, however, is the mixed forest, made up of many vari- 

 eties of trees, such as beech, oak, elm, walnut, hickory, 



