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, ete. 
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), and are usually 
developed as forerunners of forests. An iliustration 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. 
