Climate of Middle Illinois. 33 



The forest is rich in herbaceous plants (about 250), many of them 

 with conspicuous flowers of vivid colors. The first in flower are Hepatica 

 and Trillium nivale, mostly in the middle of March, and the last in fall 

 are most of the compositse, species of Aster, Solidago, Helianthus, Eupa- 

 toriuni, Helenium, etc. 



THE PRAIRIE. 



The largest prairie of the district in question does not exist any more. 

 It was where now the city of Peoria is built, and the little that is left is 

 no more a prairie, for most of the prairie plants are replaced by immi- 

 grated foreign weeds. The smaller prairies of the district are turned 

 mostly into cultivated land; still most of the prairie plants that occur in 

 Illinois are represented. 



How originated the prairie? To solve this problem many attempts 

 were made. Violent storms were accused to prevent the growth of trees. 

 But the wooded districts are exposed to the same violent storms that pros- 

 trate the largest trees but never the young pliant stems, which will grow 

 when the older ones are gone. 



There is a general belief that the annual prairie fires make the 

 prairie; a view which without hesitation even Volney adopted in his "Tab- 

 leau du climat et du sol des Etats unies" ("ou la nature du sol et plus 

 encore les incendies anciens et annuel des sauvages ont occassione de vastes 

 deserts"). A burnt forest will not be turned to a prairie, for at once 

 brambles and other shrubs grow up and defend the forest soil against the 

 invasion of the prairie, and we find no remnant stumps below the sod what 

 would prove that there was once a forest. 



Prevailing dry winds and deficiency of rain may be the cause of tree- 

 less tracts in southern Russia, but certainly not in the upper Mississippi 

 valley with an annual average of 35 inches of precipitation with about 

 one hundred rainy days. To explain the origin and existence of the prair- 

 ies we have to resort to geological causes. 



Lesquereux, in Geological Survey of Illinois (I., 238-254), demon- 

 strates that the soil of the prairie was formed under water by slow decom- 

 position of water plants. When the land emerged it was first swamp, 

 then wet prairie, at last dry prairie with a compact sod of grasses; the 

 soil had such physical and chemical qualities that no tree could grow. To 

 make trees grow it is necessary to plow deep and to expose the soil, so rich 

 of ulmic acid, to the atmosphere. Bushels of tree seeds may be thrown 

 on the surface of the prairie, they will not germinate; or even if they 

 germinate the roots will not penetrate the sod. 



Of all the theories this is one of the most acceptable. But there are 

 often small prairies in the midst of forests that have a quite difEerent or- 

 igin. They are the result of the work of the beaver that build a dam 

 5 



