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or protection of slopes from prevailing winds or the direct rays of the 
sun. In Illinois, topography was an important factor in the origin of 
prairies that developed from postglacial lakes through its effect on the 
water content of the soil. Prairies also developed on some of the well- 
drained upland soil. Forests were more common on the higher moraines, 
flood-plains, and river bluffs. 
To-sum up, climatic factors are of primary importance in determin- 
ing the general boundaries of distribution of the prairies, while the 
ever changing topography and water content of the soil are important 
in determining where sloughs, prairies, or forests will develop withiri 
these boundaries. The effect of these soil factors is more prominent 
near the boundaries of the prairie region than near its center of dis- 
tribution where we find prairies growing on the greatest range of soil 
conditions. 
Where a prairie or swamp vegetation is once established still another 
factor becomes important in keeping out the forest even after the swamps 
have become well drained, namely, the ability of the tall grasses to out- 
grow and over-shade the seedlings of forest trees. In the case of the 
drier prairies the grasses with their well-established root system have 
a monopoly of the water supply in the soil, and may prevent the de- 
velopment of seedlings of forest trees. 
Forest invasion of prairies however does occur. It may follow 
the destruction of the prairie turf by erosion or it may precede erosion. 
Forest invasion on non-eroded prairie soils in Illinois is more extensive 
on the older glaciated regions of the state than on the younger glaciated 
regions. On the Wisconsin glaciated region this invasion is most common 
on the slightly elevated prairies, but there are natural groves of forest 
on some of the non-eroded black prairie soils. 
The Ohio State University, 
Columbus, Ohio. 
