PAPERS ON BIOLOGY AND AGRICULTURE 259 



ORIGIN OF PRAIRIES IN ILLINOIS 



John Woodard, University of Illinois 



The work of other investigators has shown features 

 common to all prairies regardless of where they are 

 found. The dominant prairie species are xerophytic 

 grasses. The evaporation rate is much higher in prairies 

 than in the adjoining forests while the soil moisture con- 

 tent is much lower, often falling below the wilting point 

 of plants during the summer. This evidently explains 

 the xerophytism of the prairie vegetation, for these 

 grasses can become dormant and remain alive during 

 these unfavorable conditions which would destroy tree 

 seedling's. After a prairie grass sod is formed, it tends 

 to exclude tree seedlings. Trees can, however, invade 

 prairies along the slopes of streams, gullies and morainal 

 ridges where erosion has removed the sod and the irreg- 

 ular topography checks the wind velocity and thus re- 

 duces its dessicating action. A much slower invasion 

 takes place along forest borders where the trees check 

 the wind velocity and the shade destroys the prairie 

 grasses. 



As the Illinois prairies are in a region that was covered 

 by glacial ice they must be post-glacial in origin. During 

 the ice age there were several advances of the ice sheet 

 separated by long interglacial periods, except for the 

 period between the Early and Late "Wisconsin glacia- 

 tions, which was short. The Kansan ice sheet invaded 

 northeast Kansas and extended into Missouri as far 

 south as the Missouri river. The Illinoisan ice sheet in- 

 vaded Illinois as far as the Ozark Hills in the southern 

 part of the state. Just east of the Illinois-Indiana line, 

 the border of this ice sheet bends to th northeast, contin- 

 uing to central Indiana, then turns south to the Ohio 

 river, then east to south central Ohio, and then north to 

 central Ohio where it is buried by drift of the Wisconsin 

 ice sheets. The Iowan ice covered northeast Iowa and 

 possibly a small area in northwest Illinois. The Early 

 Wisconsin glacier only entered the east side of Illinois, 

 extending to Shelbyville and Mattoon, but covered most 

 pf the Illinoisan drift in eastern Indiana and in Ohio. A 



