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ILLINOIS ACADEMY OF SCIENCE 



Fig. 4. Graphs showing soil moisture at station 2. a: of 

 soil at the 7.5 cm. depth, b: of soil at the 25 cm. depth, c 

 and d the respective wilting coefificient of these soils. Weekly 

 rain-fall in cm. shown at base of figure. 



of soil at station 2 (fig. 4) in July. It would also be well to 

 note that at neither of the stations, except in the instance 

 mentioned above, did the water content, at the depth of 25 

 cm. fall below the critical percentage. However, there must 

 be many days at this period of the summer when the com- 

 bined action of low water content of the soil and the high rate 

 of evaporation render conditions very severe even for the 

 characteristic vegetation of the association. This may ac- 

 count for the xerophytic aspect of the prairie vegetation in the 

 late summer. 



