VESTAL: A BLACK-SOIL PRAIRIE STATION 359 
moisure therefore is rapid, though gradual. The species composi- 
tion of the prairie growth changes with the soil moisture, and a 
complete transitional series of prairie growths can be recognized. 
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It will perhaps be helpful to regard the mean and the two extreme 
conditions of this series as constituting distinct plant associations, 
which may be called (1) xerophytic prairie-grass, (2) mesophytic 
