514 BOTANICAL GAZETTE [JUNE 
considered upon the basis of its soil moisture, the association is decidedly 
mesophytic. The causes of the xerophytic character of the vegetation 
must be sought in the high evaporating power of the air and in the 
instability of the substratum. These factors, however, doubtless react 
upon the surface of the soil and tend to conserve the soil water by con- 
stantly maintaining a dry surface mulch. The constant presence of a 
sufficient amount of moisture perhaps will help to explain the readiness 
MAY JUNE JULY | AUGUST sree OCTOBER | 
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ALY NY IN [BASIN ZEW OT 
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Fic. 1.—Graphs showing the range of soil moisture in the cottonwood dune; the 
heavy line at 7.5 cm. and the light line at 25 cm. depth; wilting coefficient represented 
by a broken line. 
with which vegetative reproduction occurs within the association, while 
the shifting sand and high rate of evaporation may account for the almost 
entire absence of seedlings of any sort. 
From these scanty data it would seem that determinations of soil 
moisture, related to plant growth through the wilting coefficients of the 
soil, will afford an efficient means of making quantitative studies of the 
water supplies of the subterranean parts of plant associations, and, as 
in the present instance, enable students of ecology to analyze more 
closely the effects of the various factors influencing the production of 
any particular plant association.—Gero. D. Futter, The University of 
Chicago. 
