CAUSES OF SUCCESSION 249 
differently adapted species. The factors as expressed in terms of the 
evaporating power of the air are shown in Figs. 251, 252, and 253, which 
are graphic representations of the results of a season’s study by Fuller 
(131). The graph of the cottonwood dunes is characterized by great 
fluctuations. 
The graph for the pine dunes is decidedly lower and more regular in its 
contour than that of the association which it succeeds. Its four nearly equal 
Cottonwood dune 
Pine dune 
Oak dune 
Oak-hickory forest 
Beech-maple forest 
Fic. 252.—Showing the comparative evaporation rates (c.c. per day) in the ground 
stratum of the different animal communities from May to October (after Fuller). 
0 10 20 0 
<a 
=== === 9 ee 
eR 
area ie eee ee eae 
Fic. 253.—Showing the comparative evaporation rates (c.c. per day) in four of the 
animal communities on the basis of the maximum amount per day for any week from 
May to October (after Fuller). 
Cottunwood dune 
Pine dune 
Oak dune 
Beech-maple forest 
maxima would indicate that within its limits there was, throughout the sum- 
mer season, a continuous stress rather than a series of violent extremes. On 
the whole it shows a water demand of little more than half of that occurring 
in the cottonwood dunes. Its greatest divergence is plainly due to the ever- 
green character of its vegetation and is seen on its low range in May and the 
first part of June, and again in October when it falls below that of the oak 
dunes and is even less than that of the beech-maple forest. This would give 
good reasons for expecting to find within this association truly mesophytic 
plants [and moist forest annuals] whose activities are limited to the early 
t The words in brackets are added. 
