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 



Fig. 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). 



Fig. 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). 



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] 1 whose activities are limited to the early 

 1 The words in brackets are added. 



