1922] THONE—STARVED ROCK 367 
of the summer, and rising again with the beginning of the fall 
rains; (6) according to the mechanical composition (and therefore 
retentivity) of the soil; (c) to a minor extent according to topog- 
raphy; and (d) according to the density of the foliage canopy. 
4. The evaporating power of the air was found to vary (a) 
seasonally, increasing until midsummer and falling off afterward; 
(5) according to the state of tree foliation, declining after the forest 
had become completely clothed; and (c) topographically, being 
greatest for the same period in exposed stations and least in sheltered 
ones. 
5. The evaporating power of solar radiation was found to vary in 
the same manner as the evaporating power of the air, complement- 
ing and emphasizing the data under the latter head. 
6. Maximum temperatures were found to vary in much the 
same manner as the evaporating power of the air. Minimum 
temperatures of the air were found to be affected by topography 
in a mode inverse to that of the maxima, being highest at the low 
lying stations and lowest at the higher lying ones. 
7. Certain vegetational phenomena showed a general correla- 
tion with the instrumental observations: (a) the density of ground 
cover, number of tree seedlings, and proportion of annuals in the 
total vegetation of any given association bore an inverse relation to 
the relative xerophytism; (bd) in all but one of the stations, condi- 
tions were favorable for the development of seedlings only in spring 
and fall; (c) in the climax forest for the region (upland oak woods) 
the water-supplying power of the soil consistently fell nearly or 
quite to zero during the summer drought period; (d) the location of 
“subclimax” and ‘“‘superclimax” associations showed closer corre- 
lation with water relations than with temperature. 
The writer owes a debt of special gratitude to Professors H. C. 
Cowtes and G. D. Futter of the University of Chicago for their 
encouragement and assistance in the preparation of the data here 
presented. The courtesy of the Illinois State Park Commission 
should also be acknowledged. 
UNIVERSITY OF CHICAGO 
