362 BOTANICAL GAZETTE [DECEMBER 
3.8 cc., and 10.4 cc. Similarly, even during a period of low evapora- 
tion, three successive readings at station 7 give 15.3 CC., 5.5 CC., 
and 11.6 cc., while at station 1 the corresponding readings are 4.0 cc., 
1.4cc., and 1.8cc. Examination of the data from stations of 
intermediate exposure will show correlated results. 
The ratios between growth water and evaporation rates, pre- 
sented in fig. 4, serve to emphasize and render definite the ideas in 
the foregoing paragraphs. Of course, as FULLER (7) has pointed 
out, this ratio is a more or less artificial and arbitrary one, yet it is 
as good a method as we have at present for a quantitative statement 
of the relative xerophytism of different habitats, and inasmuch as it 
brings out strongly the cumulative effect of two factors operating 
in the same direction, it is of value. 
The foregoing considerations of the atmometric and soil moisture 
data suggest strongly the essentially prairie-like summer conditions, 
even in this woodland island in the prairie, and the intensification 
of the summer xerophytism of the soil by the summer xerophytism 
of the air. The greatest evaporating power of the air prevails 
precisely during those periods when the plants are least able to 
obtain water from the soil to satisfy it. Summer rains relieve the 
tension, to be sure, but even after heavy rains the relief is short- 
lived, and slighter in degree than might be supposed. Only in 
places at once well endowed with a supply of water and at least 
fairly well sheltered from extreme transpirational conditions (repre- 
sented in the present study by station 1) is there any chance for 
germination and growth of seedlings during the middle of the grow- 
ing season. This theoretical conclusion is borne out by the existence 
in such places of annuals as the dominant herbaceous vegetation, 
and by the presence of large numbers of tree seedlings. Further- 
more, it is here also that one finds “superclimax” trees, like Acer 
saccharum and Asimina triloba, belonging normally to more meso- 
phytic regions. In the more exposed stations (the extreme being 
no. 7) the unfavorable summer conditions set in so early and become 
so severe as to discourage seedling growth. The canyon bottoms 
therefore receive strangers hospitably and permit of relatively a 
succession, while the exposed uplands conservatively cling to 
climax vegetation they have, and permit, in places too see 
