RESEARCH METHODS IX STUDY OF FOREST ENVIRONMENT. 153 
mathematically the effects of the several components on the evapora- 
tion from the plant. One is just as far from an absolutely precise 
outcome as the other. The advantage of the atmometer is that, once 
the integration has been accomplished in the construction of the in- 
strument, the observations are relatively simple and no further com- 
plex calculations are necessary. 
As has been suggested, it is not to be presumed that the atmometer 
will show restricted evaporation due to control, as may be the case 
with the plant when it closes its stomata or when transpiration is 
automatically reduced by increasing density of the cell sap. By 
comparing the most perfect atmometer with plants, however, it 
should be possible to measure the actual effectiveness of these plant 
controls. 
It may be worth while to suggest, for the sake of more effective 
evaporation studies, that it is possibly erroneous for the student 
of plant life to look upon a large evaporation factor in the habitat 
as necessarily inimical to the plants which are there. Some theo- 
retical considerations which point to transpiration as a benefit have 
already been outlined. Xo attempt will be made, however, to decide 
the question as to whether it is beneficial to the plant or merely a 
necessary evil. Laying this question aside, it is perfectly evident 
that conditions conducive to high transpiration rates are an unavoid- 
able concomitant of the conditions necessary to active photo-synthesis. 
TVTien therefore, as in Weaver's (169) succession from prairie to 
brush or woodland types, it is found that succession produces a stead- 
ily decreasing evaporation rate, shall it be concluded that the plants 
of the brush stage are directly favored by the decreased evapora- 
tion, only relatively favored, or not helped at all, but merely able to 
succeed with less sunlight than the plants of the prairie ? Very 
likely, in a case of this kind, the rate of evaporation, while a service- 
able index to the general conditions, may not itself be a controlling 
factor, or may be a controlling factor only for a brief period in each 
season when drought occurs. It would appear to be all-important 
that evaporation rates be closely correlated with the moisture of the 
soil, as is done by Shreve (166) in giving directly, if somewhat 
crudely, the ratio of evaporation to soil moisture contents in various 
habitats. What is perhaps more important is that a clear distinc- 
tion should be made between evaporation stresses when there is an 
abundance of soil moisture, and those existing when the moisture 
supply is nearly exhausted. There are. also, critical periods brought 
about by excessive evaporation when the soil moisture is apparently 
all that it should be. These and their effects must be separatply 
analvzed. 
