1911] FULLER—EVAPORATION AND SUCCESSION 207 
7. The differences in the rates of evaporation in the various 
plant associations studied are sufficient to indicate that the atmos- 
pheric conditions are efficient factors in causing succession. 
Conclusions 
From the study of the data available, it seems evident that 
the porous-cup atmometer measures with very considerable accuracy 
the atmospheric factors which combine in making demands upon 
the water-supply of the aerial portion of the plant; the data, there- 
fore, may be directly related to the plants in an association, and 
used in determining the comparative xerophytism of plant habitats 
in so far as they are determined by atmospheric conditions. In 
such determinations it would appear that the true measure of the 
limiting atmospheric factors must be found either in the demand 
throughout the entire growing season as expressed in the average 
evaporation rate for that period, or in a maximum demand of several 
days’ duration occurring at a period when the water-supply in the 
soil is deficient, such as would be expressed in a high rate continu- 
ing for a week or more in the latter part of the summer. In the 
associations studied, these demands show practically the same ratio 
when compared with one another (figs. 5 and 6). If this be the 
case, we have in the Livingston or Transeau atmometers instru- 
ments of sufficient precision to furnish the most valuable quantita- 
tive data in the study of plant associations. 
A complete study of the water relations of a habitat may be 
obtained by combining the data supplied by the atmometer with 
quantitative determinations of the available soil-moisture. It is 
hoped that some such data may be available in the near future. 
It seems highly desirable, in investigations of this character, 
that the different investigators employ instruments standardized 
to the common unit recommended by LivinesTon (5), and further 
that a plant association of wide distribution be used as a basis of 
comparison, and that the conditions in other associations be 
expressed in terms of these units whenever it is possible to do so. 
As no association is more widely spread in the United States than 
the climax mesophytic forest which is frequently characterized 
by the presence of either Acer saccharum or Fagus grandifolia, or 
