1912] GLEASON & GATES—RATES OF EVAPORATION AgI 
2. Differences in the amount of evaporation in various asso- 
ciations are due chiefly to the nature of the vegetation, which by its 
size and density controls the evaporation beneath it. 
3. The observations indicate that successions between associa- 
tions are not caused by any conditions of evaporation. 
4. The more primitive associations have the higher rates of 
evaporation, while those most nearly like the-climax type have the 
lowest rates. This is true not only for the forest associations, in 
which low evaporation is expected, but also for the prairie asso- 
ciations, which are correlated with an arid climate and consequently 
high climatic evaporation. 
The standardization and field observation of the atmometers 
and the evaluation of the relative rates of evaporation were managed 
entirely by the junior author; for all other statements both authors 
are responsible. 
UNIVERSITY OF MICHIGAN 
Ann Arsor, Micu. 
