Jan. 3, 1916 
Hourly Transpiration Rate on Clear Days 
635 
transpiration," “transpiring power" (Livingston and Hawkins, 1915), to 
the ratio of the transpiration rate to the evaporation rate of his porous- 
cup atmometer. It has been shown 1 that the graphs representing 
transpiration and the evaporation from the porous-cup atmometer are 
similar in form, but that their maxima do not as a rule occur at the same 
time in plants exposed to extreme conditions. Furthermore, when the 
Fig. 19.—Comparison of the transpiration with the evaporation from 
a free-water surface in a shallow, blackened tank, both plotted in 
percentage of the maximum range. 
ratio of the transpiration to evaporation (the relative transpiration) is 
plotted against time, the daily graph usually shows two maxima, one in 
the morning and a second in the afternoon. 
Graphs representing the ratio of the transpiration rate of rye, alfalfa, 
and amaranthus to the evaporation rate are given in figure 20 and show 
1 See also Shreve, 1914; Bakke, 1914. 
