634 
Vol. V, No. 14 
Journal of Agricultural Research 
group of plants as a whole, 43 per cent of the transpiration took place 
before noon and 57 per cent in the afternoon, while the average radiation 
during the period was slightly greater in the forenoon. 
In the last column of the table is given the percentage of transpiration 
taking place between 11 a. m. and 3 p. m. While these figures are not 
directly comparable, owing to the difference in the length of the day— 
i. e., in the number of daylight hours— it is clear that from one-third to 
Fig. 18.—Comparison of transpiration with wet-bulb depression, both plotted in percentage of the 
maximum range. 
one-half of the transpiration during the 24-hour period takes place from 
11 a. m. to 3 p. m. 
RATIO OF TRANSPIRATION TO EVAPORATION 
Transpiration is often regarded as evaporation modified to some extent 
by plant structures and plant functions. Both are influenced by radia¬ 
tion, temperature, saturation deficit, and wind. Because of the simi¬ 
larity of the two processes, the evaporation rate has often been u^ed as a 
standard to which the transpiration is referred. 
Livingston (1906 and 1913) has given special attention to the relation 
of transpiration to evaporation, and has applied the terms “ relative 
