592 
Journal of Agricultural Research 
Vol. V, No. 14 
The maximum of 238 gm. per hour is reached about 2.30 p. m., after 
which the transpiration falls rapidly and acquires the average night 
rate soon after sunset. There is an indication from the flattening of the 
curve after 8 o’clock a. m. that from this point on to the maximum the plant 
modifies its transpiration coefficient. 1 This may be in part due to the 
closing of the stomata during this period and in part to the lowering 
of the vapor pressure of the sap of the mesophyll cells resulting from an 
increase in concentration. 
At the bottom of figure 2 is shown the mean wind velocity for each 
hour of the day. It will be seen that the maximum rate is reached about 
7.30 a. m. There is a gradual falling off until about noon, after which the 
wind velocity remains constant until 5.30 p. m. During the night the 
rate is somewhat lower. 
The transpiration graph of wheat in figure 2 is a composite based upon 
transpiration measurements of Kharkov and Turkey winter wheats, both 
being varieties of Triticum aestivum , and of one hard spring wheat, 
Kubanka, a variety of Triticum durum . 2 The transpiration graphs for 
each variety, based upon the data given in Table I, are presented in 
figure 3. It will be noted that the graphs have essentially the same form 
and that each graph after 9 a. m. shows a falling off in the transpiration 
rate below that indicated by the slope during the early morning hours. 
oats 3 * * * * 8 
The data covering the transpiration measurements of oats (Avena 
sativa) on clear days are presented in Table V and the environmental 
measurements for the corresponding period in Tables VI to IX. The 
1 If a plant in its transpiration response to its environment acted as a free physical system, it would be 
possible to express the transpiration rate in the form of an equation involving the intensity of each of the 
individual environmental factors. If the relative part played by each factor in determining transpira¬ 
tion were known, then simply by determining the transpiration rate corresponding to some given environ¬ 
ment, the transpiration rate for any other environment could be computed. The ratio of the transpira¬ 
tion rate to the environmental intensity would then be defined as the transpiration coefficient of the par¬ 
ticular plant under observation. 
a Kharkov, C. I. (Cereal Investigations) No. 1583; Turkey, C. I. No. 1571; and Kubanka, C. I. No. 1440. 
8 Swedish Select, C. I. No. 134. 
