Transpiration as a Factor in Crop Production. 115 
of the two-week period following the transfer, and averaged for 
this purpose. The results are given in Table 40. 
The results indicate that there was no significant adaptation 
of the plants under one condition of atmospheric humidity which 
would either retard or accelerate the transpiration when brought 
under markedly different conditions. 
EFFECT OF TRANSPIRATION ON LEAF TEMPERATURE. 
It is a well-known principle in physics that evaporation has a 
cooling effect upon the evaporating body. It, therefore, seems 
reasonable that the evaporation from the leaf should have a 
modifying influence upon the atmospheric temperature affecting 
it. The resultant reduction in leaf temperature must serve to 
reduce the transpiration rate. In this manner transpiration tends 
to reduce its own magnitude. Some preliminary data relative to 
leaf temperatures were secured during 1912 and 1913. In 1913 
the comparative temperatures were determined for a transpiring 
leaf of a corn plant growing in the cornfield, and of a dry, dead 
leaf having the same exposure. Being fairly similar objects, the 
two leaves should have acquired rather similar temperatures had 
not the evaporation from the living leaf exerted a cooling effect. 
Water taken in by the roots naturally acquires the relatively 
low temperature of the soil, and its movement thru the stalk 
and leaves also exerts a somewhat cooling effect. The data follow 
in Table 41. 
Table 41. — Summary of plant temperatures showing the effect of 
transpiration upon leaf temperature. (Average for 7 days, 
August 25 to 30, 1913.) 
Time of day 
Sun or shade 
Average temperatures in sun and shade 
Green leaf 
Dry leaf 
Air 
(1) 
8:00 a. m 
(2) 
Sun 
Shade 
Average 
Degrees F. 
(3) 
79.8 
76.4 
78.1 
Degrees F. 
(4) 
81.2 
77.6 
79.4 
Degrees F. 
(5) 
78.4 
75.8 
77.1 
2:00 p. m 
Sun 
Shade 
Average 
95.4 
93.8 
94.6 
104.0 
98.0 
101.0 
99.5 
97.0 
98.2 
5:30 p. m 
Sun 
Shade 
Average 
91.7 
90.2 
90.9 
96.7 
94.0 
95.3 
97.2 
94.2 
95.7 
Daily av. in sun 
Daily av. in shade .... 
Daily av 
88.9 
86.8 
87.8 
94.0 
89.9 
91.9 
91.7 
89.0 
90.3 
