36 
Journal of Agricultural Research 
Vol. XXVI, No. I 
The temperatures of the upper and lower surfaces of leaves were deter¬ 
mined for a considerable number of different plants under conditions of 
both direct and diffuse light, as shown in Table VII. The temperature 
of the lower surface of leaves exposed to direct sunlight was always con¬ 
sistently lower than that of the upper surface, although the differences 
were always less than i° C. It could not be determined whether this 
difference in temperature was due to a difference in the rate of transpira¬ 
tion of the two surfaces or whether it was caused by the upper surface 
being more highly heated on account of its more direct exposure to the 
rays of the sun. In diffuse light or during the early evening and morning 
hours little or no difference was observed between the temperature of 
the two leaf surfaces. 
Fig. 5.—Graphs showing the temperature of the air and of the turgid leaves of Pride of Saline corn and 
Dwarf milo during the day and night July 18 and 19, 1922. 
TEMPERATURE OF LEAVES IN DIRECT AND IN DIFFUSE SUNLIGHT 
A study of the temperature of leaves in direct and in diffuse sunlight 
was made at various periods of the day upon three varieties of corn, four 
varieties of sorghum, and one variety each of cowpeas, soybean, velvet 
bean, alfalfa, watermelon, and pumpkin. 
Some of the observations were made upon leaves in diffuse light which 
was artificially provided by shading the plant or some portion of it with 
an ordinary umbrella. The data obtained under these conditions are 
shown in Table VII. In a number of cases the passage of a heavy cloud 
over the sun in the middle of a series of determinations of leaf tempera¬ 
tures gave an opportunity to observe the behavior of the temperature of 
a leaf when its light exposure is suddenly changed from direct sunlight 
to diffuse light. The data for two series of observations of this kind are 
