Oct 6 , 1933 
Temperature of the Leaves of Crop Plants 
31 
cerning the influence of these observed differences in temperatures upon 
the life processes in the leaves. 
The temperature of the air during these observations on the relation¬ 
ship of the transpiration rate and temperatures of leaves ranged from 
29 0 to 38° C. The climatic conditions during July and August, 1922, 
were comparatively mild, so that the maximum temperature of the air 
during the experiments was from 2 0 to 6° C. lower than that commonly 
experienced during the most severely hot part of the growing season in 
Kansas. The data presented, however, are believed to represent the 
temperature relationships of the wilted and turgid leaves of plants under 
the conditions in the field. An increase in the temperature of the air of 
from 2 0 to 6° C. would not materially alter the above-observed temperature 
differences between wilted and turgid leaves, since, as discussion will 
show later, the heat absorbed by the type of leaves used in these experi¬ 
ments is quickly dissipated, so that their temperature rises to only a 
few degrees above that of the surrounding air. The temperature of the 
wilted leaves during the early morning and evening hours and during the 
night rapidly drops to that of the turgid leaves, which is approximately 
the temperature of the surrounding air. 
IyEAF TEMPERATURE DURING THE DAY AND NIGHT 
The greater number of the 20,000 determinations of the relative 
temperature of the air and leaves that are herein reported was made 
upon the upper surfaces of the turgid leaves of plants during the daylight 
hours from 9 a. m. to 4 p. m. The plants upon which the observations 
were made included four varieties of sorghum and one variety each of 
watermelon, pumpkin, cowpea, soybean, and alfalfa. The average of 
1,000 temperature observations of the air and of the upper surface of 
the turgid leaves of corn in direct sunlight and under a wide range of 
atmospheric conditions was 30.58° C. for the air and 30.64° C. for the 
leaves. The average of the same number of determinations made with 
the leaves of five sorghum varieties, under conditions of exposure to 
the sun similar to those of the corn leaves, showed that the temperature 
of the leaves was 30.64° C., while the average of an equal number of 
determinations of the air temperature during the same period was 
30.66° C. Five hundred observations on soybeans gave an average 
temperature of the air of 33.13° C., and an average of 33.66° C. for the 
temperature of the upper surface of the leaves, while a similar number 
of determinations on the leaves of cowpeas showed an average tem¬ 
perature of 34.4° C., and the temperature of the air averaged 34.2° 
during the observations. The determinations made upon the leaves of 
the pumpkin and watermelon were few in number, but the data obtained 
showed that the average temperature of the upper surface of the leaves 
of these plants was approximately the same as that of the surrounding 
air. The average of about 200 temperature determinations on the leaves 
of alfalfa was 27.8° C., while the air temperature averaged 28.6° C. 
These observations show that the temperature of the turgid leaves of 
com, sorghum, pumpkin, watermelon, and soybean in direct sunlight, 
under the general climatic conditions which prevail during the growing 
season in Kansas, fluctuates slightly above and below air temperature, 
but that the average temperature is the same as that of the surrounding 
air. The temperature of the leaves of cowpeas and alfalfa under the 
same conditions as those of the other plants consistently showed a 
temperature of less than 1° C. below that of the surrounding air. These 
