Oct. 6, 1923 
Temperature of the Leaves of Crop Plants 
4i 
Other investigators have observed that the temperature of turgid 
leaves in diffuse light is lower than in the direct sunlight. Blackman 
and Matthaei (2) found that the internal temperature of the detached 
leaves of cherry laurel in direct sunshine was from 4 0 to 13 0 C. higher 
than that of the air, but that when the same leaf was placed in the shade 
its internal temperature was only i° to 1.5 0 C. higher than the air that 
surrounded it. Smith (14) found that the internal temperature of leaves 
in the shade varied from 1.5 0 C. below to 4 0 C. above that of the air, but 
that the internal temperature of the same leaves placed in direct sun¬ 
shine reached a temperature of as much as 15 0 C. above that of the 
surrounding air. 
SUMMARY 
(1) Method .—By means of a thermoelectrical device approximately 
20,000 observations were made in a study of the relationships of the tem¬ 
perature of the air to that of attached leaves of corn, sorghum, cowpeas, 
soybeans, alfalfa, pumpkin, and watermelon growing under field condi¬ 
tions at Manhattan, Kans., during the summer of 1922. The tempera¬ 
ture of the leaves of these plants was studied along four lines: The relation 
of leaf temperature to the rate of transpiration, the temperature of the 
leaves during the day and night, the temperature of different portions of 
the leaf, and the temperature of the leaves in direct and in diffuse sun¬ 
light. 
(2) Influences to be considered .—The temperature of a leaf is influenced 
by the temperature of the air, by the available water supply in the soil, 
by air currents, by the type of leaf, by the intensity of the light to which 
it is exposed, and by other factors, so that any data presented in regard 
to the temperature of leaves must be considered as relative only to 
the conditions that prevailed when the temperature determinations 
were made. 
(3) Temperature fluctuations .—Under ordinary field conditions during 
the daylight hours, the temperature of the leaves and of the surrounding 
air is not constant even during so brief a period as a few seconds, but 
shows sudden and marked fluctuations which vary from a fraction of a 
degree centigrade to as much as 4 0 C. or more. Consequently, the 
average of a number of consecutive determinations gives a more exact 
index of the temperature behavior of leaves and the surrounding air 
than a single determination. Each temperature value reported for 
these experiments is the average of from 10 to 20 separate determina¬ 
tions taken during a 10 to 20 minute period. 
(4) The relation of leaf temperature to the rate of transpiration .—For a 
large number of determinations during the hours of 9 a. m. to 4 p. m. the 
average temperature of the wilted leaves of corn, sorghum, soybeans, and 
cowpeas was respectively, 1.85°, 1.55 0 , 2.8°, and 4.65° C. higher than the 
temperature of the turgid leaves of these plants under the same condi¬ 
tions, with the exception of the amount of water in the soil. During 
the transpiration-temperature experiments, the percentage of available 
water in the soil above the wilting coefficient was 2 to 4 per cent for the 
wilted plants and 10 to 12 per cent for the turgid plants. The aver¬ 
age transpiration rate of the turgid leaves was much higher than that 
of the wilted leaves. The ratio of the rate of transpiration of the turgid 
leaves to the rate of transpiration of the wilted leaves was as 2.5 to 1 in 
the case of corn and sorghum, and as 3.5 to 1 in the case of cowpeas and 
soybeans. 
