Oct. 6, 1923 
Temperature of the Leaves of Crop Plants 
19 
the relation between the rate of transpiration and the temperature of 
the leaves, experiments were conducted with both turgid and wilted 
plants. The plants designated as turgid were those which showed no 
visible signs of wilting during the transpiration experiments. The soil 
in the containers in which these plants were growing was kept at a 
moisture content of approximately 23 per cent by the frequent addition 
of water to replace that which was lost from the plants. In order to 
obtain wilted plants, certain containers were set aside to which no water 
was added to replace that lost by transpiration. The plants were con¬ 
sidered sufficiently wilted for the experiments when they did not regain 
their turgid condition during the night. The water content of the soil 
in which the wilted plants were rooted averaged during the transpiration 
experiments about 2 per cent above the wilting coefficient. The loss of 
water from the plants was determined every two hours by weighing the 
containers on platform scales sensitive to 7 gm. After each experiment 
the leaves were removed from the plants and their outlines traced on 
unruled paper. The areas inclosed by these outlines were later measured 
by a polar planimeter, and from the data thus obtained the rate of tran¬ 
spiration per unit of leaf surface was calculated. The evaporating power 
of the air during the experiments was measured by means of Livingston 
spherical porous cup atmometers. 
EXPERIMENTAL DATA 
The temperature of an intact leaf of a plant exposed to natural field 
conditions is influenced by numerous factors, the most important of 
which are the temperature of the air, the supply of available moisture in 
the soil, air currents, the evaporating power of the air, and the intensity 
of the light to which the leaf is exposed. Under identical conditions the 
temperature of one kind of leaf is different from that of another kind, 
while different regions of the same leaf have different temperatures. 
Owing therefore to the numerous factors influencing the temperature of 
leaves, any data presented upon that subject must be regarded as relative 
only to the conditions that prevailed when the temperature determina¬ 
tions were made. 
FLUCTUATIONS IN TEMPERATURE 
In direct sunlight when the temperature is relatively high and when 
the air is in motion, the temperature of the leaves of plants and the 
surrounding air is not constant, but shows sudden and marked fluctua¬ 
tions, even during so brief a period as a few seconds. These changes in 
temperature, which vary from a small fraction of a degree centigrade to 
as high as 4 0 C. or more, are easily detected by the galvanometer, but are 
of such short duration that as a rule they are not visibly recorded by a 
mercury bulb thermometer, even when it is graduated to tenths of a 
degree. These rapid fluctuations in the temperature of the air are due 
in all probability to the fact that the air is not uniformly heated through¬ 
out, but contains warmer or cooler pockets which suddenly replace the 
air surrounding the measuring instruments. The leaves of plants respond 
very quickly to the changes in temperature of the surrounding air, for 
even a very slight increase or decrease in the temperature of the air is 
almost immediately followed by a corresponding change in the tempera¬ 
ture of the leaves. When the air is still and when the temperature is 
relatively low, the fluctuations in the temperature are so few and so small 
