Oct. 6, 1923 
Temperature of the Leaves of Crop Plants 
35 
TEMPERATURE OP DIFFERENT PORTIONS OF THE EEAF 
Some determinations were made to compare the temperature of the 
base and tip of the leaves of cowpea, soybean, velvet bean, Sudan grass, 
Blackhull kafir and two varieties of com under conditions of direct and 
diffuse sunlight. The conditions necessary to produce diffuse light were 
obtained by the opportune passing of clouds over the sun or by shading 
the plant, or the portion of it under observation, by an ordinary umbrella. 
The data obtained from these experiments are shown in Table VI. The 
experiments with these leaves in direct sunlight show that the tempera¬ 
ture of the base of the leaf is always consistently lower than that of its 
tip. The differences in temperature observed varied from o.i° to 1.5 0 C., 
depending evidently upon the nature of the leaf and upon the available 
water supply. In diffuse light the temperature differences between the 
two portions were smaller than those observed in direct light, and in a 
few cases no differences in temperature were found. 
Table VI .—Difference in temperature of the upper surface of the base and tip of the leaves 
of various plants 
Time. 
Kind of plant. 
Aug. 12: 
2.35 to 2.50 p. m. 
2.5s to 3.10 p. m. 
3.45 to 4.00 p. m. 
4.15 to 4-3° P- m . 
Aug. 14: 
10.05 to 10.20 a. m... 
10.25 to 10.40 a. m... 
2.20 to 2.35 p. m. 
2.55 to 3.10 p. m. 
3.45 to 4.00 p. m. 
4.10 to 4.25 p. m. 
Aug. 15: 
10.50 to 11.05 a. m... 
11.10 to 11.25 a. m... 
2.10 to 2.25 p. m. 
Aug. 16: 
10.05 to 10.20 a. m... 
10.25 to 10.40 a. m... 
10.55 to 11.10 a. m... 
11.10 to 11.25 a. m... 
1.30 to 1.50 p. m. 
1.55 to 2.10 p. m. 
3.30 to 3-45 P- m . 
3.50 to 4.05 p. m. 
Cowpeas, Victor. 
.do. 
Soybeans, Morse. 
.do. 
Kafir, Blackhull. 
.do. 
Sudan grass. 
.do. 
Corn, Reid Yellow Dent.... 
.do. 
Corn, Commercial White. ... 
_do. 
_do. 
Bean, Velvet. 
_do. 
I Cowpeas, Victor. 
I.do. 
I Soybeans, Morse 
.do. 
Num¬ 
ber of 
deter¬ 
Condition. 
Average temperature 
of— 
mina¬ 
tions. 
Air. 
Base. 
Tip. 
20 
In sunshine. .. 
°C. 
35-5 
°C. 
35-4 
°C. 
36.8 
20 
Shaded. 
34 -o 
33 - 7 
33-8 
20 
In sunshine... 
35-9 
37-0 
3 7 - 2 
20 
Shaded. 
33-0 
32. 8 
32.8 
20 
.do. 
31-4 
30 - 6 
30. 8 
20 
In sunshine. .. 
33 - 2 
33- 2 
33-4 
20 
.do. 
33 - 1 
32- 6 
32. 7 
20 
Shaded. 
32-9 
32.5 
32. 6 
20 
In sunshine. .. 
35-4 
36-4 
37-3 
20 
Shaded. 
32. 2 
31-9 
32. 1 
20 
In sunshine... 
32. 2 
32 - 0 
32. 5 
20 
Shaded. 
30 - 2 
29.9 
30- 1 
20 
Clouds. 
30 . 7 
30. 0 
30. 0 
20 
Shaded. 
30- 6 
30. 1 
30- 2 
20 
In sunshine... 
32 - x 
32. 1 
32.4 
20 
Shaded. 
32. 2 
31-6 
3 i- 7 
20 
Clouds. 
30 - 5 
30.3 
30.4 
20 
In sunshine... 
35 - 1 
36.8 
37 - 2 
20 
Shaded. 
35- 6 
35 *o 
35 -o 
20 
In sunshine. .. 
34-4 
34-3 
34-4 
20 
Shaded. 
34 - 0 
33 - 8 
33-8 
It would seem that this temperature difference between the two 
regions of the leaf is due to a difference in the available water supply. 
Under conditions of relatively high evaporation the water supply entering 
the leaf may be so depleted by transpiration in the basal region that the 
tissue near the tip does not receive a supply of water adequate to main¬ 
tain a transpiration rate equal to that of the lower portion of the leaf, 
and consequently the temperature of the tip is higher than that of the 
base. The fact that this difference in temperature diminishes or dis¬ 
appears entirely under the milder atmospheric conditions in diffuse light 
would seem to strengthen this interpretation of the results, but the rela¬ 
tive rate of transpiration of the two regions of the leaf must be determined 
before anv conclusions can be drawn. 
