Oct. 6, 1923 
Temperature of the Leaves of Crop Plants 
17 
The other thermo junction, TC, was attached to a clamp, C, in such a 
way that it could be conveniently placed upon the surface of a leaf. This 
clamp consisted of a pair of brass tongs modified by completely inclosing 
their distal ends with heavy layers of cork, CK, shaped into a wedge form 
so that the dimensions of the edges in contact when the clamp was closed 
were only 3 by 10 mm. By means of a clamp of this kind the thermo¬ 
junction could easily be placed as desired in direct contact either with the 
upper or lower surface of the leaf, and the temperature thus determined. 
Similarly, the temperature of the air could readily be determined by hold¬ 
ing the open clamp in the air and taking precautions to shield the thermo¬ 
junction from the direct rays of the sun by the cork of the clamp. 
A portable telescopic galvanometer, G, with a sensitivity of 0.025 
microamp. and with a scale having divisions of one-half cm., graduated 
to tenths, was placed in the circuit, which, with a damping switch, S, 
completed the main part of the apparatus. In addition, a thermometer, 
T, graduated to o.i° C. was used for taking the temperature of the air 
for comparison with the temperatures obtained by means of the gal¬ 
vanometer. Since the temperature of the thermojunction, T'C', was 
known and could be kept approximately constant, the difference in tem¬ 
perature between T'C' and TC could be calculated from the swing of the 
galvanometer. It was found that for a considerable range of temperature 
a deflection of one scale division indicated a difference in temperature of 
i° C. between the two thermo junctions, and that under the conditions of 
the field a difference of o. 1 0 C. could be accurately detected. 
The apparatus when in use in the field was placed upon a small, 
specially constructed table, shaded by a heavy piece of canvas placed 
about 2 feet above its surface. When vines and other low plants were 
the subject of investigation, the apparatus was placed on a low box and 
shaded with an umbrella. The clamp and its thermo junction, as well as 
a portion of the connecting wires, were not shaded, but were always freely 
exposed to the atmospheric conditions prevailing during the experiments. 
As the temperature determinations were largely comparative ones, the 
errors that might be due to the absorptions of heat by the clamp and 
exposed portions of the wires were not taken into consideration. 
LEAF TEMPERATURE DETERMINATIONS 
The temperature determinations herein reported were made upon the 
attached leaves of plants growing either in the field or in large metal 
containers that were exposed to field conditions. Certain leaves upon 
the same plant or upon different plants were selected for a given series 
of temperature determinations, care always being taken to select leaves 
of about the same age and with approximately the same exposure to the 
incident rays of the sun. After the leaves for an experiment had been 
selected, the table containing the apparatus was brought into a position 
where the thermojunction could be conveniently placed upon tfye leaves 
to be studied. The surface of the table was always brought to a level 
position before any determinations were made, for, unless this is done, 
the galvanometer readings of one set of experiments are not comparable 
to those of another. All the temperature determinations were made by 
two persons. One by means of the clamp held the thermojunction on 
the leaf while the other pressed the key and recorded the swing of the 
galvanometer. In this manner the swing of the galvanometer in any 
given determination could be read in approximately two to three seconds. 
60372—23 - 2 
