a tC 
. * 
36 
In observing another leaf, the intervals at the same time were respectively 
fourteen and eighty-two minutes, the ratio being practically the same. 
The effect of the passing of a cloud before the sun was observed very many 
times; it naturally varied with the depth of the shading. In similar 
~ eases the test by weighing shows a depression in transpiration, but I could 
detect no additional one to be ascribed to the higher humidity. 
It is clearly in large part because the direct sunshine heats the leaf 
above the temperature of the surrounding air that the transpiration is so 
much more rapid in it than in the brightest diffuse light. The follow- 
ing table shows the extent of this overheating. ‘The temperature was 
determined by tying a leaf backward around the bulb of a thermometer: 
sae" 
Temperature. 
Hour —— 
| In shade. |. In sun. | In leaf. 
| ° ° | fe) 
| 78.0; | mF 21.8 | 21.9 | 
| 8a.m. | 24.3 | 25.2 | 27.4 | 
/ 9a.m. | 26 30.7| 38.1 
am. 26.9 | 32 | 35.4 
| 11 a. m. | 27.8 31.5 | 034.7 
12m 28.3 | 34.7 37:7 | 
1p.m 28 | 30 031.5 
2p.m 28.5 | 31.5 38 | 
3 p.m 28.8 | 31 36.7 | 
4p.m.. 28.6 30.6 36.4 
5p. m. | 27.7 | 30s 84 
6p. m. | 26.6 | 27.6 28.5 
a Light cloud. bCloudy. 
How great a difference in evaporation, as a merely physical process, these 
differences in temperature will exert is shown by a consideration of the variation 
in the tension”® of water vapor with changes of temperature. Thus, at noon the 
temperature in the shade was 28.°3; at this point the tension of water vapor is 
28.560 millimeters; at the temperature of the exposed leaf, 37.°7, the vapor 
tension is 48.463 millimeters; at 11.30 a. m., with a temperature 28.°4, the 
relative humidity was 66. The tension of vapor in the air at that time was 
18.89 millimeters, making a relative humidity for the temperature of the leaf of 
only 39; the unsatisfied possible tension of vapor in the air was 9.69 millimeters 
in the shade, while it was 29.583 millimeters for the leaf. 
The actually observed excess of transpiration in strong, direct light 
over that in the shade was greater, as a rule, than that of evaporation from 
a water surface under the same temperature conditions; the change from 
a light haze, under which the leaf is already somewhat overheated, to full 
illumination, frequently multiplying the rate of transpiration by four. 
This extra effect may in part be due to the action of the stomata, and 
must in part be ascribed to the expansion of the gas in the intercellular 
spaces, with the consequent ejection, as the leaf is warmed, of a portion 
* Tables of Landolt and Bernthsen. 
