54 
JOHN H. EHLERS 
sunlight; (2) the effect of one fourth to one half full sunlight; (3) the 
effect of diffuse light; (4) the leaf temperature at night; (5) the effect 
of strong air currents; (6) the temperature of leaves at different 
angles to the sun's rays. 
(i) The Effect of Full Sunlight. — Unfortunately no data on the 
effect of full sunlight in still air could be obtained. Low temperature, 
still air, and full sunlight are a combination of conditions that rarely 
obtains. The nearest approach to these conditions was found on 
February 24. The data obtained are given in Table IV. 
Table IV 
February 24, IQ14 
Clear. Light breeze. Relative humidity 87% 
1 
2 
3 
4 
5 
6 
7 
8 
9 
Time of Obser- 
vation 
No. of 
Obser- 
vations 
Aver. 
Bridge 
Reading 
in Cm. 
Calculated 
Temp. 
Diff. 
Shade 
Temp. 
Black 
Bulb in 
Vacu- 
um 
DifF. Col- 
umns 5 
and 6 
Actual 
Leaf Temp. 
Wind 
Velocity- 
Miles 
per Hr. 
9.00- 9.02 
9.21- 9.24 
9.28- 9.33 
10.02-10.04 
10.50-10.55 
4 
4 
7 
4 
7 
26.1 
48.4 
44.4 
60.4 
44.0 
3.26° 
6.04 
5.54 
7-51 
5.46 
-15.0° 
-I4.I 
— 14.0 
-13.0 
— 12.2 
5.6° 
14.4 
20.0 
20.6 
25.6 
20.6° 
28.5 
34.0 
33-6 
37.8 
-11.74° 
- 7.06 
- 8.46 
- 5-49 
- 6.74 
1.2 
1.2 
2.4 
.4 
.8 
11.00 
II.QO5 
II.OI 
II.OI5 
11.02 
11.025 
11.03 
11.04 
11.05 
56.0 
37.2 
50.8 
38.2 
64.0 
59.6 
46.5 
40.0 
54.0 
6.96 ' 
4-52 
6.31 
4-75 
7-95 
7.41 
5.78 
4-97 
6.71 J 
-12.5 
28.3 
40.8 - 
- 5-54 
- 7.98 
- 6.19 
- 7.75 
- 4-55 
- 5-09 
- 6.72 
- 7.53 
- 5.79 
- 1.2 
1 1. 08-1 1. 12 
II.25-II.32 
II.29-I2.35 
2.35- 2.40 
4 
8 
7 
8 
58.1 
56.5 
57.6 
41.5 
7.21 
7.00 
7.11 
5.13 
— 12.0 
— II. I 
— 10.2 
— lO.O 
28.9 
28.9 
29.4 
II.7 
40.9 
40.0 
39.6 
21.7 
- 4.79 
- 4.10 
- 3-09 
- 4.87 
1.2 
•9 
•5 
.3 
Averages 
6.04 
— 12.4 
33.8 
- 6.36 
I.O 
With the shade temperature of the air varying from — 15° to 
— 10° C. the black-bulb thermometer registering from 20.6° to 40.9° C. 
higher, and the wind velocity varying from 0.3 to 2.4 miles per hour, 
the average differential temperature between leaf and air for the 
entire time during which the readings were taken was 6.04° C, i. e., 
