64 
JOHN H. EHLERS 
rays — one at approximately 45°, the other at 90°. For this purpose 
a cut branch, partially protected from the wind by a screen of white 
cheese cloth, was exposed to solar illumination. The screen was 
open at the top to prevent the enclosed air from becoming heated. 
Doubtless there was some reflected radiation, but, since the leaves 
were side by side within the enclosure and therefore under the same 
conditions, this did not affect the results. These are given in Table 
XV. Other attempts in which the differential temperatures of the 
two leaves were read alternately in rapid succession gave similar 
results. 
Table XV 
March 5, IQ14 
Temperature of leaves at different angles 
Time of 
Aver. Bridge 
Calculated 
Shade Temp. 
^Black Bulb in 
Observation 
Reading in Cm. 
Temp. Difif. 
Vacuum 
2.15 
454 
5.29° 
49.8 
5.80 
Angle 45° 
46.3 
5.39 
48.0 
5-59 
14.0° 
36.1° 
38.6 
4.50 
43-0 
5.01 
31.0 
3.61 
2.20 
35-5 
4.14 
Average 
42.2 
4.92 
2.25 
58.9 
6.86° 
64.2 
747 
64.5 
7.51 
Angle 90° 
57.1 
6.65 
14.2° 
36.0° 
49.0 
5.7i 
47.0 
548 
54-5 
6.35 
50.0 
5.83 
2.30 
54-6 
5.36 
Average 
55-5 
6.36 
B. Direct Evidence of Photosynthesis in Winter 
An attempt was made to obtain direct evidence of photosynthesis 
under winter conditions by examining for starch content the leaves of 
the various conifers growing in the university arboretum. The leaves 
were collected in early morning and again in late afternoon of clear 
days. They were kept in 75 per cent alcohol and later examined 
microscopically. Thin sections were placed on a slide and treated 
