Carbon Assimilation. 
147 
bath to such a temperature that the E.M.F. of the combination of 
two thermocouples was zero. The two couples are then at the same 
temperature and the temperature of the water bath containing one 
junction is consequently the internal temperature of the leaf. 
In this way it was shown that the values obtained by Brown 
and Escombe for the rise in temperature of the illuminated leaf 
were much too small. The following table giving some of the 
values obtained by Blackman and Matthaei shows how much 
higher the internal temperature of the leaf may be above that of 
its surroundings when subjected to intense illumination. 
Table X. 
Effect of Light in Raising Internal Temperature of Leaves. 
Source of Light. 
Relative Intensity 
of Light. 
Temperature of 
bath containing 
Leaf Chamber. 
Internal 
Temperature 
of Leaf. 
Keith high pressure 
gas burners. 
13 
11°C 
15°C 
M 
26 
11°C 
23-7°C 
n 
45 
13-5°C 
30-5°C 
Brilliant sunlight 
in July. 
— 
18-6°C 
22-4°-30-7°C 
In all experiments with high light intensities the internal 
temperature of the leaf was therefore measured. 
In all experiments 800 c.c. of air containing from 0-8% to 2-8% 
of carbon dioxide were passed over the leaf per hour. As this was 
never used up it was supposed that carbon dioxide was not a 
limiting factor. The experiment was allowed to run for 1^ to 2 
hours before measurements were made, in order to render the 
conditions constant. The amount of carbon dioxide absorbed 
during consecutive hourly or two hourly periods was then measured. 
This gives the value of the ‘ apparent assimilation ’ to which is 
added the value found for the respiration in order to obtain the 
‘ true assimilation.’ 
Assimilation at Low and Medium Temperatures. 
As unit intensity of light was used the light from a single 
incandescent gas burner when the front of the mantle was 130 eras, 
from the leaf. With this illumination, assimilation could be detected 
at as low a temperature as —6°C. With increasing temperature 
the assimilation rapidly increased up to 3°C, above which, increase 
