Carbon Assimilation. 
i 2 i 
was taken up by the astomatic upper surface. In the case of 
Alisma Plantago , which has stomata on both sides of the leaves, 
% 
there was found in every experiment performed a constant tendency 
for the absorption of carbon dioxide to be greater by the upper 
surface where the stomata are more frequent. Confirmatory 
results were obtained with Tropceolum majns and Acer platanoides. 
From these experiments the conclusion is drawn that the 
intake and evolution of carbon dioxide takes place through the 
stomata. The only alternative explanation is that in the case of 
leaves with the stomata confined to the lower surface the cuticle 
on the lower surface is fifty to a hundred times more permeable to 
carbon dioxide than the lower surface. It seems impossible to 
suppose this the case, especially as leaves with thin cuticles gave 
results exactly similar to those obtained with leaves possessing 
very thick cuticles. 
Although accepting Blackman’s results in regard to the 
exhalation of carbon dioxide in respiration, Brown and Escombe 
(1905 a) are of opinion that his method is not so well adapted to 
investigations of the intake of carbon dioxide in assimilation, chiefly 
because the amounts of carbon dioxide dealt with seldom exceeded 
0*1 c.c. with a possible experimental error of one-tenth of that 
amount. Brown and Escombe therefore performed some experiments 
similar to Blackman’s under conditions which admitted the 
measurement of carbon dioxide taken in by the two sides of a 
leaf on which the distribution of stomata was known. 
The following tables taken from Brown and Escombe’s paper 
exhibit their results. 
Table IV. 
Respiration from the two Surfaces of various Leaves. 
CO 2 evolved 
Ratio of CO 2 
Ratio of stom- 
Plant. 
Time in 
Leaf area in 
in c.cs. 
evolved. 
atic distribution 
hours. 
sq. cms. 
Upper 
Upper 
Upper 
Lower 
Lower 
Lower 
8-41 
100 
100 
Canna indica 
4-75 
28 27 
20-76 
246 
246 
5-55 
100 
99 9 9 
5-0 
28-27 
17-90 
322 
» 9 
304 
100 
9 9 9 9 
4-23 
28-27 
6-40 
210 
9 9 
1-03 
100 
100 
Rumex alpinus 
5-5 
59-44 
3-60 
286 
269 
