Carbon Assimilation. 
1 23 
varies, the rate of intake of carbon dioxide must also vary and 
the results obtained may he due to the greater degree of opening of 
the stomata on the illuminated side. Brown and Escombe also think 
that as more energy is absorbed by the chloroplasts of the palisade 
parenchyma, carbon dioxide will he more rapidly utilised in that 
part of the leaf and consequently the diffusion gradient will be 
steeper in the intercellular spaces of the palisade into which the 
stomata of the upper surface open, which will also favour a more 
rapid intake of carbon dioxide by the stomata of the upper surface. 
Further evidence as to the path of carbon dioxide into and 
out of the leaf has been obtained by investigating the gaseous 
exchange when the stomata are artificially blocked. It had 
previously been asserted by Boussingault that the path of carbon 
dioxide intake was through the cuticle. This conclusion was 
based on an experiment in which leaves of Nerium were painted 
over with lard. In one the astomatic upper surface was so covered, 
in the other the lower surface, with the result that the leaf with its 
stomata blocked assimilated more. Blackman shows that this result 
is due to the use outside the leaf of too high a concentration of 
carbon dioxide (more than 30%). 
The results of Blackman’s own experiments with leaves of 
Nerium oleander are given in the subjoined table and show clearly 
the effect of increasing the carbon dioxide concentration. 
Table VI. 
Mean percentage 
of CO 3 present in 
each experiment. 
CO 2 in c.c. decomposed per 
unit area. 
Ratio of amount of C0 3 
decomposed per unit area. 
Normal leaf. 
Vaselined leaf. 
Normal leaf. 
Stomata blocked. 
6 
0-07 
o-oi 
1 
0-14 
6-3 
0-055 
o-oi 
1 
0-20 
7-5 
0 046 
0017 
1 
0-21 
14 
0-18 
004 
1 
0-37 
55 
0-049 
0-067 
1 
1-3 
50 
0-043 
0-069 
1 
1-5 
97 
0-033 
0-060 
1 
1-8 
Some further experiments made with Nerium oleander show 
that more carbon dioxide passes through the vaselined under 
surface than through the unvaselined cuticle of the upper surface, 
so that coating the leaf with vaseline does not render it impervious 
to the passage of carbon dioxide. 
