56 



Carbon Assimilation. 



varies, the rate of intake of carbon dioxide must also vary and 

 the results obtained may be 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 be 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. 



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. 



