54 



Carbon Assimilation. 



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 inajus 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 

 O'l 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. 



