26 



SCIENCE PROGRESS. 



In this case even in direct sunlight no CO, is absorbed 

 by the upper surface of the leaf ; all that is taken up enters 

 on the lower surface. 



From the combined consideration of the concordant 

 results obtained on respiration and assimilation, we see 

 definitely that, at least, as far as carbon dioxide is con- 

 cerned, gaseous exchange closely follows stomatic dis- 

 tribution on the two surfaces of leaves. From this it 

 results that this exchange takes place practically entirely 

 through the openings of the stomata into the intercellular 

 spaces. The only alternative hypothetical possibility, that 

 in every case the exchange takes place across the epidermis 

 cells and cuticle of those areas which bear stoma' a, and yet 

 not through the stomatal openings, can hardly be seriously 

 entertained. This supposition would involve, among others, 

 the assumption that the cuticle on the lower surface of 

 hypostomatiferous leaves is often fifty to a hundred times 

 more permeable than that on the upper surface. Mangin 

 has however shown that it is never more than four or five 

 times as permeable. The normal difference in gaseous 

 exchange, which is almost of a higher order of magnitude, 

 cannot therefore possibly be explained in this way. 



It is further of interest to note the assimilation by the 

 leaf in bright light of its own CO, formed by respiration. 

 An experiment was made with a leaf of Aav platanoides (all 

 the stomata below) by attaching a capsule to the lower 

 surface only, and supplying to it a current of air freed from 

 CO. 



The third estimation was however commenced rather 

 too soon after the period of darkness for this to be quite 

 conclusive evidence of escape of CO, in diffuse light. 



