18 SCIENCE PROGRESS. 



times, in spite of the fact that the lower is usually riddled 

 with stomata covered only with a film of glycerine jelly, 

 while there are but few of these weak spots on the upper 

 cuticle. This shows that the actual permeability of the 

 cuticle of the lower surface, apart from -the stomata, is never 

 as great as five times that of the upper cuticle, a determina- 

 tion that we shall subsequently see to be of importance. 



In the second part of his paper, from experiments on the 

 respiration and assimilation of leaves, the author comes to 

 the conclusion that the amount of osmosis that could take 

 place through the cuticle, as calculated from the above data, 

 is quantitatively insufficient to account for the whole of the 

 necessary gas exchange, but may account for a half to two- 

 thirds of it in respiration. During assimilation, however, he 

 holds that the stomata must carry on practically the whole of 

 the exchange as the tension of C0 2 in the air is too small to 

 produce much osmosis through the cuticle. 



In a note to a paper on starch formation from sugar and 

 other substances, Boehm, in 1889 (12), mentions that he has 

 performed experiments which go to prove the prepondering 

 role of stomata in the gas exchange of assimilation. He ex- 

 posed thick hypostomatic leaves, laid with their lower faces 

 on the surface of water, to bright light in an atmosphere 

 rich in carbon dioxide, and found that starch was only formed 

 in small areas. He concludes that gases diffuse only ex- 

 tremely slowly through the epidermis cells, and the stomatic 

 path of gaseous exchange must be much the more important. 



The most recent work on our subject is by Stahl, 1894 

 (13), who has anticipated myself in publication of one class 

 of experiments that go to prove the almost absolute role of 

 stomata in assimilatory exchange. 



These consist in showing the local absence of starch for- 

 mation when a limited area of a leaf is coated on its stomati- 

 ferous side with some substance that mechanically blocks 

 the stomata. This forms an easy demonstration experi- 

 ment ; Stahl tried it on numerous leaves, with the uniform 

 result that, while starch was plentifully formed in normal 

 parts of the leaf, no starch appeared in the coated parts 

 after long exposure to light. 



