288 The Physiology of Plants BOOK in 



and the partial pressure of the carbon dioxide in the air 

 bathing the leaf surface. Thus Brown and Escombe, 

 adopting a method slightly different from Blackman's, 

 which enabled them to work with leaves attached to the 

 stems of their plants in the open air, were enabled to 

 confirm his results in all essential particulars. 



The close of the century found us thus in possession of 

 evidence proving conclusively that in ordinary green leaves 

 the process of the absorption of the gases concerned in photo- 

 synthesis, is one of diffusion through the stomata into the 

 intercellular spaces of the interior, the rate depending on 

 its partial pressure at the moment. We shall see later 

 that the same means of interchange subserves the respira- 

 tory processes. 



The ultimate absorption of gas by the living protoplasm 

 of the leaf must depend, however, upon a process resembling 

 very closely Boussingault's suggested method of entry. 

 The gas in the intercellular spaces must penetrate the 

 walls of the cells lining them to reach the seats of 

 chemical change. The question of the permeability of the 

 walls by carbon dioxide has been the subject of several 

 researches. Allusion has been made to Barthelemy's 

 views based on Fremy's opinion of the composition of 

 cuticle. Though experiments have proved these to be 

 unfounded, this is not the case with the statements of 

 Wiesner and Molisch. These observers showed that so 

 long as cellulose walls are wet, they are permeable by 

 carbon dioxide, but lose the permeability as they become 

 dry. Leitzmann made a similar observation in 1887. 



It is clear that the stomatal method of absorption cannot 

 apply to plants below the Ferns, in which, with rare excep- 

 tions, and those very local, stomata do not exist. As a rule 

 these are not strongly cuticularized, so that the properties 

 of ordinary cellulose, such as we find in the intercellular 

 spaces, come into play. Sachsse showed, in 1888, that 



