The Path of Gaseous Exchange. 5 1 



Chapter III. 

 Tilt- Path of Gaseous Exchnuge. 



The passage of carbon dioxide from the outside medium into 

 the leaf in the case of submerged water plants almost certainly 

 takes place by diffusion in aqueous solution through the outer walls 

 of the epidermal cells in the same way that substances will diffuse 

 from cell to cell within the plant. 



In the case of lower plants like the mosses the path must be 

 the same, as the surface layer of the leaf is uniform throughout. In 

 the higher land plants, on the other hand, there are two paths by 

 which gases might diffuse into and out from the leaf. There might 

 be diffusion through the cuticle of the epidermal cells as in submerged 

 water plants or mosses, or the diffusion of gases might be principally 

 through the small perforations, stomata, which occur in varying 

 abundance over one or both surfaces of the leaves of higher plants, but 

 which comprise only a fraction of the total area of the leaf. It is a 

 possible alternative that both cuticle and stomata may be utilised 

 for diffusion, in which case it becomes of interest to determine the 

 relative importance of the cuticle and stomata in gaseous diffusion 

 into the leaf. 



The work on the paths of gaseous exchange before the 

 researches of P. F. Blackman, like the work on chorophyll before 

 Willstatter's, is all open to the criticism that the experimental 

 methods used were imperfect. It is therefore not to be wondered 

 at that a mass of contradictory results was obtained, and that none 

 of the views of earlier workers had been established. It will be 

 sufficient for us to refer here to the observations of Garreau (1850), 

 Merget (1877-8), Wiesner (1879), Boehm (1889) and Wiesner and 

 Molisch (1889), who have urged that the stomata are the path of 

 gaseous exchange, while Boussingault (1868) and Barth^lemy (1868) 

 have advocated the contrary view, that the intake of carbon dioxide 

 takes place through the cuticle. Mangin (1888) took up an inter- 

 mediate position that diffusion through the cuticle is insufficient to 

 account for the whole of the gaseous exchange. During assimilation 

 he concluded that practically all the gaseous exchange takes place 

 through the stomata as the pressure of carbon dioxide in the 

 external air is insufficient to cause much diffusion through the 

 cuticle. 



It is unnecessary for us to go into a detailed description of the 

 results and conclusions of these workers nor into a criticism of 



