118 Ingvar Jorgensen and Walter Stiles. 
Chapter III. 
The Path of Gaseous Exchange. 
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. P. 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 
