
1905.| On the Physiological Processes of Green Leaves. 65 
in the leaf-cells during respiration, the rate at which this will diffuse into the 
surrounding air will be quite independent of the degree of opening of the 
stomata. If the stomata partially close whilst this outward diffusion is going 
on, the partial pressure of the carbon dioxide within the leaf will increase, 
and this increase will be inversely proportional to the altered linear dimen- 
sions of the opening. This rise in “ diffusion potential” will, consequently, 
exactly counterbalance the effect of the diminished size of the stomatic 
- opening, and the same amownt of respiratory carbon dioxide will consequently 
escape from the leaf, no matter what changes in the size of the opening takes 
place, provided these are not sufficient to close the stomata completely. 
This however does not apply to the intake of atmospheric carbon dioxide 
into the leaf during the assimilatory process. Providing a sufficient amount 
of the right grade of energy is reaching the chloroplasts, the “ diffusion 
potential” in this case will remain constant, the partial pressure of the inward 
diffusing carbon dioxide varying from 3/10,000 of an atmosphere cutside the 
leaf to zero at the point where complete absorption takes place. Under 
these conditions the intake of carbon dioxide during assimilation must vary 
directly with the linear dimensions of the openings. Hence in an amphisto- 
matous leaf the relative rate of assimilation by the two sides will not only be 
influenced by the number of stomata on equal areas in each case, but also on 
the degree to which these stomata are opened, a condition which, as we have 
seen, is not operative in the gaseous exchange of carbon dioxide in 
respiration. 
Partial opening of the stomata on the upper side is extremely likely to be 
brought about when the incidence of illumination and radiant energy is on 
that side of the leaf, and in such a case we might expect exactly the results 
which are recorded in Table XII. There is, however, another factor which no 
doubt contributes to the apparent excess of assimilation by the upper surface 
of such leaves. By far the greater part of the particular grade of radiant 
energy which produces photosynthesis is doubtless absorbed by the chloro- 
plasts of the palisade parenchyma into which the stomata of the upper side 
of the leaf open. The diffusion gradient will therefore be “steeper” in the 
intercellular spaces of the palisade parenchyma than in those of the more 
deeply seated spongy parenchyma, a fact which will in itself favour a more 
rapid inflow of carbon dioxide into the leaf through the stomata of the upper 
side. 
VOL, LXXVI.—B. F 
