138 VEGETABLE PHYSIOLOGY 
into the intercellular space system, the importance of 
which we have already examined. These intercellular 
spaces contain, as we have seen, a mixture of gases which, 
though approximating to the composition of the atmo- 
sphere, yet differs from it in the relative quantities of the 
constituents. We have seen that the composition of this 
mixture of gases tends to become uniform by the currents 
which circulate in the intercellular cavities, and by the 
slower processes of diffusion, which are set up in conse- 
quence of local production or abstraction of particular 
constituents. So long as the stomata and the lenticels are 
open, the composition of the atmosphere within the plant 
tends to become identical with that of the external air. 
The actual absorption of the gases takes place almost 
entirely from this internal reservoir, very little finding 
entrance into the cells of the epidermis. A certain 
amount is, however, taken in by the very young parts 
which have not become modified by the development of a 
cuticle. 
The cells which abut upon the spaces in the leaves 
and other green parts are those which are principally 
concerned in the absorption of gases. Their walls are very 
thin and delicate, and are saturated with water. The 
different gases present dissolve in the outermost film of 
this water, according to their degree of solubility, and 
thence diffuse slowly through the membrane into the cell 
sap, which saturates the protoplasm, and fills the vacuoles. 
The quantity of each taken up depends, as in the case of 
the metallic salts already discussed, upon the ability of the 
protoplasts to make use of the gas, and thus to withdraw 
it from the sap. If it can be combined in any way with 
other bodies in the cell, or with the living substance itself, 
it is thus withdrawn from the water, and room is made for 
more to enter. If not, the limit of saturation of the sap.is 
soon reached. 
The only gas which is absorbed from the air for the 
purposes of food-construction is carbon dioxide. This 
