ABSORPTION OF FOOD MATERIALS 148 
leaves and other green parts. They enter freely through 
the stomata so long as these are open, and find their way 
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 proto- 
plasm 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 so 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the living substance itself, it is thus withdrawn from the 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 
