ABSOEPTION OF FOOD MATEEIALS 143 



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 the living substance itself, it is thus withdrawn from the 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 



