152 VEGETABLE PHYSIOLOGY 



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 it is thus withdrawn from the water, and room is made for 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 

 exists in the atmosphere in very small amount, only about 

 four parts in ten thousand being normally present. The 

 very large green surface which an ordinary terrestrial 

 plant possesses renders, however, a considerable amount 

 of absorption possible. If the general conditions are 

 favourable, the absorption is continuous, for carbon dioxide 

 is at once decomposed or made to enter into some form of 



