THE AEKATION OF PLANTS 119 



in different plants, as has already been shown, linger has 

 put on record measurements of the relative volumes of air 

 and cellular tissue in the leaves of forty-one species of 

 plants. These were found to range from 77 : 1000 in Cam- 

 phor a officinalis, where it was least, to 713 : 1000 in Pistia 

 texensis, in which it was greatest. 



The movements of the air in the intercellular space 

 systems of plants depend almost entirely upon the physical 

 processes of diffusion. The entrance and exit of air from 

 the exterior are generally possible, occasions when the 



Fio. 81. TRANSVERSE SECTION OF ROLLED LEAF OF HEATH WITH 

 STOMATA, st, IN THE GROOVE. 



orifices are completely occluded being very rare. It does 

 not, however, at all follow that the atmosphere in the 

 spaces has the same percentage composition as the external 

 air. When we consider that it is the source of the supply 

 of the gases used in the metabolism of the plant, and the 

 recipient of those which are from various causes exhaled, it 

 becomes evident that this is not the case. Nor is its com- 

 position uniform for even a short time, as the various 

 processes which subtract from or add to it take place in 

 different parts with very different rapidities. At the same 

 time there is a tendency for it to become uniform according 

 to the laws of the diffusion of gases. 



