THE AERATION OF PLANTS 119 
in different plants, as has already been shown. Unger 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- 
phora 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 
Fig. 81.—TRansveRsE Section or RoLtLeED Lear or Heata with 
Sromata, 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. 
