THE AERATION OF PLANTS 113 
of water in the plant. They serve automatically to preserve 
the plant from excessive loss of water, but they have 
no direct regulating influence upon the interchange of 
gases. Indeed, when, from flaccidity of the leaves or from 
other causes, they close, the aeration of the plant is, to a 
certain extent, interfered with, if not suspended—a con- 
sideration which will help us to understand why a plant 
needs to contain so large a reservoir of air as is afforded 
by its intercellular spaces. The volume of this reservoir 
varies considerably 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 Camphora officinalis, where it was least, 
to 718: 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 
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 
composition 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. 
The amount of nitrogen varies but little. This gas 
has a certain feeble solubility in water, and a small 
quantity goes into solution in the water which saturates 
the cell-walls; but as such nitrogen is not made use of in 
the cells, its absorption very speedily ceases, the cell-sap 
not being able to contain more than a trace of it. The 
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