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 vol-ume 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 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 

 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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