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CHAPTER VII 
THE AERATION OF PLANTS 
In the study of the vital processes carried on by the proto- 
plast we have seen so far how entirely it is dependent 
upon the free access of water. Another factor necessary 
for its existence is a supply of air. With but few exceptions, 
and those occurring among the lowliest plants, every living 
organism carries out a series of gaseous interchanges, a 
feature of which is the absorption of oxygen. In nearly all 
cases a corresponding amount of carbon dioxide is ex- 
haled. In most plants—in all, indeed, that are green— 
another gaseous interchange takes place, carbon dioxide 
being absorbed and oxygen simultaneously eliminated. 
Every protoplast must consequently be afforded facilities 
for carrying out gaseous interchanges, the nature and 
extent of which vary according to its constitution. The 
water with which it has such a close relationship serves as 
the medium through which such interchanges take place, 
for it is only in solution that gases are able to penetrate 
into the living substance. 
In the case of those protoplasts which live in a watery 
environment, the latter supplies them with the gases they 
absorb and receives those which they exhale. If all air is 
withdrawn from the water in which they are living, death 
speedily ensues. The gases enter the naked protoplasts by 
diffusion through the film of water which is in contact with 
their free surfaces. In the case of those which have a 
cell-wall the same means are made use of. Gases in solution 
