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CHAPTEE VII 



THE AEBATION OF PLANTS 



IN the study of the vital processes carried on by the 

 protoplast 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 exhaled. 

 In the case of many plants, of 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 withdrawn from the water in which they are living, death 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 can diffuse through the cell-wall, which, as we 

 have already seen, is saturated with water. If we turn to 



