THE ENERGY OF THE PLANT 303 



classed under the one term fermentation. We have seen 

 that all the katabolic changes in which the self-decomposi- 

 tion of the protoplasm is not directly involved may be 

 carried out either by the intervention of the living substance 

 itself or an enzyme secreted by it. The oxidation of various 

 matters is in some cases confined to the substance of the 

 protoplasm itself, and is in others carried out in its vacuoles 

 by an oxidase ; alcoholic fermentation is in some cells a 

 matter initiated and carried on by their protoplasm, and 

 in others is due to the enzyme secreted by them. The 

 digestive changes can similarly be conducted by enzymes 

 or by the living substance without their intervention. 



We must not however include all digestive fermenta- 

 tive changes among anaerobic respiratory phenomena, if 

 such inclusion involves the acceptance of the view that 

 this is their primary purpose. Though they do effect 

 the conversion of potential into kinetic energy, this is 

 wholly subsidiary to their function in connection with the 

 nutrition of the plant. We have seen that in the processes 

 of germination the energy they liberate is so far in excess 

 of the requirements of the cells that a large amount 

 escapes in the form of heat. For them to work indeed 

 there must be an initial supply of energy, which is pro- 

 bably supplied to them in a similar form, for at C. they 

 are incapable of effecting any decompositions. 



We must not suppose that anaerobic respiration is 

 capable permanently of taking the place of the normal 

 aerobic process. Though the stoppage of oxygen can be to 

 a certain extent compensated for, the vital mechanism 

 gradually becomes exhausted, and life ceases if the cessa- 

 tion of the supply is prolonged. In the higher plants 

 anaerobic is at the best only capable of supplementing 

 aerobic respiration, and that for but a limited period. The 

 commencing asphyxiation serves as a stimulus to the 

 protoplasm, which responds by setting up the anaerobic 

 changes, but like all stimulations the ultimate effect is 

 exhaustion and a failure to continue the response. 



