298 VEGETABLE PHYSIOLOGY 



The transformation of potential into kinetic energy is 

 associated with decomposition just as the converse process 

 is bound up with construction. Destructive metabolism 

 in the cell is then the means by which its energy is made 

 available. We have seen that the processes of this kata- 

 bolism go on in the interior of each cell. Each liberates 

 at least as much energy as it requires for the maintenance 

 of its life and the discharge of its particular functions. 



The processes associated with the utilisation of the 

 stored energy are, then, chemical decompositions in which 

 various constituents of the cell are involved. We may 

 divide them into two series, in the first of which the 

 protoplasm itself takes part, and which comprise the pro- 

 cesses in which its own breaking down takes place. In the 

 second series it effects the splitting-up of other bodies 

 without a necessary disruption of its own molecules. 



We have already discussed the first of these two series, 

 which involves the phenomena of respiration. We have 

 seen that during the life of the cell oxygen is continuously 

 absorbed by the protoplasm ; the immediate result is its 

 self -decomposition, attended by the formation of substances 

 of very different degrees of complexity, the ultimate simplest 

 ones being carbon dioxide and water. If the self-decom- 

 position of protoplasm involved such a splitting-up as would 

 lead to the formation of nothing but these, together with 

 an equally simple form of combined nitrogen, nearly all 

 the potential energy of the cell would be liberated. But 

 we have seen that this does not take place. Carbon 

 dioxide and water are accompanied by many other products 

 of a fairly complex character. There is not, therefore, an 

 evolution of nearly the possible amount of energy, and 

 what is set free is mainly employed in the reconstruction of 

 protoplasm from these products and the new food supplied. 

 As carbon dioxide and water are continually escaping from 

 the cell in appreciable quantities, it is evident that apart from 

 any increase of its fabric the store of its potential energy 

 is gradually diminished, and that it can only continue 



