THE ENERGY OF THE PLANT 289 



or its conversion into the kinetic form, during the decom- 

 positions which take place within it. 



The protoplasm itself contains a store of such potent^ 

 energy. We have seen that it can only be constructed at 

 the expense of food supplied to it. The formation of the 

 protoplasm which follows the supply of food to the cell 

 involves work, and the energy so used is partly changed 

 from the kinetic to the potential condition. When the 

 protoplasm undergoes what we have called its auto-decom- 

 position, which is continually taking place, a certain amount 

 of this potential energy is liberated and can be observed and 

 measured in various ways. When destructive metabolism 

 is active we have already noticed that there is usually a 

 rise of temperature, as in the processes of the germination of 

 seeds. A certain amount of the liberated potential energy 

 in this case manifests itself in the form of heat. A vegetable 

 cell which obtains no direct radiant energy from without 

 can consequently obtain the energy it needs from within 

 itself, by setting up decomposition either of its own sub- 

 stance or of certain materials which have been accumulated 

 within it. 



The supply of elaborated material to a cell and that of 

 available potential energy within it are not, however, 

 exactly equivalent. A certain part of the transported 

 material is devoted to the maintenance of the fabric of the 

 cell. The protoplasm in a growing cell is permanently 

 increased ; frequently its cell- wall is permanently thickened. 

 In these cases the whole of such material is not subjected 

 to subsequent decomposition, but much remains unchanged 

 during the plant's life. The cell is consequently never 

 found to be capable of giving up to the plant of which it is 

 a member the whole of the potential energy which reaches 

 it. If we consider the round of the metabolic changes 

 which take place in such a cell, we find that energy is 

 absorbed to construct its substance, and that as the latter 

 undergoes self-decomposition energy is again liberated. 

 But a certain part of what is supplied to it is permanently 



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