296 VEGETABLE PHYSIOLOGY 



The protoplasm itself contains a store of such potential 

 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 self- 

 decomposition, which is continually going on, a certain 

 amount of this potential energy is liberated and can be 

 observed and measured in various ways. When destruc- 

 tive metabolism is active we have already noticed that 

 there is usually a rise of temperature, as in the pro- 

 cesses 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 substance 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 sub- 

 sequent 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 

 retained in potential form, and hence every cell is depen- 

 dent for the maintenance of its energy upon a constant 



