288 VEGETABLE PHYSIOLOGY 



be attended by the evolution of a certain definite amount 

 of heat. This heat would, represent the energy that had 

 been applied to the construction of the material so burned. 

 Any accumulation of material in the body of the plant 

 represents, therefore, not only a gain of weight or substance, 

 but a storage of energy. This has disappeared from observa- 

 tion during the constructive processes, but can be liberated 

 again during their decomposition and applied to other pur- 

 poses. Energy which has thus been accumulated and 

 stored is known as potential energy, to distinguish it from 

 the actual or 'kinetic energy originally absorbed. The forma- 

 tion of material in the plant, therefore, involves a storage 

 of energy in the potential form, and wherever such material 

 is found there is in it an amount of energy which can be 

 liberated with a view to utilisation at any point to which 

 the material has been transferred. The translocation of 

 material, therefore, involves also a distribution of the 

 energy which, originally absorbed as the kinetic energy of 

 light or heat, has been applied to constructive processes, and 

 has consequently been made potential. 



Those exceptional plants which absorb elaborated food 

 from their environment have a source of energy therein. 

 This food is a store of potential energy which is absorbed 

 as such, and made kinetic subsequently. The salts absorbed 

 by all plants from the soil also represent a certain, though 

 very small, amount of potential energy. 



It is this potential energy on which the plant depends 

 for the various processes which go on in such cells as are 

 not the recipients of external kinetic energy. Even in the 

 cells which absorb the latter a certain amount of potential 

 energy also is present, which represents what has been 

 stored by them in the constructive processes they carry out, 

 or has reached them in the shape of complex materials 

 formed originally in other cells. 



It is mainly on this store that not only the whole organism, 

 but every cell depends for the execution of its vital processes. 

 Each cell is a seat of the liberation of this potential energy, 



