282 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 

 observation during the constructive processes, but can be 

 liberated again during their decomposition and applied to 

 other purposes. 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 

 formation 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 distribu- 

 tion 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 evironment 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 ab- 

 sorbed 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 



