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. Tach cell is a seat of the liberation of 
