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 ig 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 algo 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. 
Hach cell is a seat of the liberation of this potential energy, 
