THE ENERGY OF THE PLANT 283 
this potential energy, or its conversion into the kinetic 
form, during the decompositions which take place within it. 
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 taking place, 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 
