THE ENERGY OF THE PLANT 289 
or its conversion into the kinetic form, during the decom- 
positions 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 auto-decom- 
position, which is continually taking place, a certain amount 
of this potential energy is liberated and can be observed and 
measured in various ways. When destructive metabolism 
is active we have already noticed that there is usually a 
rise of temperature, as in the processes 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 sub- 
stance 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 ig 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 subsequent 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 ig 
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 
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