THE ENERGY OF THE PLANT 291 
by a close scrutiny of the internal work of the cell 
itself. a 
pee transformation of potential into kinetic energy is 
associated with decomposition just as the converse process 
is bound up with construction. Destructive metabolism 
in the cell is then the means by which its energy is made 
available. We have seen that the processes of this kata- 
bolism go on in the interior of each cell. Each liberates 
at least as much energy as it requires for the maintenance 
of its life and the discharge of its particular functions. 
The processes associated with the utilisation of the 
stored energy are, then, chemical decompositions in which 
various constituents of the cell are involved. We may 
divide them into two series, in the first of which the proto- 
plasm itself takes part, and which comprise the processes 
in which its own breaking down takes place. In the second 
series it effects the splittmg up of other bodies without a 
necessary disruption of its own molecules. 
The first of these two series involves the phenomena of 
respiration, to which we must now turn our attention. 
Of the gaseous interchanges which were mentioned in a 
former chapter as characteristic of living protoplasts, the 
most widespread is that which is marked by the absorption 
of oxygen. With the exception of a few of the lowlier 
organisms, all of which are members of the group of Fungi, 
every living protoplast must be constantly absorbing 
this gas in order not only that its vital activities may con- 
tinue to be discharged, but that its life itself may be main- 
tained. Withdrawal of oxygen from the environment 
of the protoplast is after a longer or shorter interva 
followed by its death. It is true that under certain con- 
ditions which we shall discuss in a subsequent chapter 
the interval may be prolonged, but death ultimately 
ensues. 
This absorption of oxygen is in most cases associated 
with an exhalation of carbon dioxide, which is generally 
given off in a volume approximately equal to that of the 
19 * 
