THE ENERGY OF THE PLANT 285 



studied by a close scrutiny of the internal work of the cell 

 itself. 



The 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 

 protoplasm itself takes part, and which comprise the pro- 

 cesses in which its own breaking down takes place. In the 

 second series it effects the splitting 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 absorp- 

 tion 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 absorb- 

 ing this gas in order not only that its vital activities may 

 continue to be discharged, but that its life itself may be 

 maintained. Withdrawal of oxygen from the environment 

 of the protoplast is after a longer or shorter interval 

 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 



