300 VEGETABLE PHYSIOLOGY 
together with the return of the carbon dioxide and water 
therefrom ; the latter is strictly comparable to the changes 
taking place in those tissues after the entry of the oxygen 
into them. 
The variation of the respiratory quotient which we 
noticed in starchy and oily seeds respectively points to a 
varied metabolism, according to the nature of the food 
supplied to the living substance. 
We see, then, that the two processes are not immediately 
connected in the sense of the carbon dioxide and water 
coming at once from the direct oxidation of carbon and 
hydrogen, but that they are ultimately associated there can 
be no doubt, though they are separated in time by a series 
of chemical changes taking place in the living substance. 
This ultimate association is shown by the fact that, if 
the access of oxygen to a plant is prevented, after a longer 
or shorter period the exhalation of carbon dioxide ceases. 
To get a true view of the nature of the process of respira- 
tion we must therefore turn our attention to the metabolic 
changes which are taking place normally in the living sub- 
stance. From the instability which we have noticed in 
the protoplasmic material, we can infer that its own mole- 
cules are in a constant state of decomposition and recon- 
struction, new material being incorporated and certain 
other substances cast off. Besides these, we are probably 
not wrong in concluding that other changes also take place 
in the various substances which are contained in it, into 
which its own molecules do not enter. Processes of slow 
oxidation and gradual reduction are taking place there 
continually, excited, however, in all probability by the 
changes in the protoplasm itself. We shall discuss these 
later, but for the present we may say that they are by no 
means simple, and the direct oxidation of either carbon 
or hydrogen has probably no place among them. An 
instance of them may be seen in the oxidation of alcohol 
in the cells of Mycodermi acett, a fungus which converts 
alcohol into acetic acid. This process, into which the 
