294 VEGETABLE PHYSIOLOGY 
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 poimts 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 
respiration we must therefore turn our attention to the 
metabolic changes which are taking place normally in the 
living substance. From the instability which we have 
noticed in the protoplasmic material, we can infer that its 
own molecules are in a constant state of decomposition 
and reconstruction, 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. Pro- 
cesses 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 aceti, a fungus which converts 
alcohol into acetic acid. This process, into which the 
molecule of protoplasm apparently does not enter, can 
