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 aceti, a fungus which converts 

 alcohol into acetic acid. This process, into which the 



