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 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 

 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 



