126 VEGETABLE PHYSIOLOGY 



the formation and exhalation of two fairly simple com- 

 pounds, carbon dioxide and water. To ascertain what is 

 the true relation of the two processes, it is necessary to 

 look closely at the nature of the chemical changes going 

 on in the protoplasm itself, or what is usually spoken of as 

 its metabolism. 



Eespiration in the strict sense is therefore a process 

 going on in the living substance itself. The gaseous inter- 

 change observed is the expression of thl beginning and the 

 end of a series of complex changes in which the molecules 

 of the living substance are involved. The details of the 

 absorption of the oxygen of the plant from its environment, 

 and its presentation to the protoplasm, together with those 

 of the ultimate evolution of the carbon dioxide and water 

 from the plant-body, should be regarded rather as belonging 

 to the mechanism of respiration than to respiration itself, 

 which is a function of the living substance only. The former 

 corresponds to the entry of the oxygen into the lungs of 

 an air-breathing mammal, and its transport to the tissues, 

 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 evolution of carbon dioxide ceases. 



To get a true view of the nature of the process of 



