THE METABOLISM OF PLANTS. 203 



ment as to the mode in which the oxygen affects this process, 

 but we may provisionally accept Pfliiger's view that the 

 absorbed oxygen enters into the protoplasm-molecule as 

 "intramolecular" oxygen, that the molecule is thereby 

 rendered unstable, and that it readily undergoes decompo- 

 sition, carbon dioxide and water being amongst the_ products 

 formed. It must not, however, be assumed that the pro- 

 toplasm-molecule which has taken up oxygen at once un- 

 dergoes decomposition, and that the carbon dioxide evolved 

 in any given short period of time actually contains oxygen 

 absorbed during that period. It has been shewn above that 

 there is not necessarily any definite relation between the 

 volumes of oxygen absorbed and of carbon dioxide exhaled 

 in a given time, and from this we may infer that there is 

 no definite relation between the taking up of oxygen by 

 the protoplasm-molecule and its decomposition. This in- 

 dependence is well illustrated by the statement made above 

 concerning the nature of the respiratory interchange of gases 

 at different temperatures, to the effect that the absorption 

 of oxygen is relatively greater at low temperatures, and that 

 the evolution of carbon dioxide is relatively greater at high 

 temperatures. Since it is known that a high temperature 

 promotes the processes of destructive metabolism, we find 

 the significance of this statement to be that at a low tem- 

 perature the storing -up of intramolecular oxygen is relatively 

 more active than the decomposition of the protoplasm- 

 molecules, whereas at a high temperature the converse is the 

 case. We can readily imagine that for each organ there 

 is a temperature at which these processes are equally active, 

 and at which, therefore, the volumes of oxygen absorbed and 

 of carbon dioxide evolved are approximately equal ; this 

 equality in the volumes of the gases has been frequently 

 determined as pointed out above (p. 197). 



It has been found, however, that the relation between the 

 volumes of the gases depends very much upon the nature 

 of the non-nitrogenous organic substances present in the 

 organ which serve as plastic material for the reconstruction 

 of protoplasm. It is only when the organ contains carbo- 



