282 Plant Physiology 



in two weeks half their original substance, the expelled 

 products being mostly CO 2 and H 2 O. 



This loss of organic substance is indicative of respiration, 

 a process of catabolism and energy-release absolutely 

 essential for the maintenance of every active cell, plant 

 or animal, in both of which the essential process is 

 substantially the same. 



162. The demonstration of respiration. It is not at 

 all requisite that respiration shall have exactly the same 

 course in the cells of different organisms, nor is it necessary 

 that it shall be dependent upon a common type of mechan- 

 ism for the usually accompanying gas exchange. 



Once properly understood, activity and growth are 

 themselves the best evidences of respiratory activity. 

 Nevertheless, there is a gas exchange commonly inseparable 

 from the process, and this gas exchange, since it is an 

 accompaniment of respiration, serves as a simple and 

 definite demonstration of the material end result, or of the 

 type of material change which goes on. This gas exchange 

 consists in the absorption and use of O 2 and the elimination 

 of C0 2 . Most important as a provisional experimental 

 demonstration of respiration is the evolution of CO 2 . 

 This is, of course, positive indication that organic matter 

 is undergoing catabolic or dissimilatory transformations, 

 and the CC>2 set free may be made, usually, a satisfactory 

 qualitative or even quantitative measure of the com- 

 parative rate of the process, especially in the higher plants, 

 with which we are particularly concerned. 



Proof of O 2 absorption and CO 2 production during the 

 activity of living tissues is most definitely shown by an 

 accurate analysis of the gases from a respiration chamber. 



