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 CO 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 C0 2 production during the 

 activity of living tissues is most definitely shown by an 

 accurate analysis of the gases from a respiration chamber. 



