102 RESPIRATION 



the ultimate elimination of the gaseous waste of physiological 

 combustion. 



This motive power commonly is obtained by the physio- 

 logical combustion of carbohydrate, by which, theoretically, 

 a molecule of sugar is completely oxidized by 6 molecules of 

 oxygen giving origin to 6 molecules each of carbon dioxide 

 and water with the liberation of a considerable quantity of 



energy : — - 



C 6 H 12 6 + 60 2 = 6C0 2 + 6H 2 + 674 K. 



The liberation of these 674 calories by the complete 

 oxidative degradation of a molecule of carbohydrate will 

 involve the production of temperatures far above these ob- 

 served in the living plant, but, as has been indicated above, 

 the same amount of thermal energy will be set free when the 

 process takes place in slow stages with a hardly perceptible 

 rise in temperature. This oxidation is effected by the plant 

 by means of enzymes, and when the respiration is intense 

 a considerable amount of heat is evolved and dissipated, but 

 these aspects will be considered later. 



RESPIRATORY QUOTIENT. 



The above equation shows that the volume of carbon 

 dioxide evolved is equal to that of the oxygen absorbed ; in 

 the plant this correlation provides a well-known avenue for 

 investigating certain aspects of the respiratory processes.* 

 This fact, first appreciated by de Saussure, is commonly called 

 the Respiratory Quotient, in the consideration of which it is 

 well to realize that it has but little value in indicating the 

 essential parts of the process, correlating as it does merely a 

 final product, carbon dioxide, of a long series of changes with 

 the absorbed oxygen, the two being but remotely related. 

 The ratio C0 2 /0 2 is variable not only in different plants but 

 also in the same plant at different phases of its existence ; 

 in other words, the value of. the ratio is subject to the con- 

 ditioning factors. Puriewicz,y for instance, found that the 



* See Bonnier and Mangin : " Ann. Sci. Nat. Bot.," 1884, vi., 19, 217 ; 

 1885, vii., 2, 315 ; 1886, vii., 3, 5. 



f Puriewicz : " Jahrb. wiss. Bot.," 1900, 35, 573. 



