72 RESPIRATION 



will come to an end.* Between the oxygen absorbed 

 and the carbon dioxide evolved there is maintained a cor- 

 relation sometimes so close as to provide a well-known 

 avenue for investigating certain aspects of the respiratory 

 processes.f This correlation, first appreciated by de Saussure, 

 is commonly called the respiratory quotient in the considera- 

 tion 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 initial oxygen, the two being but 

 remotely related. The ratio CO 2 /O 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 conditioning factors. Puriewicz,J for instance, 

 found that the amount of carbon dioxide evolved showed a 

 much greater range in variation than did the absorbed oxygen 

 in the same plant under different conditions, the figures ob- 

 tained in a series of experiments showing a variation of - 14 

 to 1 20 per cent of the average for carbon dioxide, whilst the 

 oxygen varied from o to 48 per cent of the average. Ruby 

 observed in the olive that the ratio remained practically 

 constant and was but little affected by the age of the plant 

 or of the organ examined ; generally it is rather higher in 

 leaves from fruiting branches, a fact possibly connected with 

 the greater abundance of available food, and in the early 

 season of the year it is less than unity but later rises to unity. 

 As will be seen later, the carbon dioxide evolved may have 

 different origins ; in those instances in which it is due to the 

 immediate physiological combustion of available substances, 

 it will be apparent that the respiratory quotient will vary 

 according to the nature of this food. 



Thus, in general terms, if sugar be the immediate respir- 

 able substance, the respiratory quotient will be in the neigh- 



* The irritability of plants is outside our present consideration : an intro- 

 duction to the problems regarding the minimum pressure of oxygen necessary 

 to maintain movements, the streaming of protoplasm in chlorophyll-containing 

 cells in darkness and in an atmosphere free from oxygen, and similar subjects 

 will be found in the larger text books on general plant physiology. 



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

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



JPuriewicz: "Jahrb. Wiss. Bot.," 1900, 35, 573. 



Ruby: "Ann. Sci. Nat. Bot.," 1917, 20, i. 



