254 TEXTBOOK OF PLANT PHYSIOLOGY 



is equal to unity. The same quantitative relationship exists 



between the interchanging gases in photosynthesis, where oxygen is 



liberated and carbon dioxide is absorbed. 



There are many deviations from this general rule, as very often 



the respiratory coefficient is less than unity. Sometimes, it is 



more. Various causes may be responsible for these differences. 



Besides the end products of oxidation, CO2 and H2O, frequently 



there are formed but partly oxidized compounds, such as oxalic 



acid, C2O4H2, tartaric acid, CiOgHg, etc., which contain more 



oxygen than carbohydrates normally do. When these substances 



are produced, part of the absorbed oxygen remains in the plant, 



CO. 

 and the ratio of — — is less than unity. On the other hand, when 

 O2 



oxygen is deficient, as in the respiration of bulky organs or seeds 

 immersed in water, normal respiration may be accompanied by 

 alcoholic fermentation, resulting in the liberation of carbon dioxide 

 without absorption of oxygen from the air. In such cases, the 

 respiratory coefficient will be greater than unity. 



The respiratory coefficient may diverge considerably from unity 

 in cases when the respiratory material is not sugar, but some other 

 substance which contains different quantities of oxygen and hydro- 

 gen than sugar does. When the substance is richer in hydrogen, 

 as for instance, a fat or a protein, part of the oxygen will be utilized 

 in the oxidation not only of the carbon but also of hydrogen, and 

 the respiratory coefficient will fall as low as 0.7 to 0.8. Thus, for 

 instance, in the oxidation of stearic acid, the reaction proceeds 

 according to the formula 



Ci 8 H 3 60 2 + 260 2 = I8CO2 + I8H2O, 



CO 18 



and the ratio of — — will be — or 0.69. 

 2 26 



In combustion of substances containing more oxygen than do 

 carbohydrates, such as the organic acids, the respiratory coefficient, 

 on the contrary, will be greater than unity. Thus, in combustion 

 of oxalic acid, according to the equation, 



2C2O4H2 + 2 = 4C0 2 + 2H 2 0, 

 the respiratory coefficient is 4 ; hence, it is quite clear that the lower 



