90 PLANT PHYSIOLOGY 



The respiratory quotient may diverge considerably from unity 

 in cases when the respired material is not sugar but some other 

 substance that contains different quantities of oxygen and 

 hydrogen from those that sugar contains. When the substance 

 is richer in hydrogen, e.g., 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 quotient will fall as low as 

 0.7 to 0.8. In the oxidation of stearic acid, for instance, the 

 reaction proceeds as follows: 



C18H36O2 + 26O2 -> I8CO2 + I8H2O, 



and the ratio of C02:02 will be 18:26 or 0.69. 



In the combustion of substances containing more oxygen than 

 do carbohydrates, such as the organic acids, the respiratory 

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

 the combustion of oxalic acid, according to the equation 



2C2O4H2 + 02-^ 4CO2 + 2H2O, 



the respiratory quotient is 4. Hence, it is quite clear that the 

 lower the respiratory quotient, the higher is the heat liberation 

 during combustion, and vice versa. Proteins and fats exhibit 

 therefore a high heat equivalent, organic acids a low one. 



Variations in the heat equivalent depending on the food 

 material supplied may be most readily observed in molds, which 

 possess scarcely any reserve substances of their own but are 

 endowed with a very high intensity of respiration. When such 

 fungi are grown on carbohydrates, the C02:02 ratio is almost 

 equal to 1 ; when cultivated on fats, it falls to 0.7; when on acids, 

 it rises to 2 to 3. But if these organisms are supplied with a 

 mixture of sugar and other substances, the coefficient will be at 

 first close to 1 until the sugar is used up. Then the combustion 

 of the other substances will begin with a consequent change in 

 the ratio. It is interesting to note that even in oily seeds the 

 quotient may be increased until it approaches 1, if sugar is sup- 

 plied from without (Polocev, 1910). This shows clearly that 

 sugar is the chief respiratory material. Kost5d}chev is of the 

 opinion that sugar is the only directly combustible material, the 

 other substances, such as proteins and fats, undergoing trans- 

 formation with release of sugar, previous to combustion. 



