THE RESPIRATORY RATIO 519 



if occurring simultaneously, would be the absorption of oxygen in a volume 

 in excess of the carbon dioxide evolved and the resulting respiratory ratio 

 would be less than one. 



Similarly oxidation of the hydrolytic products of the proteins results in a 

 respiratory ratio of less than one since the proportion of oxygen to carbon 

 in such compounds is less than in carbohydrates. 



2. Respiration of Compounds IVhich Are Relatively Rich in Oxygen as 

 Compared ivith Ilexoses. — In some species of plants, particularly those of 

 the succulent habit of growth, organic acids are often oxidized. Such com- 

 pounds are relatively rich in oxygen as compared with carbohydrates. The 

 equations for the complete oxidation of oxalic and malic, two of the common 

 plant organic acids are as follows: 



COOH 

 2 I + O2 -^ 4 CO2 + 2 H2O + 60.2 kg.-cal. 



COOH 



Oxalic acid 



COOH 



CHOH + 3 O2 -^ 4 CO2 + 3 HoO + 320.1 kg.-cal. 



I 

 CH2 



I 

 COOH 



Malic acid 



The theoretical respiratory ratio for oxalic acid is therefore 4, for malic 



acid -or 1.33. For tartaric acid, another organic acid which occurs in plants, 



3 

 the respiratory ratio is 1.6. Oxidation of any compound of this type results 



in respiratory ratios in excess of one. 



3. Occurrence of Anaerobic Respiration. — In this process, which may 

 occur in the higher green plants under certain conditions, carbon dioxide 

 is released without any corresponding absorption of oxjgen. Sometimes when 

 the oxygen supply is deficient, both aerobic and anaerobic respiration occur 

 simultaneously in a plant tissue. Some cells may be respiring anaerobically, 

 while others are carrying on aerobic respiration. In the early stages of 

 germination of seeds in which the seed coats are relatively impermeable to 

 oxygen a limited amount of aerobic respiration may be carried on, accom- 

 panied by a larger proportion of anaerobic respiration. Such a phenomenon 

 may be observed, for example, in pea seeds. Under such conditions the volume 

 of carbon dioxide evolved may be very large in proportion to the volume of 

 oxygen absorbed and the respiratory ratio is much greater than one. As 



