104 RESPIRATION 



can be produced from sugar both in vitro and in vivo, for all 

 have been identified in the degradation products of various 

 strains of Aspergillus niger * grown on carbohydrate sub- 

 strates, -j- It is obvious that in all such examples the re- 

 spiratory quotient will be less than unity. Oxalic, malic, 

 citric and tartaric acids arc of frequent occurrence in the 

 higher plants. True it is, that it is difficult to decide whether 

 these, in common with other products isolated from the plant, 

 are of catabolic or anabolic origin. In the ripening of fruit, 

 the disappearance of organic acids is concurrent with the 

 appearance of sugar ; it is an open question whether the 

 acids give origin to the sugar or are used up in respiration. 

 If these acids are used in respiration, it is worthy of note 

 that malic acid is the most easy to oxidize and citric acid the 

 most difficult, tartaric acid being intermediate ; the question 

 arises whether this circumstance is in any way connected 

 with the fact that apples and pears, which chiefly contain 

 malic acid, can ripen in northerly regions ; that grapes, which 

 contain tartaric acid, require a warmer climate and do not 



7 i. 



ripen so effectively as the apple in the northern temperate 

 zone, whilst the orange and lemon, which, contain citric acid, 

 are confined to tropical or sub-tropical countries. 



Succulent plants provide examples of a normal stoppage 

 of the respiratory process at an intermediate stage. Members 

 of the Crassulacese do not give off carbon dioxide when first 

 placed in darkness, although the absorption of oxygen is active ; 

 there is, however, an accumulation of organic acids, malic 

 and oxalic, and it is not until these have accumulated in 

 relatively large quantities that carbon dioxide is evolved as 

 in a normal plant, j The peculiar metabolism of these plants 

 is generally associated with and explained by their massive 

 structure rendering the movement of gases a relatively slow 

 process, the catabolism stops at an intermediate stage and 



* The respiratory activities of this and other moulds are commercially 

 exploited. 



f Franzen and Schmidt : " Ber. deut. chem. Ges.," 1925, 58, 222. 

 Amelung : " Zeit. physiol. Chem.," 1927, 166, 161. Butkewitsch : "' Bio- 

 chem. Zeit.," 1927, 182, 99. 



t See Nicolas : " Compt. rend.," 1918, 167, 131. 



