ACIDIFICATION OF SUCCULENTS 265 



supply of carbon dioxide for photosynthesis. They may have therefore 

 evolved a mechanism by which the products of respiration, formed during 

 the night, can be utihzed for photosynthesis during the next day: instead 

 of burning carbohydrates completely to carbon dioxide, they interrupt 

 the respiration at the stage of malic or citric acid and store these acids 

 in the leaves until morning. Meyer's explanation implies that the 

 plant acids are respiration intermediates; their disappearance in light can 

 be interpreted as evidence that they also are intermediates of photosynthesis. 



The first suggestion is supported by the fact that acidification occurs 

 at the cost of carbohydrates (Kraus 1873; Wolf 1931; Bennet-Clark 

 19332). Not more than one molecule of acid appears for each disap- 

 pearing carbohydrate molecule (Wolf). Bennet-Clark suggested, how- 

 ever, that (at least in Sedum) the acidification occurs by the oxidation 

 of sedoheptulose, rather than that of hexoses. Warburg (1886) asserted 

 that acidification occurs only in air, but Bendrat (1929) observed that 

 it can proceed also in absence of oxygen; it thus appears that acids may 

 be produced by fermentation rather than (or as well as) by the autoxi- 

 dation of the sugars. 



The second part of our hypothesis — the attribution of deacidification 

 to a resynthesis of carbohydrates in light — is supported by the observation 

 of Meyer (1878) that succulents deprived of carbon dioxide nevertheless 

 produce carbohydrates in light, until their reserve of acids is exhausted; 

 and by the experiments of Warburg (1886) who found that Bryophyllum 

 can synthesize carbohydrates in light from externally supplied malic acid. 

 It is, however, diflScult to choose between two possible mechanisms of 

 resynthesis — the direct photochemical reduction of malic (or citric) acid 

 (i. e., photosynthesis with organic acids as substitutes for carbon dioxide), 

 and an oxidation (or photoxidation) of the acids to carbon dioxide 

 followed by ordinary photosynthesis. The possibility of indirect re- 

 synthesis was pointed out by Spoehr (1913) and Willstatter and StoU 

 (1918). Obviously the direct mechanism (if confirmed), would provide 

 an argument (equivalent to a successful substitution test) in favor of 

 organic acids as intermediates of photosynthesis. 



Warburg (1886) and Astruc (1903) found that the rate of deacidifica- 

 tion is reduced by an increase in the pressure of carbon dioxide; this 

 may indicate a competition between carbon dioxide and the organic acids 

 for the part of oxidants in photosynthesis and thus support the direct 

 reduction theory. On the other hand, the simultaneous evolution of 

 oxygen and carbon dioxide during deacidification, first noticed by Meyer 

 (1878) and confirmed by Aubert (1890, 1891, 1892), can be quoted in 

 favor of the "indirect reduction" theory, since direct photosynthesis of 

 carbohydrates from acids, although it can reduce the carbon dioxide 

 consumption during the deacidification period to zero, could not cause a 



