8 ACIDITY AND GAS INTERCHANGE IN CACTI. 



In the work of Maquenne and Demoussy," which is very recent, we have one 

 of the most elaborate and exact investigations that has ever been attempted in 

 this field. New methods and new corrections in the evaluation of their experi- 

 ments are introduced. The bulk of the work does not touch immediately on 

 the subject in hand, but some passages are of interest in connection with succu- 

 lent plants. In general, the authors did not concern themselves, experiment- 

 ally at least, with the matter of acidity. They state, however, that the -^ 

 ratio in young tissues is greater than in mature ones. The lower ratio found 

 at night, when the acidity is rising, they ascribe largely to the increased solu- 

 bility of carbon dioxide in the juices of the plant at the lower temperatures 

 which prevail at night. This would cause the apparent fall in the ratio by 

 cutting down the amount of carbon dioxide evolved from the plant. It seems, 

 however, dubious whether this increased solubility could possibly account 

 for enough of this gas to produce the considerable effect on the gas-ratio that 

 is to be observed. 



Among the references to the importance of the acidity of the Crassulaceae 

 and other succulents in connection with the general problem of respiration 

 that are to be found in the more general works, we have the discussion given 

 by Nathanson 6 , in his recent book on the nutrition of plants. He states his 

 opinion very emphatically that the formation of large amounts of acid is 

 due not merely to the diminished oxygen attendant on the morphological 

 structure of succulents, but to a regulatory process connected with the photo- 

 synthetic activities in this type of plant. His interpretation does not really 

 seem to be very different from that of Warburg, except that is is somewhat 

 more vague; and as a result of his discussion it is perhaps no easier to dis- 

 tinguish cause and effect than in the usual statements concerning the relation 

 of respiration and photosynthesis in succulents. 



The present status of the question is about that which is expressed by 

 Pfeffer, which is based largely on the work of Kraus and Warburg, supported 

 in the main by Aubert and Purjewicz. The acids which are formed are the 

 result of oxidation of sugars, which is incomplete because of the compara- 

 tively restricted supply of oxygen consequent on the morphological structure 

 of the succulent type of plant. The disappearance of the acid, as is plainly 

 shown by Spoehr's work, is due mainly to light, which has a photolytic action 

 on the malic acid and breaks it up into simpler substances. The acids do not 

 appear to stand in direct relation with the synthesis of the carbohydrates in the 

 photosynthetic processes, but the product of their disruption, which in its final 

 form is carbon dioxide, may be of importance in this regard. Even if Bayer's 

 hypothesis as to the nature of the photosynthetic process were on a firmer 

 foundation than it is, the presence of formaldehyde as one of the degeneration 

 products of the acids, as established by Spoehr's experiments, would weaken 

 it greatly. However, even if formaldehyde is not the primary substance 

 produced in photosynthesis, the possibility of this substance being worked up 

 by the plant into sugars is not precluded. Under such circumstances, it might 

 be suggested that any formaldehyde which is formed in the process of the 



"Maquenne, L., et E. Demoussy. Nouvelles recherches sur les echanges gazeaux des plantes 

 vertes avec 1'atmosphere. Paris, 1913. See also Compt. Rend., vol. 156, p. 28, 1913; ibid., vol. 

 156, p. 506, 1913; ibid, vol. 156, p. 278, 1913. 



"Nathanson, A. Der Stoffswechsel der Pflanzen, pp. 376-394, 1910. Leipzig. 



