ACIDITY AND GAS INTERCHANGE IN CACTI. 



BY HERBERT M. RICHARDS. 



HISTORICAL. 



That the respiratory activity, or rather the phenomena connected with 

 the evolution of carbon dioxide, in cacti and in other succulents is peculiar 

 and that the acidity of the juices of these plants exhibits periodic rise and fall 

 have been known for a long time. These phenomena have been the subject 

 of a considerable number of researches, some of which have been elaborate and 

 thorough. 



Within the last 25 years there have been published the comprehensive 

 papers of Aubert and of Astruc 6 dealing directly with the problems of the 

 physiology of the succulents, in which may be found a survey of the literature. 

 These accounts, with the excellent review of the subject that can be found in 

 CzapekV Biochemie der Pflanzen, as well as the short but well-digested treat- 

 ment given by Spoehr, d perhaps make it unnecessary to go into great detail, 

 nevertheless it will be well for the sake of convenience to summarize the 

 present state of knowledge concerning the points mentioned above. 



The two principal phenomena with which we are concerned, namely, the 

 acidity with its periodicity and the carbon dioxide-oxygen quotient, are, as 

 is now known, intimately connected; but in the earlier literature they were 

 not associated. 



It was that remarkably astute investigator, deSaussure,* who first discovered 

 that in a certain cactus form (Opuntia sp.), with which he experimented, the 

 intake of oxygen might, under some circumstances, greatly exceed the output 

 of carbon dioxide. In consequence of this the so-called respiratory quotient, 

 instead of approaching unity, fell far below it. But at that tune there was no 

 knowledge of the acidity of the plant juices, let alone its periodicity, so that 

 the significance of this peculiarity was not understood. As to the acidity of 

 succulent plants, we owe the first mention to a casual observation by Benja- 

 min Heyne' in 1819, who contributed a note regarding the more acid taste of 

 the leaves of Bryophyllum (Cotyledon) calydnum in the morning than late in 

 the afternoon. This original note, with the further contribution by H. F. 



"Aubert, E. Recherches physiologiques sur les plants grasses: Part 1, Acides organiquea, 

 turgescence et transpiration des plantes grasses; part 2, Respiration et assimilation compareea 

 chez les plantes grasses et les vegetaux ordinaires. [Thesis.] Paris, 1892. See also: Sur la 

 repartition des acides organiques chez les plantes grasses, Rev. Gen. Bot., vol. 2, p. 369, 1890; 

 Recherches sur la respiration et 1'assimilation des plantes grasses, Rev. Gen. Bot., vol. 4, p. 

 203, 1892; Recherches sur la turgescence et la transpiration des plantes grasses, Ann. Sc. Nat. 

 Bot., series 7, vol. 16, p. 1, 1892. 



*Astruc, A. Recherches sur 1'acidite vegetale. Ann. d. Sci. Nat. Bot., series 8, vol. 17, pp. 

 1-105, 1903. 



c Czapek, F. Eiiiige Biochemische Verhaltnisse der Pflanzensauren. Biochemie der Pflanzen, 

 vol. 2, pp. 446 et seq., 1905. 



d Spoehr, H. A. Photochemische Vorgange bei der Entsauerung der Succulenten. Biochem- 

 ische Zeitschrift, vol. 57, p. 95, 1913. 



'De Saussure, Theod. Recherches chimiques sur la vegetation, p. 64, 1804. 



'Heyne, B. Trans. Linn. Soc., vol. n, part 2, p. 213. 



