4 ACIDITY AND GAS INTERCHANGE IN CACTI. 



Link, is printed in Kraus's" paper on the acidity of cell-sap. Link actually 

 tested with litmus paper the juice expressed from the same plant and also 

 other crassulaceous forms, in the morning and in the evening, and so by more 

 definite experimental means corroborated Heyne's report. 



It was not until 1875 and later that A. Mayer 6 published his series of papers 

 on his investigations of the organic acids found among the Crassulacese and 

 established the fact that it is malic or isomalic acid that is present in these 

 plants. 



We may regard the investigation of de Saussure and the observations of 

 Heyne and of Link as the starting-points from which have come the subsequent 

 researches into the acidity of the succulent plants and their respiratory activ- 

 ity. Indeed, the whole question of the relation of acidity to the evolution of 

 carbon dioxide had its first stimulus in these observations. Historically con- 

 sidered, Liebig's theories find place here. In connection with the disappear- 

 ance of organic acids in the ripening of fruit, he conceived the idea that these 

 acids were concerned in the upbuilding of carbohydrates by the plant, and 

 constituted a midpoint between carbon dioxide on the one hand and sugars 

 on the other. This point of view was not, indeed, accepted by all, even at the 

 time of its publication, and has since found but little favor, at least among 

 plant physiologists. For a further discussion and more detail concerning 

 the part which the investigations of A. Mayer played in regard to Liebig's 

 theory, reference may be made to Warburg's account** and to Spoehr, loc. ciL, in 

 which papers the matter is more fully discussed. It is sufficient to say that 

 practically all of the later investigators have shown Liebig's theory to be 

 untenable. 



Kraus, in his paper on the acidity of cell-sap, in general very distinctly states 

 his opinion that the acid is in no way connected with the formation of food 

 substances and that it is a by-product of respiration. He determined that the 

 loss of acid during the day was due to the direct effect of light and had no 

 immediate connection with the true respiratory activity of the tissues. He 

 regarded the acid as a secondary product of proteid splitting. 



De Vries, e in his work on the periodicity of acid formation in succulents, 

 determined that prolonged darkness also results in diminution of acidity and 

 further that exposure to high temperature produces the same effect. He con- 

 cluded that the splitting up of the acid and its upbuilding are coincident pro- 

 cesses, both of which are always taking place. At night the synthesis of the 

 acid is more rapid than its breaking down, and in the day the reverse is true. 

 He also held that no connection exists between the presence or absence of 

 acids and the formation of sugars by the plant. 



In many respects Warburg's paper, which followed shortly afterwards, is the 

 most scholarly treatment of the subject that we have. He discusses the his- 



Kraus, G. Ueber die Wasservertheilung in der Pflanze. IV. Die Aciditiit des Zellsaf tea. Abh. 

 der naturforsch. Ges., Halle, vol. 16, pp. 154-157, 1883. 



"Mayer, Ad. Landw. Versuchstat., vol. 18, p. 428, 1875; vol. 21, p. 277, 1878; vol. 30, p. 217, 

 1884; vol. 34, p. 127, 1887. 



c Liebig. Chemie in ihrer Anwendung auf Agrikultur und Physiologic, 9th ed., p. 31, 1876. 



^Warburg, O. Ueber die Bedeutung der Organischen Sauren fur den Lebensprozess der 

 Pflanzen. Untersuch., aus dem Bot., Inst., Tubingen, vol. 2, p. 57, 1886. 



*De Vries, H. Ueber die periodische Saurebildung der Fettpflanzen. Bot. Zeit., vol. 42, p. 337, 

 1884. Ibid., Ueber die Periodicitat im Sauregehalt der Fettpflanzen. Verslagen en Medeed- 

 lingen der Kononkl. Akad. van Wetenschapen. Afd. Naturkunde Reeks, Deil 1, 1884. 

 Abstract Just Bot. Jahrb., vol. 1, p. 65, 1884. 



