6 ACIDITY AND GAS INTERCHANGE IN CACTI. 



aidity he regarded as a priori improbable. Indeed, some of his experiments 

 showed an actual decrease of these substances in the tissues with diminishing 

 acidity. He, like Aubert and Astruc, whose papers will be discussed later, 

 agreed with Kraus in considering carbohydrates as the ultimate material from 

 which the acids are formed, and he did not entertain de Vries's idea of a light 

 stimulus which is inhibitory to acid formation, which consequently results in 

 the diminished acidity characteristic of the day period. He also maintained 

 that a certain amount of oxygen is necessary for the formation of the acids, 

 but the quantity is much less than that required to break them down, oxygen 

 being, nevertheless, of greater significance in the former process than in the 



f~1(~\ 



latter. As to the -Q-^ quotient, he found it lowest at the periods of maxi- 

 mum acid formation and highest when it is being broken down. In general, 

 he considered the evidence to be all in the direction of supporting the usually 

 accepted conclusion that the organic acids in plants are products of incomplete 

 oxidation. The formation of the carbon dioxide is probably not direct, but 

 by gradual steps in which simpler and simpler compounds are produced as the 

 gas is given off. His conception of respiration, indeed, is that it is just such a 

 process of gradual disintegration, first of carbohydrates and then of the acids 

 formed from them. 



Astruc also has contributed a lengthy paper covering the general field of 

 acidity in plants. Considerable attention is given, however, to the question 

 of the acidity of the Crassulacese in particular. The two forms chiefly em- 

 ployed for experimental purposes were Sempervivum and Echeveria. Unlike 

 Aubert, who found less acid in the young parts and most in the recently mature 

 tissue, Astruc reports that the acids are formed most of all in young organs 

 which show high cellular activity and a maximum of turgescence. According 

 to him the acids which are produced gradually tend to diminish in amount as 

 the tissues advance in age, either by entering into combination with alkaline 

 bases absorbed with the soil-water or by esterification, in consequence of which 

 the cell contents lose their acid properties. His conclusions are not so 

 different from those of Aubert as would appear at first sight, for the latter 

 admits that the average acidity finally diminishes after the period of early 

 maturity is passed. Probably the two writers did not have precisely the same 

 criteria for judging the relative age of the parts used. Astruc considers that 

 the formation of the acids is indirectly dependent in part upon previous photo- 

 synthetic activity, so that succulent plants form much less acid at night, when 

 they have been exposed during the previous day in an atmosphere devoid of 

 carbon dioxide. In this respect he differed from de Vries, but agreed with 

 Aubert in thinking that the formation of acid at night is dependent upon 

 assimilation in the day. These conclusions are obvious if, as is generally 

 accepted, it is from the partial breaking up of carbohydrates that the acid 

 arises. Corroborative of this position he found that etiolated tissue was much 



lower in acidity than that which came from normally illuminated plants. The 



CO 

 relation of acidification to the -Q-^ ratio was shown by the lowering of the 



latter at night-time, when acid formation is taking place. Astruc agreed with 

 Purjewicz and Aubert in finding that any cause which tends to hasten the 



Aatruc, A. Recherches sur I'acidit6 vegetale. Ana. d. Sci. Nat. Botanique, Series VII. vol. 

 17, pp. 1-64, 1903. 



