102 ACIDITY AND GAS INTERCHANGE IN CACTI. 



besides carbon dioxide, which are similar to those formed by normal respira- 

 tion. Presumably, the acid found in the tissues exposed to atmospheres 

 devoid of oxygen is the same as under normal conditions, but of this we have 

 no proof. In connection with this question it should be determined if alcohol 

 is produced by the "intramolecular respiration" of cacti, as appears to be 

 the case in all other plants in the absence of oxygen, but since the normal 

 result of partial oxidation in the cacti seems to be malic acid, their behavior 

 under these conditions may be quite different from that of the ordinary plant. 



According to Spoehr, in the paper already referred to, ethyl alcohol must be 

 the first degeneration product of malic acid when exposed to light in the 

 presence of oxygen. This then passes over by further oxidations to acids 

 which become successively simpler. Looked at from this standpoint, it would 

 seem as if the normal respiration of the cacti were of the nature of what is 

 commonly called intramolecular, except that the process is not carried far 

 enough to yield alcohol, but stops at malic acid. Alcohol is now commonly 

 recognized as an accompanying result of intramolecular respiration in forms 

 that have sugars at their disposal. Such an interpretation is in accord with 

 the idea already largely entertained, namely, that the respiratory activity 

 of succulents is influenced by the insufficient oxygen supply of the tissues. 

 It is plain, however, that in this connection an investigation should be made 

 into the nature of the gases held in the tissues of these plants. Such knowl- 

 edge might throw further light upon the question. 



That the acids are formed by reason of the comparative lack of oxygen in 

 the massive tissues of the succulents has been the conclusion of many investi- 

 gators. This interpretation has much to recommend it and is supported both 

 by the behavior of the cacti in the absence of oxygen as well as by their behavior 

 in the presence of an excess of that gas. That the acids are the only end prod- 

 uct of this respiration is not necessarily the case nor indeed, probably so 

 for some carbon dioxide is evolved even when the tissues are gaining in acid- 

 content. If we were informed of the exact nature of the substances consumed 

 in the first place, it might be possible to estimate how large a percentage of 

 the actual energy-releasing processes this oxidation to acid represents. It is 

 safe to say that it is the major part. It may be said, in this connection, that 

 in these experiments there were very few cases only one or two where 

 there was a complete absence of the evolution of carbon dioxide in the dark. 

 These cases, moreover, were anomalous in other respects. Whether the rela- 

 tively small residual output of carbon dioxide was from oxidations resulting 

 from contact with free oxygen or arose as did the carbon dioxide of intra- 

 molecular respiration would be difficult to determine. It would be necessary 

 to ascertain the exact amount of oxygen required to produce the malic acid 

 found from the materials at the disposal of the cells and then to see how it 

 compared in quantity with the actual amount of oxygen absorbed. In such 

 a way it could then be decided whether the carbon dioxide given off when the 

 acidity is rising is produced in an aerobic or an anaerobic fashion. 



In the opinion of the writer the accumulated acid, together with whatever 

 carbon dioxide is evolved during the process of accumulation, represents the 

 end-products of the respiratory activity, if we exclude the metabolic water 

 which must also be formed. But the respiratory activity is by no means the 

 direct cause of all of the carbon-dioxide which may be given off. A consider- 

 able portion, and under some conditions by far the largest portion of this gas, 



