66 THE CARBOHYDKATE ECONOMY OF CACTI. 



Opuntia versicolor shows the same behavior as Opuntia phceacantha in 

 that for at least 5 days these plants can live with no apparent untoward 

 effects in an atmosphere containing no oxygen. The rate of carbohydrate 

 consumption is higher under anaerobic conditions than during aerobic 

 respiration. This is probably due to the higher energy release and hence 

 greater efficiency of the aerobic process, as was pointed out in the intro- 

 ductory discussion. The water loss under the experimental conditions was 

 naturally low ; the plants which had consumed the larger amounts of carbo- 

 hydrate showed a higher percentage of water-content. 



In the foregoing experiment with Opuntia versicolor acidity determina- 

 tions were also made. In each case 100 grams of the plant material which 

 had been ground were thoroughly expressed in a meat-press. The juice 

 thus obtained was made up to a definite volume and aliquot portions titrated 

 with 0.1 normal potassium hydrate, using phenolphthalein as an indicator. 

 The plants representing the original condition were collected at 4 p. m., at a 

 time when the acidity was almost at its minimum. The results thus obtained 

 are as follows : 



Original condition 0.20 c. c, 0.1 normal KOH per gram fresh material 



Kept in air 0.78 c. c. " 



In oxygen-free atmosphere 0.50 c. c. " " " " " " " 



From this it is evident that acid formation is benefited by the presence of 

 oxygen. This is quite in accord with the results obtained by Warburg. 1 

 The principles underlying the phenomena of acidification and deacidifica- 

 tion have been discussed by this worker and more recently by Richards and 

 by the author. 3 It is evident also that there is a decided acidification even 

 in the absence of oxygen. Whether this acidity also represents malic acid, 

 as under normal conditions, or some other acid, the product of real intra- 

 molecular respiration is an open question. The answer hereto would throw 

 considerable light on the mode of this form of energy release. 



The respiration of succulent plants represents somewhat of a modifica- 

 tion of the process taking place in most mesophytes. The characteristic 

 formation of acids is intimately associated with the restricted oxygen supply 

 consequent on the structure of the succulent type. However, when these 

 plants are deprived of all oxygen, it is evident that the final course which 

 the respiratory process follows is further altered, and the products formed 

 are radically different. In consideration simply of the chemical reactions 

 of sugars in aqueous solution in the presence of oxidizing agents under 

 various conditions, as outlined in the introductory discussion, variations in 

 the reaction products with altered external conditions of the organism are to 

 be expected. It was indicated in this discussion in what manifold channels 

 the course of sugar disintegration may proceed, and what slight differences 

 in the nature of solution may affect the main course of the reaction. Pre- 

 cursory to oxidation of the sugars, there must take place a dissociation or 



1 WARBURG, O. Ueber die Bedeutung der Organischen Saeuren fuer den Lebenspro- 

 zess der Pflanzen. Speciell der Sog. Fettpflanzen. Unters. aus dem Bot. 

 Inst. zu Tuebingen, 2, 53-150, 1886. 



1 SPOEHR, H. A. Biochem. Zeitschr., 57, 95, 1913. 



