14 THE CARBOHYDRATE ECONOMY OF CACTI. 



but rather by those substances (or catalysts) which under physico-chemical 

 conditions existent in the cell exert on sugars an effect analogous or equiva- 

 lent to that of metallic salts or hydroxides in aqueous solution. In fact, the 

 importance and role of catalyzers in glycolysis has been established by the 

 large amount of work which has been done with various tissue and pancreas 

 extracts. Thus, for example, Walter Loeb 1 has shown that substances 

 obtained from the alcoholic pancreas extracts by treatment with iron salts 

 effect a cleavage of glucose in which the products formed were formaldehyde 

 and pentose. However, by the use of pure sugar solutions and alkalies, it 

 has been possible to follow more clearly the many complex disintegrations 

 and rearrangements which the sugar molecule undergoes. The principle 

 of chemical action is most probably the same. The nature of the products 

 formed under these conditions depends upon whether or not there is present 

 an oxidizing agent, such as air. In the organism the same differentiation 

 exists in regard to the catabolism of hexoses. Under anaerobic conditions 

 one set of products is formed, while under aerobic conditions there is 

 another set of products different from the first results. The similarity of 

 the reactions obtained in vitro to those observed in the organism of anaero- 

 biosis and aerobiosis are worthy of notice. In the succulent plants, such as 

 the cacti, conditions exist which lie between these extremes, i. e., a low or 

 insufficient oxygen supply/ This is perhaps the most important single 

 factor in the formation of acid in the plants. Under similar conditions 

 there is formed in the plant, not d-lactic acid, but d-malic acid. The genesis 

 of this latter substance in succulent plants has been a matter of much 

 speculation, which, however, has in no case been founded on chemical 

 experience. The relation between anaerobiosis and aerobiosis will become 

 evident from the chemical discussion which is to follow. 



If a solution of d-glucose is treated with alkali of higher concentration 

 than that causing the reciprocal transformations, as e. g., 8 X normal sodium 

 hydroxide in the absence of oxygen or an oxidizing agent there are formed: 

 large quantities of dl-lactic acid and dl 1-3 dioxybutyric acid, besides four 

 isomeric C 6 saccharinic acids. The importance of lactic acid in the 

 metabolism of the mammalian organism and its formation under conditions 

 of insufficient oxidation or restricted oxygen supply is well known.* As 

 will be shown later, the very large quantities of pentoses found in these 

 plants arise from the hexoses under conditions of repressed metabolic 

 activity; the further disintegration of the pentose sugars is therefore of 

 special interest in relation to the present problem. That these plant acids 

 are of great importance in many of the functions of the plant has been 

 clearly established by Richards in regard to the gaseous exchange and 



1 VON FUERTH, OTTO, and A. J. SMITH. The problems of physiological and pathological 



chemistry of metabolism. Chapt. XIX, 1916. 

 3 RICHARDS, H. M. Acidity and gas interchange in cacti. Carnegie Inst. Wash. Pub. 



No. 209, p. 32, 1915. 

 ' DAKIN, H. D. Oxidations and reductions in the animal body. Pages 56-68, 85-87, 



1912. 

 VON FUERTH-SMITH. L. c., p. 452. 



