24 THE CARBOHYDBATE ECONOMY OF CACTI. 



II. HISTORICAL. 



Beyond the proximate analyses incident to investigations of the food 

 value of cacti for agricultural purposes, very little work has appeared on 

 the carbohydrates of the cacti. 1 Nor is there any extensive information on 

 the subject available for other succulent plants. However, on account of 

 the complex character of its respiratory processes, its structural peculiari- 

 ties, and the rather extreme climatic conditions under which many of its 

 members live, this group of plants has been subjected to extensive physio- 

 logical investigations. Some of the earliest and now classical researches on 

 respiration were carried out with this material. 



The interest has centered primarily around the gas interchange and the 

 periodic acidification and deacidification of the plants. De Saussure 2 was 

 the first to study thoroughly the respiratory relations of these plants. The 

 question of acidity in succulents has been subjected to extensive investiga- 

 tions ; most recent are the very thorough researches of Richards * and those 

 of Jenny Hempel. 4 In the publication of Richards is given a clear discus- 

 sion of the historical development of the subject, and the conditions induc- 

 ing acidification and deacidification, together with the effect on the respira- 

 tory quotient, are there elaborately formulated, based upon wide observation 

 and comprehensive experimentation. 



The relation of the plant acids to the carbohydrates has been the subject 

 of much speculation ever since the appearance of Liebig's theories of the 

 origin of sugars from hydroxy acids in ripening fruits. Although the 

 weight of evidence seems to be against the Liebig theory, and it is con- 

 sidered as untenable by most plant physiologists, the principle thereof has 

 been frequently revived and has found expression in modified forms. 8 The 

 formation of organic acids as catabolic products of carbohydrate metabo- 

 lism has received very little attention from plant physiologists. From the 

 chemical evidence available, many of the intermediate products of sugar 

 degradation are acids of the nature of those found in the plant, as has been 

 discussed in the introduction. However, the manner in which the carbo- 

 hydrates are first affected, the actual mode of acid formation in the plant, 

 and the further transformations are all questions toward which very little 

 physiological inquiry has been directed. As prerequisite knowledge for 

 investigation of these problems, we must have some understanding of the 

 interrelation and transformations of the carbohydrates from the simple to 

 the complex, and vice versa, the conditions inducing such changes and also, 



1 GRIFFITH, D., and R. F. HARE. Prickly pear and other cacti as food for stock. Bull. 



60, New Mexico College of Agriculture and Mechanic Arts, 1906. Ibid., 72. 

 HARRINGTON, H. H. Analysis of cactacus. Tex. Expt. Station Report 1, 28, 1888. 

 8 DE SAUSSURE, THEO. Recherche chemique sur la v6ge"tation. 1804. 

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



Pub. No. 209, 1915. 



* HEMPEL, JENNY. Buffer processes in the metabolism of succulent plants. Comptes- 



rendus des travaux du Laboratoire de Carlsberg, 13, 1917. 



* BAUB, E. Ueber die Genesis der Kohlenhydrate. Naturwissenschaften 1, 474-477, 



1913. Zeit. physik chem., 63, 683-710. 



