8 THE CARBOHYDRATE ECONOMY OF CACTI. 



amounts of almost all plants. In the cacti conditions are unusually favor- 

 able for the study of the formation and nature of these substances. It is not 

 claimed that the pentosans in themselves represent the composition of 

 protoplasm. The interest lies in the transformations which the carbo- 

 hydrates undergo in the course of metabolism under varying external con- 

 ditions and the nature of the chemical changes which are involved in the 

 economy. Nor are these phenomena to be considered as a complete account, 

 but at the most as a single chapter in a very extensive and involved story. 



It would seem that the material used in respiration of most of the higher 

 plants is essentially of carbohydrate nature. Whether plants use only 

 carbohydrates in the course of normal respiration has not been established. 

 Fats, of course, are used extensively by plants, especially in seeds; but 

 whether the fat is converted into carbohydrate before it is used in the 

 respiratory process is still unsettled. As to the role of proteins, especially 

 in the higher plants, we are quite in the dark, but it can be assumed with 

 safety that these substances do not play nearly so important a part in the 

 economy of higher plants as they do in animals. As will be shown, of 

 greatest importance to the mode of the breaking down of the simpler carbo- 

 hydrates is the nature or condition of solution. In the investigations to be 

 cited this condition refers more especially to the degree of alkalinity. There 

 is considerable reason for believing that the proteins and their simpler 

 derivatives, incorporated in this cell substratum or medium, produce therein 

 the necessary conditions under which glycolysis may proceed, or through the 

 production of acid and alkaline products regulate the enzymatic activities. 

 These products are in turn again synthesized into the protoplasmic proteins, 

 so that relatively small amounts of protein suffice for the conversion of large 

 quantities of carbohydrate and (as actually seems to be the case) the major 

 portion of the energy released is thus to be ascribed to the catabolism of the 

 carbohydrates. Also, the remarkable property possessed by the proteins 

 and their salts of neutralizing acids or bases by the opening up of suc- 

 cessive IST.H-O-C groups gives them the power of maintaining the H* or 

 OH'ion concentration of the medium within definite limits, usually close 

 to neutral. In view of the fact that the products of carbohydrate catabo- 

 lism are mostly acid, it might well be expected that this property is of prime 

 physiological importance, and that herein lie some of the fundamental 

 differences between cells of animals and those of plants with their vacuolar 

 fluid. Some workers, in fact, have ascribed to the proteins themselves 

 enzymatic properties which may be interpreted that the zymogen as well as 

 the co-enzyme is a product of the protein. These conceptions evidently 

 depend upon a clearer understanding of the nature of enzyme action and 

 catalysis before any substantiation or further elucidation may be hoped for. 



Deleano, 1 working with mature leaves of Vitis vinifera at 18 to 22 

 in the dark, found that these utilize only carbohydrates during the first 



1 DELEANO, NICOLAS T. Studien ueber den Atmungsstoffwechsel abgeschnittener 



Laubblatter. Jahr. f. wiss. Bot., 51, 541-592, 1912. 



BORODIN, J. Ueber die physiologische Rolle und die Verbreitung des Asparagins im 

 Pflanzenreiche. Bot. Zeitg., 36, 801-832, 1878. 



