334 MACDOUGAL AND SPOEHR— GROWTH AND IMBIBITION. 



and in the liberation of energy. Although these two activities, 

 imbibition and metabolism, are so closely interrelated in the growth 

 processes they are nevertheless of such a widely different nature that 

 it cannot be assumed, as will be shown, that they are equally in- 

 fluenced by external conditions, as for instance, temperature ; the 

 conditions under which one affects the other depending, in turn, 

 upon several other factors. 



In general, chemical inversion, or the transformation of the 

 highly condensed to the simpler molecules capable of oxidation and 

 translocation, takes place only under conditions of ample water 

 supply. However, these reversible enzymatic reactions never run 

 entirely in one direction. Only differences between the two are 

 observable. We are dealing with a delicate compound dynamic 

 equilibrium, involving probably dozens of steps and many more 

 substances. The very interesting investigations of Lobry de Bruyn 

 and Van Ekenstein" and of Nef on the rearrangements of the 

 hexose molecule demonstrate the extreme complexity of such equi- 

 libria. Thus Nef^- has shown that when the relatively simple 

 hexose sugar, dextrose, is dissolved in a weak alkaline solution there 

 are formed no less than 93 different substances which constitute a 

 system in dynamic equilibrium. Any number of these can react 

 selectively and shift the equilibrium, by oxidation, condensation 

 or the like, the course of the reaction depending upon the condition 

 of solution as to concentration, temperature, etc. How much more 

 complex must the condition be in the living cell with the numerous 

 delicate enzymatic equilibria each with its own temperature and con- 

 centration coefficient? 



The following results (which are a portion of an extensive in- 

 vestigation of the carbohydrate economy of cacti now in progress) 

 throw some light on the relation of carbohydrate metabolism to 

 growth. 



The carbohydrates predominate in the general food economy of 

 the cacti. There is no reason for believing that the metabolic 

 processes concerned in the growth of such plants consist chiefly of 



^1 Lobry de Bruyn and Van Ekenstein, Rcc. trav. chim. de Pays-Bas, 14, 

 158, 203 ; 15, 92 ; 16, 257. 



12 Nef, J. U., Annalcn dcr Chcmic, Licbig, ./oj, 204-383, 1913. 



