22 THE CARBOHYDRATE ECONOMY OF CACTI. 



the utmost importance in the study of the problem of metabolism in the 

 plant. 



The special attention which has been given here to results so far obtained 

 from the investigations of the reactions of sugars in combination with salts 

 and alkalies seem warranted in that they represent a system in dynamic 

 equilibrium; the breaking down is always accompanied by the synthesis 

 of more complex substances, and while such a complex system is still beyond 

 the experimental control of physical chemical methods, it simulates the 

 dynamic nature of living things, it emphasizes process rather than single 

 substances, and brings these processes within the range of experimental 

 science by centering energy changes on the fundamental properties of the 

 carbon atom. Furthermore, these reactions give a clear indication of the 

 nature of one form, at least, of enzyme action in a homogeneous system and 

 argue strongly for the theory of the formation of intermediate compounds. 



But before such a chemical system can be rationally applied to the f unc- 

 tionings of a living plant, some further knowledge is essential of the nature 

 of the carbohydrates and the transformations which these undergo in the 

 plant under various conditions. Here again the aspect of a dynamic 

 equilibrium is evident. It is a matter of common knowledge that most 

 plants contain monosaccharides, disaccharides, and the condensation prod- 

 ucts of these (the polysaccharides) in various proportions. Each of 

 these types of carbohydrates is what might be termed a " physiological 

 group," and the proportion in which they are present in a plant leaf, 

 for instance, is the result of certain factors, and in turn controls to a large 

 measure the physiological activity of the organism. An effort has been 

 made to determine some of the conditions affecting this equilibrium of mono- 

 saccharide^l^disaccharide^Z^polysaccharide and to study the effect of this 

 on metabolism. 



In general, chemical inversion, or the transformation of the condensed 

 to the simpler molecules capable of oxidation, translocation, and the forma- 

 tion of other substances, takes place 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 delicate compound dynamic equilibria involving probably dozens of 

 steps and many more substances. The enormous importance of water in 

 various functions of the organism, such as growth, has long been recognized. 

 When it is realized that it is to a great extent only under conditions of 

 sufficient water that the simpler and more reactive carbohydrates are 

 present in sufficient amounts in the cell, this fact becomes understandable. 

 It has been found that variations in the water-content of the cactus joints 

 greatly influence the proportion of the various groups of sugars. As a 

 consequence, the course of metabolism is also decidedly affected. This 

 undoubtedly also is true for other plants. With high water-content the 

 polysaccharides are converted into monosaccharides, and from all evidence 

 available under normal conditions, the latter are the carbohydrates most 



