RECENT ADVANCES IN SCIENCE 577 



strate metabolism proceeds, and may be viewed as a complex 

 of interrelated chemical changes, the substances concerned 

 being characterised by their instability. 



In considering respiration and the carbohydrate meta- 

 bolism of plants, the preconception as to the great stability 

 of the carbohydrates as a class, a preconception founded on 

 knowledge of them only as pure substances in simple solution, is 

 erroneous. Recent chemical investigations have shown that 

 monosaccharides, when associated with the salts of various 

 metals in alkaline solution, undergo spontaneously an extra- 

 ordinary series of complex and balanced changes leading to 

 the appearance of a very large number of products which 

 range up in complexity from C0 2 and formic acid. For example, 

 Nef has found that in a system containing initially dextrose 

 and sodium hydroxide no less than ninety-three different 

 substances are finally in equilibrium. Concentration, oxidation 

 potential, and temperature control the nature and propor- 

 tion of the final products. In the presence of dilute alkali 4 

 sugar is not decomposed, but the structural arrangement of 

 the sugar molecules is affected. It is a striking and puzzling 

 fact that but a small number of the thirty-two possible sugars 

 which are now known as synthetic products are ever found 

 in nature in the plant, and therefore it may be regarded as 

 suggestive that the composition and proportion of the various 

 sugars in equilibrium found in these experiments with dilute 

 alkali closely approaches those found in nature. The explana- 

 tion of the great instability and variability of reaction of the 

 sugars lies in the fact that the sugars, as weak acids, form 

 easily dissociated salts with metals and that the sugar ions 

 are unstable and highly reactive. Dissociation may be a 

 conditio sine qua non before oxidation can take place. The 

 colloidal mixture forming the substrate or medium of sugar 

 metabolism and the presence of a large variety of inorganic 

 salts in the plant appear to constitute conditions under which 

 we should expect a dissociation of the sugar molecule and a 

 very large range of possible consequent reactions. 



In the work which Spoehr himself has conducted he has 

 elaborated methods for isolating the carbohydrates of cacti. 

 During the cycle of the year cacti go through periods charac- 

 terised by low water content of the plant and high water 

 content, and by high and low temperatures associated with 

 both the above conditions. From a study of the seasonal 

 variations in the carbohydrates and from a study of the effect 

 of water content and temperature under experimental con- 

 ditions, it is found that — 



(i) Lower water content and high temperatures are 



