PENTOSE SUGARS IN PLANT METABOLISM 367 



especially in small quantities can be determined accurately only 

 by first removing all the hexoses by fermentation. ^ 



The proportion in which these two groups of sugars appear 

 varies greatly in different plants, some containing no pentoses 

 at all; while in others, such as the platyopuntias at times over 

 half of the total sugars present are pentoses. 



On account of their direct economic value and their great im- 

 portance in animal nutrition, fermentation, etc., special atten- 

 tion has been paid to the hexoses and the chemistry and physio- 

 logical role of this group has been extensively studied. When 

 dissolved in pure water these monosaccharides are comparatively 

 stable substances. Recently it has been found, however, that 

 even small traces of other substances, such as inorganic salts, 

 acids or alkalies in solution induce profound changes. These 

 investigations have aided greath^ in an understanding of the 

 manner in which the sugars are rendered so exceedingly unstable 

 and capable of such multifarious reactions in protoplasm and 

 have throw^n much light on the causes of the instability of living 

 matter. For example, an aqueous solution of dextrose is barely 

 afTected by the oxygen of the air, if however, a trace of an iron 

 salt or of a caustic alkali is added to the solution, oxidation 

 takes place rapidly and with the evolution of much heat. This, 

 very briefly stated, is due to the fact that sugars are actually 

 weak acids, capable of salt formation, that these sugar-salts are 

 far more reactive than the sugars themselves and in aqueous 

 solution spontaneously decompose into smaller molecules capa- 

 ble of oxidation, rearrangement and condensation.- 



The number of products of such a decomposition is enormous 

 and the nature of the oxidation reaction depends upon tempera- 

 ture, concentration, the inorganic salts present, etc. 



^ The complete methods and results as well as a more extensive discussion of 

 this subject are to appear in a publication on "The Carbohydrate Economy of 

 Cacti" now in preparation. See also: W. A. Davis and S. O. Sawyer, The esti- 

 mation of Carbohydrates, IV. The presence of free pentoses in plant extracts 

 and the influence of other sugars on their estimation. Jour. Agri. Sci. 6: 406-412, 

 1915. 



2 Nef, J. U., Dissoziationsvorgonge in der Zuckergruppe. Ann. d. Chem. 

 (Liebig) 357: 214, 1907; 375: 1, 1910; 403: 294, 1918. 



