;o THE CARBOHYDRATES 



For example, in cherry or wood gum they occur in the form of 

 pentosanes or pentosides, which substances may be regarded 

 as anhydrides of pentoses, since they give rise to these sugars 

 on hydrolysis in much the same way that starch on hydrolysis 

 yields glucose. 



Pentoses, although commonly looked upon as waste pro- 

 ducts, do not accumulate as the plant grows older ; they 

 fluctuate in amount according to the conditions of growth, 

 such as the amount of available water. In the case of Par- 

 thenium argentatum, a high production of pentose coincides 

 with the period of growth during which the production of 

 rubber is at its highest Spoehr considers that the pentoses 

 are definite stages in the elaboration of substances such as 

 rubber and nucleic acid.* 



GENERAL PROPERTIES OF PENTOSES. 



1. They are not fermentable by yeast. 



2. On distillation with hydrochloric or sulphuric acid, they 

 are converted in furfural, which may be detected by its turning 

 a solution of aniline acetate red. 



C 6 H 10 5 3 H 2 0= C 4 H 3 O.CHO 



This may easily be seen by boiling some wood shavings with 

 concentrated hydrochloric acid in a test tube and allowing the 

 escaping steam to impinge upon a piece of filter paper moistened 

 with aniline acetate ; f a pink colour is produced. 



It should be noted that hexosesj will also produce this 

 reaction, though to a much smaller extent, since the quantity 

 of furfural produced from them is much less (not more than 

 O'2 per cent) than in the case of the pentoses. The chief 

 product of the action of concentrated hydrochloric acid in 

 hexoses is levulinic acid. 



3. Warmed with concentrated hydrochloric acid (sp. gr. I *2) 

 and a little orcinol, they produce a greenish-yellow colour which 

 is soluble in amyl alcohol to a clear green solution having a 

 characteristic absorption spectrum with bands between the C 

 and D lines. 



* Spoehr : " Plant World," 1917, 20, 365. 



f Prepared by mixing together equal parts of aniline, water and glacial acetic 

 acid. 



:Cf. Tollens: " J. f. Landw.," 1901, 49, 39. 



