The Carbohydrates of the Leaf. 1 05 



as the only hexose sugars fermented by ordinary yeasts are rf-glucose, 

 rf-mannose, rf-fructose and less easily f?-j<alactose, while numeious 

 disaccharides with cupric-reducing power are not fermented by 

 yeasts, as well as the pentoses, while there is always the possibility 

 of the presence of substances with cupric-reducing properties other 

 than sugars. 



The further evidence of the presence of pentoses is derived 

 from the fact that these purified plant extracts on subjection to 

 distillation with hydrochloric acid according to the Krober-Tollens 

 process (see Tollens, 1914) yield a weight of phloroglucide which 

 would be given by practically the same amount of pentose calculated 

 as a mixture of ^arabinose and Z-xylose. It must be admitted that 

 the concordance is not very striking and Kluyver (1914) has pointed 

 out that the presence of hexoses and disaccharides in such a solution 

 is a source of error, as on distillation with hydrochloric acid these 

 also give small quantities of furfural-like compounds which yield an 

 insoluble phloroglucide, so that the method could not give any very 

 accurate value for small quantities of pentose in presence of large 

 quantities of other sugars. Davis and Sawyer admit the truth of 

 this criticism, but point out that the error actually introduced in 

 this way is small (about 18%). 



Nevertheless the evidence that the only sugars present in the 

 leaf are sucrose, rf-glucose, ff-fructose and pentoses does rtot carry 

 complete conviction. 



Besides sugars and starch Davis, Daish and Sawyer have 

 estimated the complex derivatives of the pentoses, the pentosans, 

 by distillation of the leaf-matter insoluble in alcohol by the Krober- 

 Tollens method. 



The table overleaf may therefore be regarded as summing up 

 our knowledge in regard to the presence of carbohydrates in the 

 leaf. 



The absence of li-mannose, which is closely related to rf-glucose 

 and rf-fructose in chemical constitution and in its behaviour as 

 regards fermentation by yeasts appears to have been generally 

 accepted without the production of any sound evidence in support 

 of the opinion. It is also generally assumed that the / forms of the 

 hexoses are completely absent from the leaf. Thus E. F. Armstrong 

 (1913) says: " In spite of frequent search it has never been possible 

 to detect /-glucose or Z-fructose in the leaves of plants, and the 

 work of Brown and Morris leaves hardly any doubt that hexoses of 

 the rf-series and their polysaccharides are the only products of 



