i8o THE CARBOHYDRATES 



Properties. 



Schulze * first proposed for this group of substances the 

 term hemicellulose, characterized by their insolubiUty in 

 water, solubiUty in alkaH, and precipitation from their alkaUne 

 solution by acid or by alcohol. On hydrolysis by means of 

 dilute acid they give origin to one or more monosaccharides 

 which may be either hexoses or pentoses, whilst cellulose, 

 which is more difficultly hydrolysed, yields glucose only. 



Although hemicelluloses are normally insoluble in water, 

 and are not extracted from wood by hot water, they become 

 soluble in water after extraction by means of alkali ; the 

 galactan of coniferous wood is, however, an exception, being, 

 according to Schorger, completely extractable from the wood 

 by hot water. 



From alkaline solutions some, but not all hemicelluloses 

 are precipitated as copper compounds on the addition of 

 boiling Fehling's solution ; among those not precipitated are 

 the arabans of the beet and cherry gum. f 



Constitution. 



The earlier workers looked upon hemicelluloses as poly- 

 merized anhydrides of pentoses or hexoses or of mixed sugars 

 as is indicated by the names xylan, araban, mannan, galactan, 

 galactoaraban, galactoxylan, etc. It has, however, been shown 

 by O'DwyerJ for the two hemicelluloses of beech wood that 

 they are not true polysaccharides in that they contain acid 

 groups, one in the form of a galacturonic acid and the other 

 a glycuronic acid residue, from which facts these compounds 

 would appear to be more closely related to the pectins (see 

 below) than to cellulose. 



By extraction of various starches with normal sodium 

 hydroxide, after a preliminary digestion with taka-diastase, 

 Schryver and his co-workers have obtained solutions from 

 which they were able to precipitate by acetic acid a hemi- 



* Schulze : " Zeit. physiol. Chem.," 1892, 16, 391. 



t Salkowski : id., 1901, 34, 171. 



X O'Dwyer : " Biochem. Journ.." 1926, 20, 656. 



