HEMICELLULOSES 183 



from coniferous wood is much less. Xylan also occurs in 

 corn cobs and in straw, from which latter source it may be 

 conveniently prepared in a state of purity * by extraction with 

 caustic soda and precipitation by means of an alkaline solution 

 of copper sulphate. Xylan was formerly thought to be a 

 polymerized anhydride of xylose, since it gives rise only to 

 this sugar on hydrolysis ; the observations of O'Dwyer on 

 the hemicelluloses of American white oak render this view 

 no longer tenable (see p. 181). 



Xylan, precipitated from alkaline solution by the addition 

 of acid, is soluble in hot water ; but water will not extract 

 it from wood in the first instance. 



ARABAN. 



This substance, which on hydrolysis gives rise to arabinose, 

 is associated with xylan in wood ; it is not clear whether it 

 occurs as a distinct individual, or whether it is combined 

 with other material ; according to O'Dwyer beech wood 

 contains two hemicelluloses, one of which contains xylose in 

 combination with glycuronic acid, while the other contains 

 arabinose combined with galacturonic acid. 



Another substance yielding arabinose on hydrolysis and 

 which has been regarded as a true carbohydrate or poly- 

 saccharide form of arabinose is cherry gum ; this substance 

 is exuded from the bark of the cherry and yields on hydrolysis 

 chiefly arabinose with a small quantity of xylose, whereas the 

 wood of the cherry extracted with alkali yields a product 

 which produces chiefly xylose. 



Gum-arabic likewise contains an araban since it yields 

 chiefly arabinose on hydrolysis. 



An araban has also been described by Ehrlich as arising 

 from the hydrolysis of protopectin (see p. 202). 



WOOD GUM. 



This term is applied to the hemicelluloses extractable from 

 wood by caustic soda. A true gum should at least swell up 

 if not dissolve in water, but many of the so-called wood gums 



* Salkowski : " Zeit. physiol. Chem.," 1902, 34, 162. 



