HEMICELLULOSES 179 



Irisin in Iris pseudacoriis. 



Phlein in Phleiim pratense and Phalaris arundmacea. 



Sinistrin in Scilla maritima. 



Triticin in Triticum repois, Draccena australis and Dra- 

 ccBna rubra. 



Of these fructosans, graminin, and triticin are not precipi- 

 tated from neutral or acid solutions by heavy metal salts. 

 With barium hydroxide they give insoluble compounds, but 

 the corresponding calcium and strontium compounds are. 

 soluble. They do not reduce Fehling's solution and yield only 

 fructose on hydrolysis. 



All these compounds possess the same characteristics ; they 

 are laevo-rotatory, yield fructose on hydrolysis, and are fairly 

 soluble in cold water. The majority are difficult to crystallize, 

 and their solutions yield a gum-like substance on evaporation. 

 It is possible that some, at any rate, of these substances may 

 bear the same relation to inulin as dextrin does to starch. 



HEMICELLULOSES. 



Whilst it is possible on a physiological basis to distinguish 

 between food reserve polysaccharides such as starch, inulin, 

 and glycogen, on the one hand, and the typical structural 

 polysaccharide cellulose on the other, there occur in the plant 

 a number of related compounds whose physiological role is 

 less sharply defined. These substances are associated with the 

 structural elements of the plant and form part of the cell wall 

 but they may, on occasion, be attacked by appropriate 

 enzymes, secreted in the plant, and be utili-'.ed as food. 

 Owing to their dual function, and to emphasize their relation- 

 ship to cellulose, they are commonly termed reserve celluloses. 

 Whilst they resemble cellulose in many of their physical 

 properties, they differ from cellulose in their chemical pro- 

 perties. Included in the hemicelluloses are mannan, galactan, 

 and pentosan, which have been isolated from wheat and rye 

 bran, from beans and pea pods, and from lichens, and wood 

 gums which have been isolated from wood. 



T2 



