260 Cellulose 



seeds of these and allied species are of complex constitution. The 

 tissues, in addition to being lignified more or less (see Lignocelluloses* 

 p. 92), contain, as an essential constituent, the characteristic com- 

 pound known as wood gum (Holzgummi). Wood gum is the 

 colloid anhydride of the pentaglucoses, yielding the crystallisable 

 pentoses, xylose and arabinose, when hydrolysed by boiling dilute 

 acids. The pentosans are dissolved by dilute solutions of the alka- 

 line hydrates in the cold, from which solutions they are precipitated 

 on acidification, and more completely in presence of alcohol. The 

 pentosans, when boiled with hydrochloric acid (ro6 sp.gr.), of 

 course yield furfural, and it is sometimes assumed that the quan- 

 tity obtained from straw is a direct measure of the pentosans pre- 

 sent. This conclusion requires qualification, and it is necessary to 

 divide the furfural-yielding constituents into the two groups : (a) 

 the pentosans, easily hydrolysed, and obtained as above described ; 

 (b] the oxycelluloses, resisting hydrolytic actions of some intensity. 

 Together with the investigations above cited, the student should 

 read an account of the researches of Schulze and Tollens (Landw. 

 Vers.-Stat. 40, 367) upon the composition and constitution of the 

 substance of * brewers' grains.' This material consists obviously 

 of the more resistant seed-envelopes of the barley, the greater por- 

 tion of which remains unaffected by the malting and mashing pro- 

 cesses. The following scheme represents the method of examination 

 pursued by the authors with indications of the results obtained: 



Acid Hydrolysis. Boiling with 4 p.ct. aqueous H 2 SO 4 



