20 



THE PLANT CELL WALL 



Diagrammatically, a wheat straw hemicellulose, DP 71-76, 

 is constructed in the following manner: 



xylose 1—4 xylose 1—4 xylose 1—4 xylose 1—4 xylose 



3 3 3 



I I I 



1 1 1 



xylose 1—4 xylose glucuronic arabinose 



acid 



Among the hemicelluloses of beech species, two quite distinct 

 polymers which have been described are: 



xylose 1—4 xylose 1—4 xylose 1—4 xylose 1—4 xylose 



2 



1 



methylglucuronic acid 



and 



xylose 1—4 xylose 1—4 xylose 1—4 xylose 1—4 xylose 



2 2 



1 



1 



xylose 1—4 xylose methylglucuronic acid 



i 



By comparison with celluloses with molecular weights rang- 

 ing up to 10 6 or more and pectic substances in the range of 

 60,000-300,000, the acid hemicelluloses are relatively small mo- 

 lecules. The degrees of polymerization and approximate molec- 

 ular weights {DP x 160) of some representative polyuronides are: 



