CONSTITUTION AND ARCHITECTURE IN THE CELL WALL 



21 



There is a natural gradation between the polyuronide hemicellu- 

 loses and the non-cellulosic polysaccharides. Indeed, both groups 

 are sometimes treated collectively as acidic and neutral forms of 

 hemicellulose. The major hemicellulose of the maize cob, which 

 constitutes three-fourths of the alkali-soluble components consists 

 of a linear xylan with only one glucuronic acid per molecule. Such 

 polymers will not differ greatly from xylans themselves. Among 

 these neutral hemicelluloses or non-cellulosic polysaccharides, 

 there are homopolymeric mannans, galactans, arabans, xylans, 

 and copolymeric glucomannans, galacto mannans, arabinogalactans 

 and arabinoxylans. These polymers which may be termed "glycans" 

 vary considerably from species to species. Representative glycan 

 contents of various plants are: 



As in the acidic polymers the neutral compounds show diversity 

 in residues, linkages, branching and size. Ivory nut mannan is 

 a linear homopolymer of DP = 7 5 in which the residues are joined 

 by 1,4-linkages. A high degree of crystallinity is indicated by 

 X-ray diffraction. Lupin galactan is also a cellulose analog, con- 

 sting of linear 1,4-linked /9-D-galactose chains. 



Hardwood arabinogalactan is a branched polymer of molecular 

 weight 16,000 for which the following structure has been proposed: 



