30 THE PLANT CELL WALL 



ary alcohol groups and the less frequent free phenolic group are 

 accounted for, and provision is made for branching via the open 

 oxy-group on ring 6. The phenolic OH at ring 5 might provide an 

 alternative branch point, but stereochemical limits would restrict 

 branching to one of these rings only. The end monomer unit (ring 7) 



ch,oh 



H I 



H 



0CH 3 0H <faZ 



Representation of a theoretical guaiacyl propane heptamer. After Adler, 

 E., Newer Views of Lignin Formation. No. 40, p. 294. Technical Associa- 

 tion of Pulp and Paper Industries, Easton, Pennsylvania, 1957.) 



corresponds to coniferaldehyde. The unsaturated aldehyde group 

 by undergoing condensation with phenols, amines, and the like 

 forms the chromogens which account for some lignin color reactions. 

 Other aldehydic groups may replace primary alcohols, but not 

 extensively. 



Among the variations of this pattern, the closure of the open 

 glyceryl-^-phenyl ether chains to form additional ring structures 

 as shown in the 2->3 linkage is not unlikely. Some lignins may 

 be more homogeneous, consisting, for example, of repeating units 

 containing the 2->3 linkage. Others are more diversified in monomer 

 types. 



As the main guide lines to an understanding of lignin structure 

 become established, more attention is directed toward the organic 

 and physical chemistry of the cellular synthetic processes which 

 lead to the formation of such unusual yet widespread biopolymers. 

 These aspects of lignin research will be examined when we consider 

 some aspects of cell wall dynamics and surface chemistry. 



Important Minor Wall Components 



The ash constituents of cell walls commonly make up 1-5 per 

 cent of their dry weight. Many elements in varying quantities are 

 to be found among the mineral substances of the wall, including 



