24 THE PLANT CELL WALL 



of 4500 x 19 A. Although there are many variations in the structures, 

 sizes and specific monomers, these polymers fall, in principle, into 

 a common category with the complex polysaccharides which have 

 already been discussed. 



Before leaving the wall polysaccharides, it would be well to 

 summarize in a broad way their salient features as a group of 

 natural structural materials. 



In the construction of polysaccharides, it is clear that most 

 of the possible ways in which any two monomers can be linked 

 together have been employed, albeit to varying degrees. The 

 specific monomeric units themselves are not great in number, 

 although numerous enough to call for great skill and knowledge 

 in the development of analytical techniques. Thus, the great array 

 of polymers which are to be found in nature exist as a result of 

 biosynthetic versatility. The combinations of monomer types and 

 linkages, the degree of polymerization and the branching of the 

 polymer together must account for the variety of wall polysaccha- 

 rides which are now known. By virtue of its unusual size and 

 high degree of molecular order, cellulose seems to be set off from 

 all of the other wall substances as the primary frame substance. 

 That the framework should be embedded in or encrusted with 

 other polymers is not surprising, but the functional significance 

 of the diversity which we have observed among the non-cellulosic 

 polymers, and of their unequal, and sometimes unique distribution 

 among various species or their parts remains a matter of physio- 

 logical investigation. 



Lignin 



The cell wall polysaccharides are of interest as a specific part 

 of cellular structure. Of even greater importance historically is 

 the "Baustein" concept which has led in the past century to modern 

 concepts of polymer structure both in biological and purely chemical 

 pursuits. The polysaccharides together with the proteins and 

 nucleic acids have provided the basic picture of macromolecules 

 composed of repeating units, periodicity in organic molecules, 

 and the coexistence of structural diversity with uniformity of 

 overall pattern. 



