CONSTITUTION AND ARCHITECTURE IN THE CELL WALL 25 



Relative to the polysaccharides and other biopolymers of 

 importance, the lignins stand out as the unique aromatic polymers 

 of the higher plant. They are also distinguished in their mode 

 of synthesis, as will be discussed later, and in the manner in which 

 monomers are joined together. 



There is good reason to believe that our current picture of the 

 cell wall polysaccharides as physical entities is moderately accurate 

 at least. Careful isolation of cellulose and other carbohydrates and 

 controlled solvent treatment and hydrolysis have sufficed to provide 

 reasonably complete analytical data about these polymers. In 

 contrast, the methods of historical importance when applied to 

 the lignins are far more likely to modify the molecule and its 

 monomers extensively. 



The successful application of hydrolytic procedures to the 

 polysaccharides has no true parallel among the lignins inasmuch 

 as the linkage of monomers involves carbon-to-carbon bonds, 

 rather than readily cleaved acetal or glycosidic oxygen bridges. 

 Further, the phenolic and benzylic character of the lignin monomers 

 renders them susceptible to undesirable oxidations during isolation 

 and subsequent handling. 



The lignins of some tissues dissolve to a limited extent in alcohols 

 and other neutral organic solvents, but frequently form colloidal 

 sols rather than true solutions. Dioxane is exceptional among 

 organic liquids for its ability to dissolve the lignins; however, 

 there is some question as to its inertness as a solvent. With dioxane 

 as a possible exception, lignin extractants are commonly reaction 

 solvents. Lignins are generally extractable with sulfites, bisulfites, 

 alkalis, and acidic alcohols (for example, by alcoholysis), but are 

 modified chemically by such processing. Lignin is resistant to water, 

 dilute acids and cold concentrated acids. 



The degradation accompanying isolation of soluble lignins has 

 prevented extensive development of their detailed physical picture. 

 Some concept at least of the molecule can be obtained from the 

 study of these lignin derivatives in solution. 



By isothermal distillation, lignins isolated from spruce and 

 other woods yield molecular weights of 800-1000. From osmotic 

 pressure measurements, molecular weights as high as 3800-4500 



