CHAPTER 3 



COMPARATIVE CHEMISTRY OF INTER- 

 CELLULAR SUBSTANCES AND WALLS 



I. Phylogenetic Aspects of Lignins and Lignin 



Synthesis 



Among the secondary substances of the plant kingdom, lignin 

 is one of the few which have been assigned a definite role in evolu- 

 tion. The mechanical integrity of the higher plant cell has been 

 associated with its lignification. The deposition of lignin upon 

 and within the polysaccharide framework may be expected to 

 increase considerably its resistance to compression forces, allowing 

 development of more massive structures. Thus, the capacity for 

 lignification has been implicated in the rise of the upright vascular 

 land plant. 



A close relation between the compression force generated by 

 the mass of the plant and its lignin content has been established, 

 even for a small woody stem (Euphorbia, Fig. 1). In contrast, 

 a hollow-stemmed Equisetum shows no correlation between silica 

 or combined silica-lignin content along an axis of comparable 

 length. In the hollow structure, the pressure exerted by the plant 

 upon itself is approximately 700 dyne-cm- l , one-tenth of the figure 

 obtained for the solid woody stem. 



Such considerations, emphasizing as they do the evolutionary 

 significance of lignins, call for further knowledge of the phylo- 

 genetic features. These features may be studied in two ways: 



(a) by chemical measurements on fossil plant materials; and 



(b) by chemical and biochemical measurements upon living members 

 of representative taxa. 



We shall consider first the second of these approaches, in which 

 experimental biosynthesis of lignin plays an important part. 



