INTERCELLULAR SUBSTANCES AND WALLS 73 



Second, among the angiosperms, lignification is a constant 

 feature, save in the case of the aquatic Elodea which produces 

 appreciable lignin only when it is supplied with precursor. This 

 may be considered a secondary characteristic resulting from 

 a genetic loss in the antecedents of such plants. 



Such a viewpoint is supported elsewhere in the Pteridophyta: 

 These plants characteristically have well lignified vascular tissues, 

 however exceptions include the water ferns Salvinia and Cerato- 

 pteris. The former, of course, occurs as a surface aquatic. Al- 

 though Ceratopteris extends its axis well into the air, its succulent 

 tissues are supported by hydrostatic forces. 



Third, chemical diversity among the lignins obviously depends 

 upon the chemical constitution (methoxyl content, for example) 

 of the precursors available. In addition, however, tissue factors 

 may modify the course of oxidative polymerization of a single 

 precursor in a way that allows different products to be formed. 

 The matrix phenomena noted above and discussed elsewhere, 

 are offered as a possible example of tissue factors. 



The presumption that precursor supply, precursor structure 

 and/or tissue factors operate among different tissues as well as 

 among different species is supported by instances of lignins 

 localized to special tissues such as peristome teeth, and by observa- 

 tions on the variations (increases) in the methoxyl content of 

 lignins with age. 



Current concepts in aromatic biochemistry suggest that bio- 

 genetic relationships exist among the several major types of phe- 

 nolic carbon skeletons: 



C 6 -C 3 -C 6 flavonoids 



/ 



/ 



C 6 -C 3 f--^(C 6 -C 3 ) 2 lignanes 



, , , pre-aromatiC/ 



carbohydrate -► . 



t 



C 2 \ (C 6 -C 3 ) n lignins 

 * C 6 — Ci ► tannins, depsides 



\ 



In this scheme, substances at the carbohydrate level must 

 undergo cyclization and desaturation, forming intermediates such 

 as prephenic acid, and in turn phenols with one— or three— carbon 

 side chains. These, in turn, may lead to several terminal products 



