INTERCELLULAR SUBSTANCES AND WALLS 75 



sphere. Accordingly, the littoral ancestors of the upright land 

 plant may have been "primed" for lignification before free oxygen 

 became a significant part of their environment. 



Before ending our phylogenetic considerations, the status of 

 lignins in fossils themselves deserves brief review. The lignins of 

 angiosperms are distinguished from others by their high methoxyl 

 content. Analytically, the lignins of gymnosperms and other "lower" 

 forms yield, on nitrobenzene oxidation, /?-hydroxybenzaldehyde and 

 vanillin (4-hydroxy-3-methoxy-benzaldehyde). Angiosperm lignins 

 yield, in addition, highly methylated syringaldehyde. In lycopsids, 

 sphenopsids, and older members of the Pteropsida, vanillin-yield- 

 ing lignins may be traced at least to the Devonian. Vanillin-yielding 

 moss remains seem to have originated in the Upper Carbonife- 

 rous. 



The syringyl group is predominantly associated with the 

 Cretaceous angiosperms but, interestingly, is also found in the 

 gnetales, dating from the Jurassic. 



Betulinium wood (Pliocene-Miocene) gives intense lignin color 

 reactions and assays about 35 per cent lignin with a methoxyl 

 content of 11-4 per cent Podocarpoxylon lignite from the upper 

 cretaceous contains 69 percent lignin containing about 7 per cent 

 OCH 3 . 



Among Carboniferous forms, specimens of Lepidodendron and 

 Calamites contain small amounts of dioxane-soluble materials 

 giving the characteristic lignin color reactions. Lepidodendron 

 extracts also have a typical (non-angiosperm) lignin ultraviolet 

 spectrum. 



Such evidence shows clearly that the lignins as chemical enti- 

 ties have considerable antiquity, dating back to the period associa- 

 ted with the origin of the land plant. 



Although lignins are comparatively resistant to the conditions 

 associated with simple burial of vegetable matter, they are neverthe- 

 less modified in fossils. Decline in methoxyl content is one such 

 change, but more extensive modification to the aromatic nucleus 

 also occurs. 



If, for example, we examine related fossil and modern forms 

 for extracts absorbing in the aromatic region of the ultraviolet, 



