CELL WALL DYNAMICS 45 



In contrast, there has been a remarkable consistency in the 

 very basic notions of lignin synthesis. The outstanding degrada- 

 tive studies by pioneers such as Klason and Freudenberg, and 

 others, led early to the "Baustein' , concept of lignin structure, and 

 thereby provided some notions about lignin precursors. Again, 

 chemical syntheses of simple models with lignin-like degradation 

 patterns came comparatively early in the hands of Klason, Erdtman, 

 Freudenberg, and others. In principle, the biochemistry of lignin 

 synthesis as it is now understood is in large measure a confirmation 

 of the organic chemical picture established before. 



The study of lignin biosynthesis has been carried out in two 

 types of systems. First, of course, are those procedures in which 

 living tissue is provided, with the presumed lignin precursor and 

 the appearance of polymer determined. Such "feeding" experiments 

 are divisible, in turn, into long-term studies with labelled precur- 

 sors and short-term studies with high concentrations of unlabelled 

 precursors. 



Second is the in vitro model which again has followed two 

 subordinate lines, the purely enzymological and the enzyme- 

 polysaccharide (matrix) system. 



This classification of approaches omits histochemical procedures 

 which bear upon the more physiological study of lignification as 

 a differentiation phenomenon. But these approaches provide informa- 

 tion about precursors, their proximity to lignin in the biogenetic 

 scheme, the enzymes which are involved, and something of the 

 conditions under which experimental lignin formation takes 

 place. 



The list of species used in experimental lignin synthesis has 

 grown considerably in recent years. Woody species such as spruce, 

 eucalyptus, bamboo, willow and maple and herbaceous plants 

 including bean, pea, potato, elodea, celery, buckwheat, clover, 

 wheat, and timothy. 



Among the substances which have been established as precur- 

 sors in various experiments are the amino acids phenylalanine, 

 tyrosine, and dihydroxyphenylalanine (DOPA); phenols such as 

 eugenol, woeugenol, coniferyl alcohol, and sinapyl alcohol; the 

 aldehydes, vanillin, /?-hydroxybenzaldehyde, coniferaldehyde ; caf- 



