64 THE PLANT CELL WALL 



models, the vascular strands of celery (Apium graveolens) have 

 been employed as efficient lignifying tissues. When these strands are 

 pretreated with the enzyme cellulase prior to use, the yield of 

 lignin under standard conditions was reduced by one-third. When 

 the strands are treated with the enzyme pectinase their yield of 

 lignin was lowered by more than two-thirds. The pectic acid model 

 also agrees with other histochemical facts. Thus, the middle lamella 

 and parts of the primary wall which are rich in pectic substances 

 lose their affinity for the specific stain ruthenium red as lignifica- 

 tion proceeds. Further, during differentiation of wood fibers, the 

 same ruthenium red-staining regions are the first to show the presence 

 of ultraviolet absorbing substances and lignin color reactions. 



Having established that the sequence of reactions in the ligni- 

 fication process involves the preparatory action of peroxidase 

 followed by a polysaccharide-directed polymerization, we may 

 now inquire into the nature of this polysaccharide-dependent 

 cell wall reaction. Chitin which formed only minute quantities of 

 polymer in its native state, was markedly increased in activity 

 after removal of acetyl groups. Similarly, partial acetylation of 

 cellulose reduces its activity substantially; when only one-fourth 

 of the cellulosic hydroxyl groups are thus blocked, more than 

 half of the lignin-forming capacity is removed. Thus, acetylation 

 introduces a blocking group which abolishes lignifying capacity 

 out of proportion to the percentage of functional groups thus 

 obscured. Clearly, an uninterrupted polysaccharide surface is of 

 importance in the reaction. 



The highly active methyl celluloses when mixed with eugenol 

 and some other phenols form gel-like precipitates from which the 

 phenol may be recovered by simple solvent treatment. Thus, complex 

 formation occurs between representative reactants. 



These findings, together with the established adsorbent prop- 

 erties of polysaccharides and the known interactions of sugar 

 and phenolic hydroxyl groups all point to the importance of hydro- 

 gen bonding in the precursor- or monomer-polysaccharide inter- 

 action. 



The experimental behavior of other polymeric substances such 

 as asbestos minerals and Dowex-50 provide further evidence for 



