CELL WALL DYNAMICS 51 



while insoluble glucosan, galactan and araban decreased in the 

 hemicellulose fraction. These changes suggest that a dynamic 

 relationship exists between wall and cytoplasm during the ripening 

 process. 



One of the high points in cellular differentiation and maturation 

 is the process of lignification. The various biochemical theories 

 of lignification fall conveniently into three categories: (a) lignin 

 arises in the cell wall by direct transformation of other wall compo- 

 nents; (b) lignin arises from precursors which diffuse centripetally 

 from their point of origin in the cambium and become incorporated 

 into the walls of xylem and phloem; and (c) lignin arises from 

 cytoplasmic precursors formed in differentiating cell and is subse- 

 quently incorporated in the wall. 



The first hypothesis is of historic interest ; direct aromatization 

 of formed polysaccharide can be rejected without reservation. 

 Of the remaining hypotheses, the second is the more widespread 

 and the popular one at present, and is supported by a great deal 

 of experimental data. This data can be represented by experiments 

 showing that radioactive lignin arises in xylem from labeled pre- 

 cursors supplied to the cambium. The position of label is unchanged 

 from precursor to residues in the lignin polymer. Further, pre- 

 cursor type molecules such as coniferin and syringin do indeed 

 occur in the cambial zone in many species. On the other hand, 

 the pattern of lignification shows several features which are more 

 consistent with the relatively neglected hypothesis of cytoplasmic 

 (or intracellular) origin. By ultraviolet microphotometry it is 

 evident that lignin deposition begins at primary wall corners thence 

 spreading through the intercellular layer and inward. Further, 

 lignified cells or cell aggregates may be completely surrounded by 

 lignin-free tissues. Thus, morphological and chemical observations 

 may be adduced as evidence in support of either hypothesis. 

 It has been proposed, therefore, that a precursor may arise in 

 the cambium and diffuse therefrom but can undergo further neces- 

 sary transformations only in particular, suitable cells. This compro- 

 mise provides a biochemically and physiologically reasonable way 

 in which the two hypotheses can be merged to account for diffusion 

 gradients and localization patterns alike. 



