DIRECT VISUALIZATION OF LIGNIFYING SECONDARY WALL THICKENINGS IN ZINNIA-ELEGANS CELLS IN CULTURE

The lignifying secondary wall thickenings of tracheary elements that w ere differentiating from Zinnia mesophyll cells in suspension culture were examined by a freeze-etch replica technique. Cellulose microfibri ls in primary and secondary wall thickenings differed in terms of both width and arrangement. The primary wall was observed as a randomly ar ranged network of microfibrils. By contrast to microfibrils in the sec ondary wall thickenings were highly organized, with many pores and spa ces between them. Numerous filamentous and granular cross-links were o bserved in both primary walls and secondary wall thickenings. As ligni fication proceeded, the cellulose microfibrils in secondary wall thick enings became obscure as a result of the deposition of large numbers o f spherical bodies around and between the microfibrils. This material had completely covered the fibrous matrix by the end of lignification. It might have been composed of the products of the dehydrogenation of monolignols. We also noted that the microfibrils appeared to be sligh tly irregular or wavy just after the start of lignification but were s traighter and appeared to be more rigid when lignification was complet e.