Abstract
The lignifying secondary wall thickenings of tracheary elements that were differentiating from Zinnia mesophyll cells in suspension culture were examined by a freeze-etch replica technique. Cellulose microfibrils in primary and secondary wall thickenings differed in terms of both width and arrangement. The primary wall was observed as a randomly arranged network of microfibrils. By contrast to microfibrils in the secondary wall thickenings were highly organized, with many pores and spaces between them. Numerous filamentous and granular cross-links were observed in both primary walls and secondary wall thickenings. As lignifica-tion proceeded, the cellulose microfibrils in secondary wall thickenings became obscure as a result of the deposition of large numbers of 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 slightly irregular or wavy just after the start of lignification but were straighter and appeared to be more rigid when lignification was complete.
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