In-vitro tracheary element formation: structural studies and the effect of tri-iodobenzoic acid.

The process of tracheary element formation by cultured isolated mesophyll cells of Zinnia elegans has been investigated by the use of scanning and transmission electron microscopy. Freshly isolated cells are typified by a thin primary wall, starch-filled chloroplasts and a nucleus with a small nucleolus and peripherally located chromatin. During culture the nucleolus expands and becomes vacuolate, and the chromatin in the nucleus is dispersed. Wall thickening is accompanied by a localisation of microtubules over the thickenings and a slight increase in dictyosome activity. Wall hydrolysis appears to proceed by a previously undescribed mechanism, and gives rise to a granularparticulate appearance in the hydrolysed wall. Tri-iodobenzoic acid inhibits the process of differentiation and induces a polar pattern of growth in the cultured cells.