ULTRASTRUCTURAL SPECIALIZATIONS OF THE CELL-WALL SLEEVE AROUND PLASMODESMATA

Specialized structures were observed in the sleeve of wall surrounding plasmodesmata in the rhizome tips of Nephrolepis exaltata and root ti ps of Spirodela oligorrhiza, Azolla pinnata, and Hordeum vulgare. Mate rial was prepared either by fixation in glutaraldehyde/paraformaldehyd e/tannic acid followed by cell wall digestion, or by rapid freezing an d freeze substitution, prior to resin embedding. Two structures were i dentified: rings of electron-opaque material encircling the neck regio n of plasmodesmata and electron-opaque helices spiraling around the le ngth of plasmodesmata. In some instances, particle-like subunits were observed in the electron-opaque rings. The possible role of these stru ctures as external sphincters involved in the control of permeability of plasmodesmata is discussed.