Torus-Bearing Pit Membranes in Cercocarpus

Abstract

Intervascular pit membranes of Cercocarpus possess torus thickenings. The thickenings, or pads, consist of lignified, secondary wall material. Torus pad deposition occurs late in cell ontogeny and is not associated with a microtubule plexus. Half-bordered pit pairs between tracheary elements and parenchyma cells often have a torus pad on the membrane surface facing the conducting cell. In contrast, a thick protective layer fills the pit cavity on the side of the parenchyma cell. Ontogeny of the torus thickenings in Cercocarpus represents a third mode of torus development in eudicots when compared to that occurring in Osmanthus/Daphne and Ulmus/Celtis.