PIT MEMBRANE-STRUCTURE AND DEVELOPMENT IN GINKGO-BILOBA

Pit membrane ontogeny in radial walls of Ginkgo biloba tracheids was f ollowed using transmission electron microscopy. Torus initiation occur s prior to initiation of the pit border and without benefit of a micro tubule plexus. The developing pit membrane is associated with masses o f wall material located within plasmalemma invaginations. Wall materia l is added in such a manner as to form a torus with highly irregular s urfaces. Margo and torus are traversed by plasmodesmata, whose channel s are connected by extensive median cavities. Matrix material is remov ed from both margo and torus shortly after hydrolysis of the adjacent cytoplasms. Matrix removal begins at the pit membrane surface and is n ot preferentially associated with the plasmodesmata. Tori in aspirated pit membranes have compacted fibrils, and their fibrillar compaction might reduce permeability to air embolisms.