STRUCTURE AND ONTOGENY OF VERTEBRARIA FROM SILICIFIED PERMIAN SEDIMENTS IN EAST ANTARCTICA

Silicified peat of Permian age from Amery Group sediments near Beaver Lake, East Antarctica represents a swamp dominated by Glossopteris, wi th little other floristic diversity. Exceptional preservation of small Vertebraria axes has allowed derailed investigations into their struc ture and early ontogeny. Vertebraria axes have an exarch actinostele, and are clearly roots. The similarity in tracheid anatomy between Vert ebraria and abundantly co-occurring Araucarioxylon wood is confirmed, further supporting the generally accepted link with the Glossopteris p lant. Two intergrading types of Vertebraria are described: ''polyarch' ' Vertebraria and ''solid cylinder'' Vertebraria. These two forms may be conspecific roots with different functions, or they may be differen t species. The pattern and initiation of primary and secondary tissues are described for the first time. Primary roots consist of a single-l ayered epidermis surrounding a differentiated outer and inner cortex, and a stele with two to nine protoxylem points delimited by an endoder mis. Secondary growth results from a discontinuous vascular cambium th at produces radial bands of secondary xylem in the regions alternate t o the protoxylem points. Primary stelar parenchyma cells adjacent to t he protoxylem points disintegrate under stresses caused by root expans ion. These areas are considerably expanded by continued secondary xyle m production, forming schizogenous lacunae. These air spaces were pres ent in the living root, and were not filled with aerenchymatous tissue .