COMPARATIVE ANATOMY AND SYSTEMATICS OF WOODY SAXIFRAGACEAE: TETRACARPAEA

Vegetative and reproductive anatomy and morphology are described for the first time for Tetracarpaea tasmannica Hook., a small shrub endemic to Tasmania. Tetracarpaea, a monotypic genus, has many characteristics of other woody Saxifragaceae, such as wood with solitary pores, scalariform perforation plates, sparse axial xylem parenchyma, tracheids, spiral thickenings in tracheary elements, and perforated ray cells. The tracheary elements of Tetracarpaea are much smaller than those characteristic of the Escallonioideae, a feature probably related to its montane forest habitat. Other features of Tetracarpaea inconsistent with most Escallonioideae include dark-staining deposits in the ray cells; a unilacunar, one-trace nodal pattern; lack of unicellular foliar trichomes; simple craspedodromous venation; areole development that is lacking or incomplete; straight and tapered veinlets; abaxial fibers associated with the foliar vascular bundles; and lack of bundle sheaths. The genus is further characterized by complete, hypogynous, tetramerous flowers. The essentially apocarpous gynoecium has multiovulate carpels, each supplied by three veins that reach the stigma. Ovules are anatropous, bitegmic, and crassinucellate. Lateral sepal bundles are derived either from the sepal midrib or from the petal-plane bundles; stamens are supplied by independent traces or by bundles originating from compound traces in both sepal- and petal-planes. The follicular fruits possess a sclerenchymatous endocarp and contain winged seeds that have a membranous testa, a ridged surface, and a cellular endosperm. Reproductive morphological and anatomical features are more consistent with features of the Saxifragoideae than with the Escallonioideae or the Cunoniaceae, although the essentially apocarpous gynoecium with multiovulate carpels is not found in these groups. Vegetative and reproductive characteristics indicate that Tetracarpaea is more closely related to Saxifragaceae than to Cunoniaceae. It is possible this isolated genus should have separate familial status.