Floral morphology and embryo sac development in Burretiodendron kydiifolium Y. C. Hsu et R. Zhuge (Tiliaceae) and their systematic significance

Floral morphology and development of megaspore and megagametophyte in Burretiodendron kydiifolium are described. The ovule is anatropous, crassinucellate and bitegmic. The archesporium is multicellular and usually two cells develop to megasporocytes. The tetrad is linear or very rarely T-shaped. The chalazal megaspore functions and develops into a Polygonum-type of embryo sac. The antipodals degenerate very early and the fertilizable embryo sac consists of only one egg cell, two synergids and two polar nuclei. In a fertilizable embryo sac the micropyle is formed by both outer and inner integuments, which are three to four and five cells thick, respectively. The epidermal cells of the nucellus divide to form a nucellar cap. A group of special cells with large nuclei and less staining are observed below the sporogeneous cells. Comparison of the embryological features of Burretiodendron s.s. with those of Tiliaceae indicates that this genus differs greatly from other members of Tiliaceae and no close relationship is established among Burretiodendron and other tiliaceous taxa. Most embryological features of Burretiodendron are shared by the genus Pterospermum of Sterculiaceae although they differ considerably in gross morphology. (C) 1998 The Linnean Society of London