Microsporogenesis and microgametogenesis of Excentrodendron hsienmu (Malvaceae s.l.) and their systematic implications

Flowers, microsporogenesis and microgametogenesis of Excentrodendron hsienmu in opening-functional flowers and non-opening flowers were studied to investigate the evolutionary relationships of Excentrodendron. E. hsienmu is a dioecious species that blossoms every 3-4 years, although large numbers of flower buds develop every year. The anther is tetrasporangiate, the tapetum is of the secretory type, the microspore tetrads are mainly tetrahedral, and the pollen grains are two-celled when shed. Four to six microsporocytes are seen on the transverse section of the anthers, and cytokinesis is simultaneous. The development of the anther wall conforms to the basic type and the anther wall is five or six cells thick, with a fibrous endothecium. The difference between the opening-functional and the non-opening flowers is mainly in the thickness of the anther wall. Early megasporogenesis in staminate flowers up to megaspore mother cell or megaspore tetrads has been observed. Excentrodendron shares with Dombeyeae only plesiomorphic features, but differs in anther wall development type and thickness. Most features of Excentrodendron are shared with Pterospermum, including such synapomorphic features as basic type of anther wall development, five- to six-cell-thick anther wall, biseriate tapetum at some places, and degeneration of microsporocytes, suggesting placement near Pterospermum. (c) 2006 The Linnean Society of London