Outstanding questions on floral structure in the small family Stemonaceae (Pandanales) include whether the gynoecium is monocarpellary, whether they possess floral nectaries, and how dimery and pentamery evolved in the family. To help resolve these issues we present new data on comparative floral anatomy in three genera of Stemonaceae in the context of a revised phylogenetic analysis using a "total evidence" approach with combined data from three genes (plastid rbcL, atpB and nuclear 18S rDNA) plus morphology. The monocarpellary condition is a synapomorphy for a clade comprising Croomia, Stemona and Stichoneuron, and the unilocular tricarpellary condition is an autapomorphy for Pentastemona. Nectaries are absent, but the densely papillate tepals may function as osmophores, especially in Croomia and Stichnoneuron. The hypothesis that the pentamerous condition in Pentastemona evolved as a result of suppression of an outer tepal and stamen, and thus represents an intermediate stage between trimery and dimery, is not contradicted by its phylogenetic placement as sister to other Stemonaceae, which are dimerous. However, since pentamery also occurs sporadically in the closely related family Triuridaceae, and dimery (and/or tetramery) occur in other Pandanales, this indicates that merism is relatively labile within the order, and that dimery and pentamery have both evolved separately and iteratively from a trimerous condition.
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