Flower structure and development in Vietnamese Sciaphila (Triuridaceae: Pandanales): refined knowledge of the morphological misfit family and implications for taxonomy

Abstract

The monocot family Triuridaceae is a morphological misfit with respect to several traits of floral morphology, including the uniformly apocarpous polymerous gynoecium and the famous inside-out flowers of Lacandonia. Although Triuridaceae are crucially important for understanding the floral evolution of Pandanales and angiosperms in general, significant knowledge gaps exist which hamper adequate morphological analysis of flowers in this family. The scarcity of morphological data is also reflected in numerous taxonomic inconsistencies. Here we provide a comprehensive study of reproductive organs of four species of Sciaphila occurring in Vietnam (S. arfakiana, S. densiflora, S. nana and S. stellata) including the first investigation of early floral development and gynoecium phyllotaxis. Our observations are mainly based on SEM images. We confirm the perianth (studied in male flowers) to be two-whorled and report a rare sequence of initiation of perianth parts: the outer tepals show a late congenital fusion, as their free lobes appear before the common perianth tube, whereas the inner tepals show an early congenital fusion, with their free lobes initiating on the tube rim. We interpret the stamen appendages as basal adaxial outgrowths of the stamen filaments. We discuss the number of thecae and locules in anthers of Sciaphila, and conclude that 3- and 4-, but not 2-locular anthers are characteristic of this genus. We describe the carpels as consisting of both ascidiate and plicate zones, the former being extremely short and completely obscured by anthesis. The single ovule is attached in the cross-zone. The style is non-plicate. We analyze gynoecium phyllotaxis by estimating its contact parastichies, and by investigating the number and arrangement of the outermost carpels. The carpel arrangement in a given gynoecium is a result of the balance between whorled and irregular (but not spiral) phyllotaxis. We recognize the following figures of gynoecium merism: 6, 9, 10, 10.5, 11 and 12, with the prevalence of those divisible by three. We discuss our results in the light of general diversity of floral structure of monocots. Our data allow to clarify several issues in taxonomy of Asian Sciaphila and indicate directions of further studies. We report a significant range extension for S. densiflora, which is newly recorded for the flora of Vietnam. We describe for the first time staminodes in female flowers of this species. We reveal two distinct morphs of S. nana in Vietnam. We highlight the need of a thorough revision of S. secundiflora species complex in order to verify the species boundaries and, in particular, to test the identity of the Vietnamese S. stellata.