Monocot Pseudanthia revisited: Floral structure of the mycoheterotrophic family Triuridaceae

The floral units of Triuridaceae, commonly regarded as flowers, may equally plausibly be interpreted as pseudanthia, i. e., modified and highly reduced partial inflorescences. This hypothesis is new for Triuridaceae, but earlier authors have noted strong similarities between the inflorescences of Cyclanthaceae and Pandanaceae ( Pandanales) and the putatively pseudanthial floral units of some Alismatales ( e. g., Potamogeton). The morphology of the floral units of Triuridaceae, particularly the female and hermaphrodite structures, closely resembles a smaller version of a Pandanus pistillate inflorescence. In some species of Pandanus, the female flower consists of a single uniovulate carpel with a single stigma, as in Triuridaceae. This striking morphological similarity is remarkable in the context of recent evidence from a single 18S nuclear rDNA sequence for Sciaphila that placed it as sister to Pandanaceae within Pandanales. However, because pseudanthia have been reported in both Alismatales and Pandanales, a pseudanthial interpretation does not in itself support or refute a close relationship with either of these orders, and a more detailed analysis of the phylogenetic placement of Triuridaceae is desirable. A pseudanthial interpretation provides a possible explanation for the inside- out flowers of Lacandonia schismatica, in which the three stamens are distal to the numerous carpels; these may not be flowers but highly reduced partial inflorescences. Pseudanthia have apparently evolved several times in monocots and other basal angiosperms, and it may be useful to assess both inflorescences and flowers more flexibly in these groups in which developmental pathways are less canalized than in more derived angiosperm lineages.