Morphology of Maundia supports its isolated phylogenetic position in the early-divergent monocot order Alismatales

According to recent molecular phylogenetic data, the rare Australian endemic Maundia triglochinoides does not form a clade with taxa traditionally classified as members of Juncaginaceae. Therefore, views on the morphological evolution and taxonomy of Alismatales require re‐assessment. As the morphology of Maundia is poorly known and some key features have been controversially described in the literature, the flowers, fruits, inflorescence axes and peduncles were studied using light and scanning electron microscopy. Inflorescences are bractless spikes with flowers arranged in trimerous whorls. Except in the inflorescence tip (where the flower groundplan is variable), flowers possess two tepals in transversal‐abaxial positions, six stamens in two trimerous whorls and four carpels in median and transversal positions. Fruits are indehiscent. The shared possession of orthotropous ovules supports the molecular phylogenetic placement of Maundia as sister to a large clade including Potamogetonaceae and related families. Maundia and Aponogeton spp. share the same highly unusual floral groundplan, a homoplastic similarity that can be explained by spatial constraints in developing inflorescences. The nucellar coenocyte of Maundia appears to be unique among monocots. As Maundia exhibits a mosaic of features characteristic of other families of tepaloid core Alismatales, its segregation as a separate family is plausible. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2013, 173, 12–45.