Flowers and inflorescences of the seagrass Posidonia (Posidoniaceae, Alismatales)

Premise of the study: The predominantly aquatic order Alismatales displays a highly variable fl ower groundplan associated with a diverse range of developmental patterns. We present the fi rst detailed description of fl ower anatomy and development in Posidonia , the sole genus of the seagrass family Posidoniaceae. Existing accounts provide confl icting interpretations of fl oral and infl orescence structure, so this investigation is important in clarifying morphological evolution within this early-divergent monocot order. • Methods: We investigated two species of Posidonia using light microscopy and scanning electron microscopy. Our observations are interpreted in the framework of a recent molecular phylogeny. • Key results: Partial infl orescences are bracteate spikes, which are arranged into a botryoid or a panicle. The fl owers are perianthless. The gynoecium is monomerous with the ventral carpel side oriented abaxially. The carpel contains a single pendent bitegmic ovule with a nucellus and long chalaza, both extending along the carpel wall. The ovule develops an integumentary outgrowth. Each fl ower is supplied by a vascular bundle, whereas the fl ower-subtending bracts are nonvascularized. • Conclusions: Our data support a racemose interpretation for the partial infl orescence of Posidonia and the presence of fl owersubtending bracts. In common with some other Alismatales, Posidonia has simultaneous development of the fl ower and its subtending bract and loss of the bract vascular supply accompanied by innervation of the fl ower by a single vascular strand. The unusual carpel orientation could be an evolutionary reduction of a formerly tricarpellate gynoecium. The ovule of Posidonia is campylotropous and unusual within Alismatales in possessing an integumentary outgrowth.