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

• Premise of the study: The predominantly aquatic order Alismatales displays a highly variable flower groundplan associated with a diverse range of developmental patterns. We present the first detailed description of flower anatomy and development in Posidonia , the sole genus of the seagrass family Posidoniaceae. Existing accounts provide conflicting interpretations of floral and inflorescence 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 inflorescences are bracteate spikes, which are arranged into a botryoid or a panicle. The flowers 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 flower is supplied by a vascular bundle, whereas the flower‐subtending bracts are nonvascularized. • Conclusions: Our data support a racemose interpretation for the partial inflorescence of Posidonia and the presence of flower‐subtending bracts. In common with some other Alismatales, Posidonia has simultaneous development of the flower and its subtending bract and loss of the bract vascular supply accompanied by innervation of the flower 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.