Posidonia K.D.Koenig

Rhizome cylindric to laterally compressed; internodes variable in length. Roots 1 or 2; root hairs rare. Leaf sheath open, persistent after blade is shed. Leaf blade flat or biconvex to terete, entire; apex rounded to truncate, or spathulate; longitudinal veins 3–21. Inflorescence subtended by 2 bracts on a long leafless peduncle; each spike with several flowers, terminating in an acuminate spur. Flowers sessile; anther connective linear-lanceolate to oblong-lanceolate; ovary oblong-ellipsoid; stigma variable in shape. Fruit variable in shape; pericarp fleshy, splitting irregularly into 3 or 4 valves from base releasing 1 seed. Seed oblong, compressed, convex on one side with the other side grooved; basal end obtuse, bearing root primordia; apical end crowned with a well-developed plumule. Testa extremely thin, transparent, with an extending ventral wing.