Common name. Posidonia Family.
Habit and leaf form. Marine herbs. Perennial. Young stems flattened. Rhizomatous. Hydrophytic; marine; rooted. Leaves submerged; medium-sized; alternate; distichous; sessile; sheathing. Leaf sheaths with free margins. Leaves simple. Leaf blades entire; flat, or solid; linear; one-veined, or parallel-veined; without cross-venules. Leaves ligulate. Axillary scales present. Leaf blade margins entire, or serrate. Stem anatomy. Secondary thickening absent.
Reproductive type, pollination. Fertile flowers hermaphrodite. Unisexual flowers absent. Plants hermaphrodite. Pollinated by water.
Inflorescence and flower features. Flowers aggregated in ‘inflorescences’. The terminal inflorescence unit cymose, or racemose (hard to determine). Inflorescences scapiflorous; clusters of spikelike inflorescences, each with three to five flowers, each ‘spike’ terminated by a flower. Flowers minute. Perianth absent. Androecium 3. Androecial members all equal; free of one another; 1 -whorled. Androecium exclusively of fertile stamens. Stamens 3; with sessile anthers (the two thecae borne dorsally near the midvein at the base of a broad, shieldlike connective). Anthers dehiscing via longitudinal slits; extrorse; bilocular; tetrasporangiate (the thecae widely separated); appendaged (in that the connective has an apically prolonged midrib). Pollen shed as single grains. Pollen grains lacking exine, and dispersed in the sea as long filaments. Gynoecium seemingly 1 carpelled. The pistil 1 celled. Gynoecium monomerous; of one carpel; superior. Carpel non-stylate (irregularly many-lobed); apically stigmatic; 1 ovuled. Placentation apical. Ovules pendulous; orthotropous.
Fruit and seed features. Fruit somewhat fleshy (the pericarp spongy). The fruiting carpel dehiscent. Seeds non-endospermic. Cotyledons 1. Embryo straight.
Geography, cytology, number of species. World distribution: coastal Mediterranean and southern Australia. 3 species.