Posidonia K.D.Koenig

Scientific Description

Family Posidoniaceae.

Habit and leaf form. Marine herbs. Perennial. Young stems flattened. Rhizomatous (rhizomes 1 or 2 per node, initially herbaceous, usually becoming lignified and covered by scales and leaf sheaths, irregularly branched). 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 (and strap-like); one-veined, or parallel-veined; without cross-venules. Leaves ligulate (with auricles present). 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; of 2–4 spikes arranged in a raceme, each spike terminated by a narrowing vegetative apex. Flowers bracteate (bracts leaf-like, but progressively reduced towards apex of axis); minute to small. Perianth absent (apparently), or vestigial (scales). Androecium 3(–4). Androecial members all equal; free of one another; 1 -whorled. Androecium exclusively of fertile stamens. Stamens 3(–4); 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 (filamentous). 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 (stigma sessile, oblique and irregularly lobed); apically stigmatic; 1 ovuled. Placentation apical. Ovules pendulous; orthotropous.

Fruit and seed features. Fruit somewhat fleshy (the pericarp spongy). The fruiting carpel dehiscent (eventually opening by longitudinal slits to release the single seed). Seeds non-endospermic. Cotyledons 1. Embryo straight.

Geography, cytology, number of species. Native of Australia. Not endemic to Australia. Australian states and territories: Western Australia, South Australia, New South Wales, Victoria, Australian Capital Territory, and Tasmania. Eremaean Botanical Province and South-West Botanical Province.