Family Posidoniaceae

Posidoniaceae (Kunth) Lotsy

Habit and leaf form. Marine herbs. Perennial; rhizomatous. Hydrophytic; marine; rooted. Leaves submerged. Leaves medium-sized; alternate; distichous; flat, or terete; sessile; sheathing. Leaf sheaths with free margins. Leaves simple. Lamina entire; linear; one-veined, or parallel-veined; without cross-venules. Leaves ligulate. Axillary scales present. Lamina margins entire, or serrate.

Leaf anatomy. Stomata absent. Hairs absent.

The mesophyll without calcium oxalate crystals. Vessels absent.

Stem anatomy. Young stems flattened. Secondary thickening absent. Xylem without vessels.

Root anatomy. Root xylem without vessels.

Reproductive type, pollination. Fertile flowers hermaphrodite. Plants hermaphrodite. Pollination by water.

Inflorescence, floral, fruit and seed morphology. Flowers aggregated in ‘inflorescences’. The ultimate 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. Pollen grains nonaperturate.

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 somewhat fleshy (the pericarp spongy). The fruiting carpel dehiscent. Seeds non-endospermic. Cotyledons 1. Embryo straight. Testa without phytomelan.

Physiology, biochemistry. Alkaloids absent. Proanthocyanidins present. Saponins/sapogenins absent.

Geography, cytology. Temperate (warm), or sub-tropical. Coastal Mediterranean and southern Australia.

Taxonomy. Subclass Monocotyledonae. Dahlgren et al. Superorder Alismatiflorae; Zosterales. APG 3 core angiosperms; Superorder Lilianae; non-commelinid Monocot; Order Alismatales.

Species 3. Genera 1; only genus, Posidonia.