| # The Families of Angiosperms |

L. Watson and M.J. Dallwitz

### Posidoniaceae (Kunth) Lotsy

Habit and leaf form. Marine herbs. Perennial; rhizomatous. Hydrophytic; marine; rooted. Leaves submerged. Leaves medium-sized; alternate; distichous; flat, or terete; quite leathery (in some species), or ‘herbaceous’, or membranous; sessile; sheathing. Leaf sheaths not tubular; flattened, with free margins (the thin lateral margins narrowly infolded, to form flaps which overlap towards the sheath bases, with auricles adjacent to the ligule). Leaves simple. Lamina entire; linear; parallel-veined; cross-venulate (not reticulate in the conventional sense, but numerous transverse veins very conspicuous in at least some species), or without cross-venules (?); auriculate at the base. Leaves ligulate. Axillary scales present. Lamina margins entire, or serrate.

Leaf anatomy. Stomata absent. Hairs absent. The mesophyll without crystals. Foliar vessels absent.

Axial (stem, wood) anatomy. Young stems flattened. Secondary thickening absent. The axial xylem without vessels.

Root anatomy. Root xylem without vessels.

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

Inflorescence, floral, fruit and seed morphology. Flowers aggregated in ‘inflorescences’. The ultimate inflorescence units 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. Carpels reduced in number relative to the perianth. The pistil 1 celled. Gynoecium monomerous; of one carpel; superior. Carpel non-stylate (irregularly many-lobed at the tip); apically stigmatic; 1 ovuled. Placentation apical to marginal (the ovules attached laterally but towards the apex). Ovary sessile. Ovules pendulous; non-arillate; orthotropous (?).

Fruit somewhat fleshy (the pericarp spongy). The fruiting carpel tardily dehiscent (after commencement of germination, subsequent to elongation of the plumule into the fruit apex, and emergence of the root system: the fruit eventually opening longitudinally along the placenta, and sometimes with several additional, shorter splits from the base). Dispersal unit in material seen, variously the seed (or the seedling), or the fruit, or the inflorescence (or fragments of the latter). Dispersal in sea water. Fruit 1 seeded. Seeds generally stated to be non-endospermic (but ostensibly endospermic, with abundant food reserves in the much enlarged hypocotyl: see Belzuncea et al., 2005); small to medium sized (12–18 mm in P. angustifolia). Cotyledons 1. Embryo straight. Testa without phytomelan.

Seedling. Hypocotyl internode present (becoming very swollen, to occupy much of the seed). Seedling macropodous (food reserves mainly in the hypocotyl).

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

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

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

Species 3. Genera 1; only genus, Posidonia.

General remarks. This description assumes that the large food storage component occupying most of the seed (see photos) is hypocotylar rather than endospermic, cf. Belzuncea et al. 2005.

Illustrations. • Posidonia oceanica, P. australis: Nat. Pflanzenfam. II (1889). • Posidonia sp.: tidal deposits of fruits and vegetative material. • Posidonia sp.: tidal deposits of inflorescence and leaf material. • Posidonia sp.: opened fruits, showing seeds germinating prior to dehiscence.

We advise against extracting comparative information from the descriptions. This is much more easily achieved using the DELTA data files or the interactive key, which allows access to the character list, illustrations, full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, distributions of character states within any set of taxa, geographical distribution, genera included in each family, and classifications (Dahlgren; Dahlgren, Clifford, and Yeo; Cronquist; APG). See also Guidelines for using data taken from Web publications.

Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 1992 onwards. The families of flowering plants: descriptions, illustrations, identification, and information retrieval. Version: 8th June 2026. delta-intkey.com’.

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