Electronic Flora of South Australia Genus Fact Sheet

Genus POSIDONIA Konig 1805: 95, nom. cons. "Tapeweed".

Phylum Magnoliophyta – Subphylum Seagrasses – Class Liliopsida – Subclass Alismatidae – Order Potamogetonales – Family Posidoniaceae

Perennials, forming extensive marine meadows. Rhizomes herbaceous at first, usually becoming lignified, frequently compressed laterally, bearing a scale at each node, with age becoming covered with senescent scales and leaf sheaths. Roots 1 or 2 at each node, arising in 2 alternate series, simple or branched, root hairs infrequent. Prophylls undifferentiated, conspicuous in some species. Leaves alternate; sheath open, more or less amplexicaul, ligulate, auriculate, persistent after the fall of the blade, the line of abscission frequently conspicuous; blade linear and flat, or biconvex to terete, longitudinal parallel veins 5–21, united near the apex, connected by numerous transverse veins (which are frequently conspicuous due to the accumulation of tannin on either side); apex rounded to truncate, or spathulate. Tannin cells present in all parts but especially in the epidermis, appearing as dark dots or stripes, increasing with senescence of the plant. Squamules numerous, palisade-like. Inflorescence borne on a flattened, leafless peduncle, subtended by 2 unequal bracts; each spike-like unit of 2–6 sessile flowers terminating in an acuminate spur. Flowers bisexual (or male only); perianth absent; stamens usually 3 (or 4), sessile, 2 bilocular anthers longitudinally dehiscent, caducous, separated by a broad, apically elongate connective which persists at the base of the fruit; carpel one, 1-ovuled, stigma lobes 1–3, fleshy, verrucose, sometimes with spur-like processes, persistent on the developing fruit. Fruit a drupe with a spongy pericarp, free floating for a time, eventually dehiscent. Seed with a ventral wing, remaining attached to the young plant for 1–2 years after germination.

Type species: P. oceanica (Linnaeus) Delile.

Taxonomic notes: At least 8 species in temperate waters, all endemic to Australia except P. oceanica, which is restricted to the Mediterranean region.

Encrusting coralline algae frequently cover the older leaves of Posidonia spp.. Many other algae are epiphytic either on the encrusting corallines or directly on the Posidonia leaves.

"Posidonia balls" or "marine balls", 7–10 cm in diameter, composed principally of drift material of P. oceanica, have been recorded from Mediterranean beaches since ancient times. Similar balls (Pl. 16 figs 2,3), common on southern Australian shores near Posidonia marine meadows, are produced from detrital remains, particularly fibres, of P. australis. The combs of the grappling apparatus of Amphibolis spp., as well as fragments of other seagrasses, algae and shells, are often firmly enmeshed in the balls. Cannon (1979) ingeniously demonstrated the formation of "Posidonia balls" using the oscillating action of a domestic washing machine to simulate the action of waves and water eddies.

* aft. = affinis i.e. a species having an affinity with, or close to another species.


CANNON, J.F.M. (1979). An experimental investigation of Posidonia balls. Aquat. Bot. 6, 407–410.

KONIG, C. (1805). Addition to M. Carrolini's treatise on Zostera oceanica L. In Ann. Bot. (Konig & Sims, London) 2, 91–99.

The Marine Benthic Flora of Southern Australia Part I complete list of references.

Author: H.B.S. Womersley

Publication: Womersley, H.B.S. (31 May, 1984)
The Marine Benthic Flora of Southern Australia
Part I
©Board of the Botanic Gardens and State Herbarium, Government of South Australia


1. Leaf blade thick and tough, biconvex to round in cross section

2. (P. ostenfeldii group)

1. Leaf blade flat, or somewhat concavo-convex, with upper and lower faces parallel

3. (P. australis group)

2. Leaf blade terete, round in cross section when fresh, 1–1.5 mm in diameter near the base, becoming somewhat flattened then tapering higher up, and terminating in a spathulate apex (remaining terete only near the base when dry); leaf blade with usually 5 longitudinal vascular bundles

P ostenfeldii

2. Leaf blade biconvex (in cross section) throughout its length, not terete in any part, (1.5–) 2–5 (–7) mm broad when fresh; leaf blade with 5–9 (–11) longitudinal vascular bundles.

P. aff.* ostenfeldii

3. Leaf sheaths brown to dark reddish-brown, remaining entire and not disintegrating into hairy fibres, straw-like in texture when dry; leaf blade epidermal cells in surface view with sinuose margins

P. sinuosa

3. Leaf sheaths pale-yellowish to grey, becoming shaggy and fibrous, disintegrating into a mass of pale hair-like fibres; leaf blade epidermal cells in surface view with straight margins


4. Leaf blade (6–) 10–15 mm broad, epidermal cells in surface view isodiametric, L/B usually about 1

P australis

4. Leaf blade 4–6 mm broad, epidermal cells in surface view elongate, L/B 3–6

P. angustifolia

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