Electronic Flora of South Australia Family Fact Sheet
Phylum Magnoliophyta – Subphylum Seagrasses – Class Liliopsida – Subclass Alismatidae – Order Potamogetonales
Dioecious; marine perennial herbs with creeping, monopodially or sympodially branched rhizomes. Leaves distichous, with distinct blade and sheathing base; leaf sheath ligulate, auriculate; leaf blade flat and linear or terete and subulate. Squamules present in each leaf axil. Tannin cells numerous. Flowers solitary or in cymose inflorescences. Perianth absent or rarely of 1-few hyaline bracts. Male flowers subsessile or pedicellate, 2 anthers, longitudinally dehiscent. Pollen grains filiform and tightly coiled within the anther; pollination hydrophilous. Female flowers (1–) 2 carpels, free, sessile or stipitate; style 1; stigmas 2–3, filiform. Fruit a 1-seeded nut, viviparous in Amphibolis and Thalassodendron.
Selected specimens: Whitfords Beach, W. Aust., 5 m deep (Lipkin, 2.ii.1982; ADU, A52953). Mangles Bay, Point Peron, W. Aust., drift (Royce 3051, 28.v.1949; PERTH s.n.). Seven Mile Beach, Dongara, W. Aust., low eulittoral to upper sublittoral (Womersley, 17.ix.1979; ADU, A51311).
Taxonomic notes: The five genera, with mainly tropical distribution, are all found on the Australian coast, with two genera occurring in southern Australia.
* The two widespread tropical species Syringodium isoetifolium (Ascherson) Dandy (1939, p. 116) and Halodule uninervis (Forskal) Ascherson (1882, p. 24) also occur in restricted areas on the temperate Western Australian coast.
Syringodium isoetifolium occurs to a depth of 5 m in mixed seagrass beds as far south as Rottnest I. and suburban Perth beaches. It may be distinguished by its cymose inflorescence (Fig. 29A) and its subulate-terete leaf blades (Fig. 29B, C), which in transverse section show a central vascular strand surrounded by 6–8 air canals and 7–10 (–15) pericentral vascular bundles.
Halodule uninervis (Fig. 29D) is a pioneer species and a rapid colonizer and may sometimes establish itself in temporarily denuded areas around Port Denison, prior to the establishment of other seagrass species (Kirkman, pers. comm.). It may be distinguished by its linear, three-veined leaves, with the submarginal veins ending in teeth (Fig. 29E).
ASCHERSON, P. (1882). Flora orientalis. Vol. 5 (ed. P. E. Boissier) (Georg: Basle.)
DANDY, J.E. (1939). In Dandy, J.E. & Tandy, G. The identity of Syringodium Kuetz. J. Bot. 77, 114–116.
The Marine Benthic Flora of Southern Australia Part I complete list of references.
Womersley, H.B.S. (31 May, 1984)
The Marine Benthic Flora of Southern Australia
©Board of the Botanic Gardens and State Herbarium, Government of South Australia
KEY TO GENERA OF CYMODOCEACEAE
1. Leaf blades flat, linear; flowers solitary
1. Leaf blades terete, subulate; flowers in conspicuous cymose inflorescences
2. Leaf blade shed before the persistent sheath; blade narrow (usually less than 3 mm broad); blade veins 3, longitudinal, a midrib and 2 submarginal veins
2. Leaf blade and sheath shed together leaving conspicuous annular scars on the stems; blade usually more than 5 mm broad; blade veins
3. Leaf blades with denticulate margins; apex rounded and coarsely denticulate. Stems unbranched or little branched, arising regularly at every fourth internode. Roots unbranched, arising at the internode preceding the stem-bearing internode. Flowers surrounded by four floral bracts
3. Leaf blades entire; apex bidentate. Stems much-branched, arising at irregular intervals of (1–)
State Herbarium of South Australia