Habit and leaf form. Aquatic herbs. Perennial; rhizomatous (rhizome herbaceous, monopodial, well-differentiated, with 1 or more unbranched roots and a bladeless prophyllum or a complete leaf at each node; branches usually 1–4 cm long, straight). Hydrophytic; marine; rooted. Leaves submerged; alternate; distichous; sessile; sheathing (with ligule and auricle present; sheath open or closed, persisting after distal part of lamina has fallen off). Leaf sheaths with free margins, or with joined margins. Leaves edgewise to the stem, or with ‘normal’ orientation; simple. Leaf blades entire; flat; linear (ribbonlike); 3–9(–11) -nerved; parallel-veined; without cross-venules. Leaves ligulate. Axillary scales present. Leaf blade margins entire. Leaves with a persistent basal meristem, and basipetal development. Stem anatomy. Secondary thickening absent.
Reproductive type, pollination. Fertile flowers functionally male and functionally female. Unisexual flowers present. Plants monoecious. Pollinated by water.
Inflorescence and flower features. Flowers aggregated in ‘inflorescences’; in spikes. The terminal inflorescence unit racemose. Inflorescences axillary (enfolded in the subtending sheath, exserted only when fruiting); 4–12 male and 4–12 female flowers together in flattened spikes or spadices, the flowers on one side — males and females alternating in two ranks. Flowers ebracteate; ebracteolate; small. Perigone tube absent. Perianth vestigial (if the marginal outgrowths from the axis — retinacula, which, when present, enclose the stamens — are interpreted as perianth), or absent. Fertile stamens present, or absent (female flowers). Androecium 1 (when androecium interpreted as 2 bisporangiate thecae), or 2 (if interpreted as 2 bisporangiate stamens). Androecial members the ‘thecae’ joined at their bases by a narrow flap of ‘connective’ material. Androecium exclusively of fertile stamens. Stamens 1, or 2 (depending on interpretation); with sessile anthers. Anthers dehiscing via longitudinal slits; unilocular; bisporangiate, or tetrasporangiate (depending on interpretation of the androecium). Pollen shed as single grains (becoming filamentous and up to 2 mm long when released). Pollen grains lacking exine, and dispersed in the sea as long filaments. Fertile gynoecium present, or absent (male flowers). Gynoecium ostensibly 1 carpelled; (?pseudo) monomerous; ostensibly of one carpel; superior. Carpel apically stigmatic; 1 ovuled. Placentation apical. Styles forked. Ovules sessile; pendulous; non-arillate; orthotropous.
Fruit and seed features. Fruit non-fleshy (pericarp scarious). The fruiting carpel indehiscent; an achene, or drupaceous (ovoid to ellipsoid). Fruit 1 seeded. Seeds ovoid to ellipsoid; non-endospermic. Cotyledons 1. Embryo curved. Testa smooth, ribbed or ridged. Seedling. Hypocotyl internode present. Mesocotyl absent. Seedling collar conspicuous. Cotyledon hyperphyll elongated; assimilatory; more or less circular in t.s. Coleoptile absent. Seedling macropodous. Seedling cataphylls absent. First leaf dorsiventral. Primary root ephemeral.
Geography, cytology, number of species. Native of Australia. Not endemic to Australia. Australian states and territories: Western Australia, South Australia, New South Wales, Victoria, and Tasmania. South-West Botanical Province.
Additional characters Fruit rostrate. Cortex with 2 vascular bundles.
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