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Habit and leaf form. Aquatic herbs. Annual, or perennial; plants with neither basal nor terminal concentrations of leaves; rhizomatous (rhizomes much-branched bearing roots, scale leaves and erect leafy shoots at nodes). Hydrophytic; non-marine (in brackish and alkakine water, not the sea); rooted. Leaves submerged; alternate (except near the flowers, frequently crowded at distal ends of branches); distichous; ‘herbaceous’, or membranous; sessile; sheathing (sheath with auricles well developed, initially enclosing the flowers). Leaf sheaths not tubular; with free margins. Leaves simple. Leaf blades entire; linear; one-veined; without cross-venules. Leaves ligulate. Axillary scales present (two per node). Stem anatomy. Secondary thickening absent.
Reproductive type, pollination. Fertile flowers functionally male and functionally female, or functionally male, or functionally female. Unisexual flowers present. Plants monoecious, or dioecious. Male flowers without pistillodes. Pollinated by water.
Inflorescence and flower features. Flowers solitary, or aggregated in ‘inflorescences’; when solitary axillary; when aggregated, in clusters. The terminal inflorescence unit cymose. Inflorescences terminal. Flowers pedicellate (pedicel often elongating in fruit); minute to small. Perigone tube absent. Hypogynous disk absent. Perianth vestigial; 3; 1 -whorled; joined; persistent. Fertile stamens present, or absent (female flowers). Androecium 1 (always, ostensibly), or 2–3 (forms with 8 or 12 microsporangia perhaps representing 2 and three concrescent anthers). Androecial members coherent (if the androecium comprises more than one stamen), or free of one another. Androecium exclusively of fertile stamens. Stamens 1, or 2–3 (depending on interpretation); with sessile anthers. Anthers bilocular; tetrasporangiate, or multisporangiate (4, 8 or 12 sporangia, affording evidence of coalesced anthers?); appendaged, or unappendaged. Fertile gynoecium present, or absent (male flowers). Gynoecium 3 carpelled; apocarpous; eu-apocarpous; superior. Carpel stylate; apically stigmatic (styles simple, stigma asymmetrically peltate); 1 ovuled. Placentation apical. Styles persistent. Ovules pendulous; non-arillate; orthotropous to anatropous.
Fruit and seed features. Fruit non-fleshy; an aggregate. The fruiting carpel indehiscent; an achene. Seeds more or less oblong; non-endospermic. Cotyledons 1. Embryo coiled. Seedling. Hypocotyl internode present. Mesocotyl absent. Seedling collar not 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, Queensland, New South Wales, Victoria, Australian Capital Territory, and Tasmania. South-West Botanical Province.
Additional characters Perianth of male flowers vestigial; 3 (scales minute, or fused and cup-like). Perianth of female flowers vestigial; 3 (scales membranous and persistent).