Habit and leaf form. Aquatic herbs. Perennial; plants with neither basal nor terminal concentrations of leaves; rhizomatous (rhizome well-developed, slender). Stem growth conspicuously sympodial. Hydrophytic; rooted. Leaves submerged, or submerged and floating. Heterophyllous (with clearly distinct submerged and floating leaves), or not heterophyllous. Leaves small, or medium-sized; usually alternate (at most nodes), or opposite (at base of each peduncle); when alternate, distichous; petiolate to sessile; sheathing (sheath free and more or less encircling the stem or sometimes adnate to petiole or base of blade; open, membranous, often decayed on older stems). Leaf sheaths tubular, or not tubular; with free margins. Leaves simple. Leaf blades entire; linear (broad and ribbon-like or narrow and filiform), or elliptic to orbicular; one-veined, or parallel-veined; cross-venulate, or without cross-venules. Leaves ligulate, or eligulate; with stipules. Axillary scales present. Leaf blade margins entire. Stem anatomy. Secondary thickening absent.
Reproductive type, pollination. Fertile flowers hermaphrodite. Unisexual flowers absent. Plants hermaphrodite (protogynous). Anemophilous, or entomophilous, or pollinated by water.
Inflorescence and flower features. Flowers aggregated in ‘inflorescences’. Inflorescence few-flowered to many-flowered. Flowers in racemes and in spikes. The terminal inflorescence unit racemose. Inflorescences pedunculate; terminating an axillary peduncle; spikes dense, simple, cylindric, emergent, somewhat succulent. Flowers ebracteate; small; regular; 4 merous; cyclic; tricyclic. Perigone tube absent. Hypogynous disk absent. Perianth of ‘tepals’ (‘tepals’ valvate, thickened, brownish, shortly clawed, obtuse), or absent (if the ‘perianth’ members are interpreted as staminal appendages); 4 (if the staminal appendages are interpreted as perianth). Androecium 4. Androecial members adnate (to the claws of the ‘perianth’ members, should these be so interpreted); all equal; free of one another; 2 -whorled (fairly clearly). Androecium exclusively of fertile stamens. Stamens 4; oppositiperianthial (opposite the perianth, when this is present and regarded as such); shortly filantherous, or with sessile anthers. Anthers dehiscing via longitudinal slits; extrorse; bilocular; appendaged (if the ‘tepals’ are interpreted as outgrowths from the connective), or unappendaged. Pollen more or less spherical. Gynoecium 3–5 carpelled. Carpels isomerous with the perianth. Gynoecium apocarpous; eu-apocarpous; superior. Carpel non-stylate, or stylate; apically stigmatic; 1 ovuled. Placentation marginal. Stigmas dry type; non-papillate; Group II type. Ovules pendulous; non-arillate; orthotropous to campylotropous.
Fruit and seed features. Fruit fleshy, or non-fleshy; an aggregate. The fruiting carpel indehiscent; drupaceous, or an achene. Dispersal usually by floating of the head of fruits. Fruit 1 seeded. Seeds looped; non-endospermic; with starch. Cotyledons 1. Embryo achlorophyllous (two species); slightly curved. Testa smooth to ridged or tuberculate. 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. 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, Northern Territory, Queensland, New South Wales, Victoria, Australian Capital Territory, and Tasmania. Northern Botanical Province, Eremaean Botanical Province, and South-West Botanical Province.
Additional characters Fruit rostrate.
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Western Australian Herbarium (1998–). FloraBase—the Western Australian Flora. Department of Biodiversity, Conservation and Attractions. https://florabase.dpaw.wa.gov.au/