Habit and leaf form. Aquatic herbs. Annual, or perennial; plants with neither basal nor terminal concentrations of leaves; rooting from base and basal nodes. Hydrophytic; non-marine; rooted. Leaves submerged; (sub-) opposite to whorled, or alternate (‘subopposite or subverticillate’); when in verticils, 3 per whorl; sessile; sheathing (more completely so in the lower members of each sub-pair or verticil). Leaf sheaths not tubular; with free margins. Leaves simple. Leaf blades entire; linear; one-veined; without cross-venules. Leaves somewhat with stipules (at the sides of the sheath), or without stipules. Axillary scales often present (two, small). Leaf blade margins serrate to dentate (usually toothed), or entire. Leaf anatomy. Hairs absent. 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. Pollinated by water (elongation of the anther stalk ruptures the surrounding scale-tube, the pollen is then released apically and transported through the water to the female flowers).
Inflorescence and flower features. Flowers solitary, or aggregated in ‘inflorescences’ (then several flowers together in the axil); axillary (at base of branches); bracteate, or ebracteate (male flowers enclosed by an ‘involucre’ of 2 cup-like structures, an outer whorl (sometimes absent) which is usually fused into a tube, and an inner whorl which is membranous, fused, flask-shaped and 2-lobed distally - this inner whorl is sometimes interpreted as the perianth); minute to small. Perigone tube present, or absent (depending on interpretation). Hypogynous disk absent. Perianth of ‘tepals’, or vestigial, or absent; if interpreted as such, sepaloid. Fertile stamens present, or absent (female flowers). Androecium 1. Androecium exclusively of fertile stamens. Stamens 1; very shortly filantherous, or with sessile anthers. Anthers dehiscing irregularly; unilocular, or four locular; tetrasporangiate (or unisporangiate). Pollen shed in aggregates. Fertile gynoecium present, or absent (from male flowers). Gynoecium 1 carpelled; ostensibly monomerous; ostensibly of one carpel; superior. Carpel stylate (style subconical to cylindric, with 2–4 stigmatic branches, sometimes also with shorter sterile lobes, rarely simple); 1 ovuled. Placentation basal. Ovules ascending; non-arillate; anatropous.
Fruit and seed features. Fruit non-fleshy. The fruiting carpel indehiscent; an achene (pericarp thin). Seeds non-endospermic; with starch. Embryo well differentiated. Cotyledons 1. Embryo chlorophyllous; straight. 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.
Physiology, biochemistry. Photosynthetic pathway: C3.
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, and Australian Capital Territory. Northern Botanical Province, Eremaean Botanical Province, and South-West Botanical Province.
Additional characters Perianth of male flowers of ‘tepals’, or absent (depending on the interpretation of the ‘involucre’). Perianth of female flowers absent (usually), or vestigial (rarely with an inconspicuous membranous ‘involucre’).