Habit and leaf form. Much-branched aquatic herbs. Perennial. Leaves cauline. Plants with neither basal nor terminal concentrations of leaves. Hydrophytic; non-marine; rooted. Leaves submerged. Heterophyllous (either scale-like or elongate). Leaves whorled (mostly very crowded), or opposite (basal leaves only); (2–)4–5(–8) per whorl; membranous, or ‘herbaceous’; sessile; simple; epulvinate. Leaf blades entire; one-veined, or parallel-veined; cross-venulate, or without cross-venules. Leaves without stipules. Axillary scales present (2 or 4 per leaf, ovate to circular, entire). Leaf blade margins minutely toothed. Leaves with a persistent basal meristem, and basipetal development. Leaf anatomy. Hairs absent. Stem anatomy. Secondary thickening absent.
Reproductive type, pollination. Fertile flowers functionally male, or functionally female. Unisexual flowers present. Plants dioecious. Female flowers solitary (usually, the floral parts of the flower borne above the surface of the water by an elongate perigone tube), or aggregated in ‘inflorescences’ (rarely); with staminodes (3). Male flowers aggregated in ‘inflorescences’ (2–5 per spathe, born on long pedicels and opening above the water surface 1(-2) at a time). Floral nectaries present. Nectar secretion from the androecium (from staminodial nectaries). Entomophilous.
Inflorescence and flower features. Flowers solitary, or aggregated in ‘inflorescences’; in cymes. The terminal inflorescence unit (when flowers clustered) cymose. Inflorescences scapiflorous, or not scapiflorous; axillary; spatheate (the spathe around the male infloresence axillary, tubular, sessile; bracts 2, connate). Flowers pedicellate (male flowers), or sessile (female flowers); bracteate (forming the ‘spathe’ around the solitary female flower); small; regular; 3 merous; partially acyclic. The gynoecium acyclic. Perigone tube present (threadlike in female flowers). Perianth with distinct calyx and corolla; 6; 2 -whorled; isomerous; different in the two whorls; white. Calyx 3; 1 -whorled; polysepalous; regular; green (to translucent, often with a reddish tinge). Corolla 3; 1 -whorled; polypetalous; white. Petals clawed, or sessile. Fertile stamens present, or absent (female flowers). Androecium 9(–10). Androecial members free of the perianth; free of one another; 3 -whorled (the whorls trimerous). Androecium exclusively of fertile stamens. Stamens 9(–10); triplostemonous. Anthers basifixed; dehiscing via short slits; latrorse; tetrasporangiate. Pollen shed in aggregates, or shed as single grains. Fertile gynoecium present, or absent (male flowers). Gynoecium 3 carpelled. The pistil 1 celled (with 3 placentae). Gynoecium syncarpous; synovarious to synstylovarious; inferior. Ovary unilocular; 1 locular. Styles 3; free, or partially joined; simple; apical. Stigmas 3; 2–4 - lobed; dry type; papillate; Group II type. Placentation laminar-dispersed, or parietal. Ovules in the single cavity up to 9; pendulous to ascending; non-arillate; orthotropous.
Fruit and seed features. Fruit fleshy; dehiscent; a capsule. Capsules splitting irregularly (underwater). Dispersal by water. Fruit 3–6 seeded (usually). Seeds ellipsoidal; non-endospermic; with starch. Cotyledons 1. Embryo straight. Seedling. Hypocotyl internode present. Mesocotyl absent. Seedling collar not conspicuous. Cotyledon hyperphyll elongated; assimilatory; dorsiventrally flattened. Coleoptile absent. Seedling macropodous. Seedling cataphylls absent. First leaf dorsiventral. Primary root ephemeral.
Geography, cytology, number of species. Adventive. Australian states and territories: Western Australia, Queensland, New South Wales, and Victoria.
Additional characters Perianth of male flowers with distinct calyx and corolla; 6. Perianth of female flowers with distinct calyx and corolla (smaller than the male perianth); 6.