Common name. Hornwort. Family Ceratophyllaceae.
Habit and leaf form. Submerged, brittle, aquatic herbs (the plant decaying behind as it grows in front). Plants rootless. Perennial; plants with neither basal nor terminal concentrations of leaves. Hydrophytic; free floating. Leaves submerged; small to medium-sized; whorled; 3–12 per whorl; becoming brittle with age; petiolate (petioles inconspicuous, light-coloured); simple. Leaf blades dissected (usually), or entire; when entire, linear; usually dichotomously dissected (segments linear to terete, often dentate). Leaves without stipules. Leaf anatomy. Hairs absent. Stem anatomy. Secondary thickening absent.
Reproductive type, pollination. Fertile flowers functionally male and functionally female. Unisexual flowers present. Plants monoecious. The unisexual flowers at different nodes or occasionally at the same node. Female flowers without staminodes. Male flowers with pistillodes (ovary rudimentary). Pollinated by water.
Inflorescence and flower features. Flowers solitary (subtended by an involucre of basally connate, foliaceous bracts), or aggregated in ‘inflorescences’ (with up to several per node); axillary. Inflorescences with involucral bracts. Flowers ebracteate; ebracteolate; minute; regular; cyclic (female), or partially acyclic (male). The androecium acyclic. Hypogynous disk absent. Perianth sepaline (bractlike); 8–15; 1 -whorled; joined (basally). Calyx 8–15; 1 -whorled; gamosepalous (basally); green to white; non-fleshy (transparent). Fertile stamens present, or absent (female flowers). Androecium (3–)10–20(–27). Androecial members free of the perianth; all equal; free of one another; spirally arranged on a flat receptacle. Androecium exclusively of fertile stamens. Stamens (3–)10–20(–27); filantherous to with sessile anthers (‘subsessile’). Anthers oblong; adnate; non-versatile; dehiscing irregularly, or dehiscing via longitudinal slits; extrorse; appendaged (via the often coloured, apically projecting connective). The anther appendages apical. Pollen shed as single grains. Fertile gynoecium present, or absent (male flowers). Gynoecium 1 carpelled. The pistil 1 celled. Gynoecium monomerous; of one carpel; superior. Carpel stylate; apically stigmatic. Style spine-like. Carpel 1 ovuled. Placentation apical. Ovary sessile. Ovules pendulous; non-arillate; orthotropous.
Fruit and seed features. Fruit non-fleshy; black. The fruiting carpel indehiscent; an achene (hard, slightly compressed, the margins sometimes spiny or winged). Seeds endospermic (mucilaginous), or non-endospermic. Cotyledons 2. Embryo chlorophyllous; straight. Seedling. Germination phanerocotylar.
Physiology, biochemistry. Aluminium accumulation not found.
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. X = 12. A genus of 6 species; 1 species in Western Australia.