Common name. Hornwort Family.
Habit and leaf form. Submerged, brittle, aquatic herbs (the plant decaying behind as it grows in front). Plants rootless. Plants with neither basal nor terminal concentrations of leaves. Hydrophytic; free floating. Leaves submerged; medium-sized; whorled; 3–10 per whorl; becoming brittle with age; petiolate; simple. Leaf blades dissected; dichotomously dissected. Leaves without stipules. Stem anatomy. Secondary thickening absent.
Reproductive type, pollination. Fertile flowers functionally male and functionally female. Unisexual flowers present. Plants monoecious. Female flowers without staminodes. Pollinated by water.
Inflorescence and flower features. Flowers solitary (one per leaf axil, in whorls); ebracteate; ebracteolate; minute; regular; cyclic (female), or partially acyclic (male). The androecium acyclic. Hypogynous disk absent. Perianth sepaline (bractlike); 9–10; 1 -whorled; joined (basally). Fertile stamens present (male flowers), or absent (female flowers). Androecium (5–)10–20(–27) (in male flowers). Androecial members free of the perianth; all equal; free of one another. Androecium exclusively of fertile stamens. Stamens (5–)10–20(–27); filantherous to with sessile anthers (‘subsessile’). Anthers 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 (female flowers), or absent (male flowers). Gynoecium in female flowers 1 carpelled. The pistil 1 celled. Gynoecium monomerous; of one carpel; superior. Carpel stylate; apically stigmatic; 1 ovuled. Placentation apical. Ovules pendulous; non-arillate; orthotropous.
Fruit and seed features. Fruit non-fleshy. The fruiting carpel indehiscent; an achene. Seeds non-endospermic. Cotyledons 2. Embryo chlorophyllous; straight. Seedling. Germination phanerocotylar.
Physiology, biochemistry. Aluminium accumulation not found.
Geography, cytology, number of species. World distribution: cosmopolitan. X = 12. 10 species.