1. Hydrilla L.C. Rich., Mem. Cl. Sci. Math. Phys. Inst. France (Paris). 12(2): 9, 61, 73, t. 2. A-K dated. 1812. pulished 1814; Benth. & Hook. f., Gen. Pl. 3: 450. 1883; Hook. f., Fl. Brit. Ind. 5: 659. 1888.
Submerged, monoecious or dioecious, perennials with slender, branched stems. Leaves in whorls of 3-8 (-12), cauline, sessile, linear to lanceclate with conspicuous midrib, subentire or serrate. Flowers unisexual, minute, actinomorphic, solitary, without nectaries. Male spathe ± sessile, solitary axillary, flattened, globose, ± muricate, Male flower: shortly pedicellate; sepals 3, ovate-oblong to obovate, scarious, green, imbricate; petals 3, linear, white; stamens 3, alternating with the petals; pistillode present. Female spathe sessile, cylindrical, apically bifid, membranous, rostrate. Female flowers: sepals as in the male, petals somewhat narrower than sepals; ovary 3-carpellate, shorter than the spathe, cylindrical to narrowly conical with filiform beak; styles 3, entire. Fruit cylindrical to narrowly conical, glabrous or softly muricate. Seeds 2-6, oblong-ellipsoidal.
A monotypic genus, distributed in Eurasia and Africa to Australia; introduced in the New World.