Description from Flora of China
Herbs perennial, aquatic or semi-aquatic, glabrous throughout. Rhizomatous, root fibrous. Stem suberect, usually stoloniferous. Arial leaves 1-pinnate, submerged leaves more dissected. Umbels compound, usually opposite leaves; bracts and bracteoles several, oblong or lanceolate, margins membranous, usually reflexed. Calyx teeth subulate or triangular, caducous. Petals white, obovate, base short-attenuate, apex retuse with small inflexed lobule. Stylopodium conic; styles longer than the stylopodium, recurved. Fruit ovoid, subdidymous, slightly compressed laterally, glabrous; exocarp corky and slightly thickened; dorsal and intermediate ribs filiform, lateral ribs obscure; vittae numerous, small, forming a continuous band encircling the seed. Seed face plane. Carpophore 2-cleft to base.
Two species: Africa, C and SW Asia, Australia, Central America (Mexico), Europe, North America; one species in China.
(Authors: Pu Fading (溥发鼎 Pu Fa-ting); Mark F. Watson)