Glabrous herb, sometimes forming dense patches; stems lax, often creeping or floating, sometimes submerged, rooting at nodes, often reddish; pneumatophores 0. Lvs opposite, ± decussate; petiole to 1.5 cm long. Lamina (0.5)-1-4 × (0.2)-0.5-2 cm, lanceolate to ovate or rhombic, often linear when submerged; base attenuate to rounded; apex obtuse or subacute, acute on submerged lvs. Fls solitary, 4-merous, sessile or subsessile. Sepals 1-1.7 mm long, triangular or broad-triangular, accrescent; apex acute. Petals 0. Stamens 4; filaments equal. Style to c. 0.3 mm long, stout; stigma ± subcapitate or obscurely lobed. Capsule 3-4.5 × c. 2.5 mm, obovoid, with green ribs. Seeds 0.5-0.6 mm long, ± oblong, rather rounded on 1 side, shining, free from endocarp, in several rows in each locule.