Soft herb of water or damp places, stems slender, us. simple, rooting from lower nodes, 5-40-(100) cm. tall. Lvs in whorls of c. 5, of many sizes and shapes; submerged lvs us. c. 2 cm. long, pinnatisect with fine segs; upper emergent lvs becoming shorter and narrower, the pinnules reducing to teeth and finally disappearing leaving lvs linear and entire, 5-15 × < 1 mm. Fls solitary in axils of upper lvs, us. unisexual, bracteoles c. 0·3 mm. long, triangular, minutely serrulate; ♀ us. lower but ♀ and ♂ may occur in same whorl; ♀ sessile, c. 1 mm. long, sepals and petals 0, styles stout, stigmas with very long hairs; ♂ 2-3 mm. long, abortive ovary forming small pedicel; sepals < bracteoles, petals pale, stamens 8, anthers c. 2 × 0·5 mm.; fr. pale, 1·2 mm. long, mericarps cylindrical, faces rough with sparse short papillae.

2n = 14

Weak aquatic or semiaquatic or terrestrial herb (5-)10-25(-40) cm tall; stems weak, slender, 1.0-1. 5( -3.0) mm diam., rooting at lower nodes; glabrous. Leaves distinctly dimorphic. Submerged leaves in whorls of 3-4(-5), ovate to orbicular in outline, (0.6-) 1.0-2.0(-2.5) cm long, 1.0-1.6(-2.5) cm wide, pectinate with 10-22 filiform pinnae 5-6(-20) mm long, with distinct petiole 1-2 mm long. Emergent leaves in whorls of (2-)3-4(-5), linear, (2-)5-7(-12) mm long, 0.5-1.0(-1.5) mm wide, usually flattened but occasionally terete to filiform, spreading or slightly curved upwards, margin entire or sometimes weakly toothed. Rarely, all or most emergent leaves pinnatifid, 4-5 mm long, 1.5-3.5 mm wide with pinnae to 1.5 mm long. Transitional leaves usually 1-2(-4) whorls only. Inflorescence an indeterminate spike with the unisexual flowers borne singly in axils of the emergent leaves. Each whorl contains flowers of 1 sex only, male in upper 1-8 whorls, female in lower 2-6 whorls, with usually 1-2 sterile whorls between. Occasionally all whorls (up to 20) contain female flowers only. All flowers subtended by 2 bracteoles. Bracteoles ovate, (0.4-)0.5-0.8 mm long, (0.2-)0.4-0.5 mm wide, slightly laciniate towards tip or entire. Male flowers 4-merous, sessile or becoming shortly pedicellate. Sepals 4, ovate, 0.4-0.6 mm long, 0.3-0.4 mm wide, weakly toothed near apex. Petals 4, hooded, very weakly keeled, 1.8-2.7(-3.0) mm long, 1.4 mm wide, becoming reflexed and inrolled after anthesis. Stamens 8; filaments 1-2 mm long, elongating after anthesis; anthers yellow, linear-oblong, 1.7-2.4 mm long, 0.4-0.5 mm wide, antisepalous anthers c. 0.2 mm longer than antipetalous ones. Styles 0. Ovary vestigial. Female flowers 4-merous, sessile. Sepals, petals and stamens 0. Styles 4, clavate, 0.2-0.3 mm long, stigmas occupying most of length of style, white, fimbriate. Ovary 4-merous, oblong, 0.7-0.8 mm long (excluding styles), 0.7 mm wide; carpels cylindrical, smooth, with styles on outermost corners. Fruit pale yellow-brown, cylindrical, sessile, splitting at maturity into 4 mericarps. Mericarps 0.8-1.1 mm long, 0.4-0.5 mm diam., smooth apart from scattering of very small asperities on outer face, crowned by persistent base of style.

Perennial herb; stems weak, slender, (5)-10-25-(40) cm long, prostrate or erect, usually simple. Lvs usually distinctly dimorphic, sometimes monomorphic. Submerged lvs in whorls of 3-4-(5), ovate to orbicular, (6)-10-20-(25) × 10-16-(25) mm, pectinate; pinnae 10-22, filiform, 5-6-(20) mm long. Aerial lvs in whorls of (2)-3-4-(5), linear, (2)-5-7-(12) × 0.5-1-(1.5) mm, usually flattened but occasionally terete to filiform; margin entire or sometimes weakly toothed. Fls unisexual, ♂ in upper 1-8 whorls, ♀ in lower 2-6 whorls, with usually 1-2 sterile whorls between, occasionally all whorls ♀. Bracteoles ovate, (0.4)-0.5-0.8 × (0.2)-0.4-0.5 mm. ♂ fls sessile or nearly so; sepals 4, ovate, 0.4-0.6 × 0.3-0.4 mm; petals 4, hooded, 1.8-2.7-(3) × c. 1.4 mm. ♀ fls sessile; sepals and petals 0. Frs 0.8-1.1 × 0.8-1 mm, pale yellow-brown or greenish, ± cylindric, smooth apart from scattering of very small tubercles on outer face of mericarps.