Aquatic or semi-aquatic herb, rooted from lower nodes; erect stems robust, to 40 cm. tall and 5 mm. in diam., sparsely branched. Lvs verticillate in whorls of c. 5, c. 3 cm. × 6-8 mm., often crowded, all pinnate to pinnatifid, segs longer and finer in lowest lvs. Fls solitary in the axils of upper lvs, sessile, hermaphrodite and protandrous; bracteoles very short, pinnati- or palmati-sect; sepals small, triangular, minutely serrate; petals pale, c. 2.5 mm. long, falling early; stamens 8, anthers 2 mm. long; ovary c. 1 mm. long, styles 4, divergent, stigmas plumose; fr. pale when ripe, often broader than long, c. 2 × 3 mm., mericarps often developing unevenly; surface smooth and rounded or occ. with a few sharp tubercles towards base.

2n = 28

Stout aquatic or terrestrial herb; stems pink in upper parts, green below, to 30 cm tall, 5-6 mm diam. at base, rooting only at lower nodes, glabrous. Leaves all whorled, slightly dimorphic. Submerged leaves often decaying early and absent; if present, in whorls of 5-7, ovate in outline, rounded at tip, 5 cm long, 4 cm wide, pectinate with 24-30 filiform pinnae 1.5-2.5 cm long. Emergent leaves yellow-green to subglaucous, crowded towards tips of stems, spreading, in whorls of (4-)5-7, oblong in outline with acute tip, (1.5-)2.0-3.5 cm long, (0.4-) 0.6-1.0 cm wide, becoming smaller near water level, pectinate, with 26-32 pinnae; pinnae linear, (2-)6-7 mm long, 0.3 mm wide, tips acute, usually incurved. Inflorescence an indeterminate spike with the flowers borne singly in the axils of the upper emergent leaves and each subtended by two bracteoles. Fertile emergent leaves in whorls of 4-6, pectinate, equalling other leaves in size and shape. Bracteoles ovate to orbicular in outline, 0.7-0.8 mm long, 0.7-0.8 mm wide, 6-8-pinnatifid or -palmate, the pinnae cut 2/3 of way to centre of lamina. Flowers hermaphrodite, 4-merous. Sepals 4, green to pinkish, ovate to deltoid, 0.6-0.8 mm long, 0.5-0.6 mm wide, minutely denticulate on margin, smooth. Petals 4, yellowish-green, weakly hooded, not or only weakly keeled, 2.5-3.4 mm long, 1.1-1.2 mm wide, on unguiculum 0.5 mm long. Stamens 8; filaments 0.3 mm long, anthers yellow, linear-oblong, 1.7-2.3 mm long, 0.2-0.3 mm wide, antisepalous anthers c. 0.2 mm longer than antipetalous ones, 4-locular, very shortly apiculate. Styles 4, clavate, 1 mm long, stigmas capitate. Ovary pink, ovoid, c. 1.0 mm long, 0.7 mm wide, 4-ribbed opposite petals, 4-locular, with 1 ovule per locule. Fruit 4-merous, on pedicel to 1 mm long, globular to slightly obturbinate, 1.5-2.0 mm long (excluding sepals), 2.0-2.5 mm wide, sepals persistent, deltoid, 0.6 mm long, 0.6 mm wide, styles persistent, reflexed; mericarps slightly flattened, dorsally rounded especially in upper part, often ± keeled in lower part, with a few small tubercles on keel.

Stout perennial herb; stems to 30 cm tall, prostrate to erect. Lvs all whorled, slightly dimorphic. Submerged lvs often decaying early, in whorls of 5-7. Aerial lvs yellow-green to subglaucous, crowded towards tips of stems, spreading, in whorls of (4)-5-7, oblong, (15)-20-35 × (4)-6-10 mm, becoming smaller near water surface, pectinate, with 26-32 filiform to linear pinnae. Fertile aerial lvs similar to other lvs. Fls ☿. Bracteoles ovate to orbicular, 0.7-0.8 × 0.7-0.8 mm. Sepals 4, ovate to deltoid, 0.6-0.8 × 0.5-0.6 mm; petals 4, weakly hooded, 2.5-3.4 × 1.1-1.2 mm. Frs globular to slightly obturbinate, 1.5-2 (excluding sepals) × 2-2.5 mm, with a few small tubercles on keel of mericarps.