Weak aquatic herb; stems 50-100 cm long, rooting mainly at base, 1-2(-3) mm diam.; leaves dimorphic, verticillate. Submerged leaves in whorls of (2-)3(-4), ovate in outline, sessile (petiole 0.1-0.2 mm long), (6-)10-15 mm long, (5-)7-10(-15) mm wide, pectinate with 12-18 filiform pinnae 8 mm long. Leaves lacking hydathodes at base. Emergent leaves red-purple, in whorls of 3(-4), very rarely becoming alternate in upper part of inflorescence, narrowly ovate to obovate, (2.5-)5-6(-12) mm long, (1.5-)2.0-2.5(-4.0) mm wide, entire (or those of lower whorls incised), obtuse, shorter than internodes at flowering. Transition from submerged to emergent leaves abrupt (1-2 whorls). Inflorescence all indeterminate spike, sometimes with 2-4 lateral inflorescences arising from axils of the upper whorl of submerged leaves. Flowers 4-merous, borne singly in axils of the emergent leaves, upper flowers male, lower ones female. Bracteoles cream, lanceolate, (1.0-)1.4-1.8(2.0) mm long, (0.2-)0.3-0.4 mm wide, entire or minutely 3-4 toothed, acute. Male flowers. Sepals 4, cream, deltoid, 0.5-0.6 mm long, 0.4-0.5 mm wide, weakly 2-3-toothed. Petals 4, yellow to reddish, hooded, not or only very weakly keeled, 2.0-2.8 mm long: 2.0 mm wide. Stamens 8; filaments 0.1 mm long, lengthening after anthesis; anthers linear-oblong, 1.8-2.4 mm long, 0.4 mm wide, antisepalous anthers c. 0.3 mm longer than antipetalous ones, non-apiculate. Styles 4, clavate, c. 0.2 mm long, vestigial. Ovary ± cubiform, 0.7 mm long, 0.6 mm wide, lobed opposite petals, non-functional. Petals and stamens oflowermost whorl of male flowers sometimes smaller than in upper whorls. Female flowers. Sepals 4, deltoid, 0.2 mm long, 0.2 mm wide, minutely serrate, vestigial. Petals 4, 0.5 mm long, vestigial. Stamens 0. Styles 4, clavate, 0.5 mm long, stigmas fimbriate, cream. Ovary ± cubiform, 1.0 mm long, 0.8 mm wide, 4-lobed opposite petals. Fruit reddish-purple, ± cubiform, 1.3-1. 4(-1 .5) mm long, 1.4-1.5 mm wide, styles persistent and erect, fruit prominently 4-lobed opposite styles, splitting at maturity into 4 mericarps. Mericarps cylindrical, 1.4 mm long, 0.8 mm diam., smooth and rounded on dorsal surface, sometimes with a faint dorsal line, planar on faces where joined to other mericarps.

Herba aquatica infirma, caulibus 50-100 cm longis, 1-2(-3) mm diametris, basibus radicantes praecipue. Folia dimorpha verticillata. Folia submersa (2-)3(-4) verticillata, ovata, (6-)10-15 mm longa, (5-)7-10(-15) mm lata, plusminusve sessilia, pectinata. Folia emersa 3(-4) verticillata, rarissimo alterna in parte superiora, anguste ovata ad obovata, (2.5-)5-6(-12) mm longa, (1.5-)2.0-2.5(-4.0) mm lata, integra vel folia inferiora incisa, obtusa, folia quam internodia breviora sub anthesi. Inflorescentia spica terminalis, ramis lateralibus 2-4 interdum.Flores 4-meri, in axillis foliorum emersorum, flores superiores masculini, flores inferiores feminei. Flores masculini 4-meri. Sepala 4, deltoidea, 0.5-0.6 mm longa, 0.4-0.5 mm lata, infirme 2-3-dentata. Petala 4, cucullata, 2.0-2.8 mm longa, 2.0 mm lata. Stamina 8; filamenta 0.1 mm longa, crescens post anthesin; antherae anguste oblongae, 1.8-2.4 mm longae, 0.4 mm latae, non apiculatae. Styli 4, clavati, 0.2 mm longi, vestigiales. Ovarium plusminusve cubiforme, 0.7 mm longum, 0.6 mm latum, vestigiale. Flores feminei 4-meri. Sepala 4, deltoidea, 0.2 mm longa, 0.2 mm lata, minute serrata, vestigialia. Petala 4, vestigialia. Stamina 0. Styli 4, clavati, 0.5 mm longi, stigmata fimbriata. Ovarium plusminusve cubiforme, 1.0 mm longum, 0.8 mm latum. Fructus plusminusve cubiformis, 1.3-1.4(-1.5) mm longus, 1.4-1.5 mm latus, stylis persistens et erectis, ubi maturis in mericarpia 4 findens. Mericarpia cylindrica, 1.4 mm longa, 0.8 mm lata, laevia et rotundata in pagina dorsali.

Perennial herb; stems 50-100 cm long, weak, simple or branched. Lvs dimorphic, whorled. Submerged lvs in whorls of (2)-3-(4), ovate, subsessile, (6)-10-15 × (5)-7-10-(15) mm, pectinate; pinnae 12-18, filiform, c. 8 mm long. Aerial lvs red-purple, in whorls of 3-(4), narrowly ovate to obovate, (2.5)-5-6-(12) × (1.5)-2-2.5-(4) mm, usually entire (lower or transitional lvs sometimes incised), obtuse. Fls solitary in axils of the aerial lvs, unisexual, the upper ♂, the lower ♀. Bracteoles lanceolate, (1)-1.4-1.8-(2) × (0.2)-0.3-0.4 mm. ♂ fls: sepals 4, 0.5-0.6 × 0.4-0.5 mm, deltoid; petals 4, hooded, 2-2.8 × c. 2 mm. ♀ fls: sepals 4, c. 0.2 × 0.2 mm; petals 4, c. 0.5 mm long. Frs ± globular or cubiform, 1.3-1.4-(1.5) × 1.4-1.5 mm, smooth and rounded on dorsal surface of mericarps, sometimes with a faint dorsal line.