Stout aquatic or marsh-dwelling herb; stems to 2 m long, 4-5 mm diam. near base, glaucous, rooting freely from lower nodes, glabrous. Leaves all whorled, slightly dimorphic. Submerged leaves in whorls of (4-)5-6, oblanceolate in outline, rounded at apex, (1.7-)3.5-4.0 cm long, (0.4-)0.8-1.2 cm wide, pectinate, with 25-30 linear pinnae up to 0.7 cm long, the lower leaves usually decaying rapidly. Emergent leaves glaucous, in whorls of (4-)5-6, erect near apex, ± spreading in lower parts, narrowly oblanceolate in outline, rounded at apex, (1.5-)2.5-3.5 cm long, (0.4-)0.7-0.8 cm wide, pectinate, with (18-)24-36 pinnae in the upper 4/5 (lower 5-7 mm of leaf rachis naked), pinnae linear-subulate, 4.5-5.5 mm long, 0.3 mm wide, tips very shortly apiculate, slightly incurved. Numerous hydathodes at base of emergent leaves. Plant dioecious. Inflorescence an indeterminate spike with the flowers borne singly in the axils of the upper emergent leaves and subtended by 2 bracteoles. Leaves subtending flowers in whorls of 6, pectinate, ± identical to other emergent leaves in size and shape. Bracteoles subulate, 1.2-1.5.mm long, with (1-)2 short teeth in lower 1/3, sometimes almost trifid. Flowers (in New Zealand) only female, 4-merous, on pedicel c. 0.2-0.4 mm long. Sepals 4, white, deltoid, 0.4-0.5 mm long, 0.3 mm wide, denticulate with 1-several small teeth on each margin, smooth. Petals absent. Styles 4, clavate, 0.1-0.2 mm long, stigmas white, densely fimbriate. Ovary pyriform, 0.6-0.7 mm long, 0.6 mm wide, 4-ribbed longitudinally between sepals. Male flowers and fruits not developed in New Zealand.

Perennial mat-forming herb, aquatic to terrestrial, stems green, erect or prostrate, 2-10(-15) cm long, rooting at lower nodes, usually ± unbranched apart from rhizomatous portions at base. Leaves all entire, linear, 2-6 mm long, 0.5 mm wide, decussate, tip blunt to acute, sometimes almost acuminate, usually with a ± well defined midrib. Hydathodes 0-2. Inflorescence a terminal spike with the flowers borne singly in axils of the upper leaves which are indistinguishable from the other leaves. Each flower subtended by 2 bracteoles which are deltoid, 0.6-0.7(-1.3) mm long, 0.2-0.3mm wide, acuminate. Flowers unisexual, males and females often borne on separate stems, but if together, then males borne in upper axils, females below. Male flowers 4-merous, sessile or on pedicel up to 1 cm long. Sepals 4, (ob)lanceolate to narrowly (ob)ovate, 0.9-1.4 mm long, 0.3-0.5 mm wide, acute or obtuse, sometimes shortly mucronate, margins entire or minutely denticulate, Petals 4, red, hooded, weakly keeled, 1.3-1. 7(-2.5) mm long, 0.8 mm wide, reflexed at anthesis. Stamens 8; filaments short, lengthening to 2.5 mm at anthesis; anthers linear-oblong, 0.9-1.1(-1.6) mm long, 0.4 mm wide, 4-locular, shortly apiculate. Styles and ovary 0. Female flowers 4-merous, on pedicel 0.2-0.5(-1.0) mm long. Sepals 4, deltoid, 0.2 mm long, 0.2 mm wide. Petals 0. Stamens 0. Styles 4, alternating with sepals, 0.4 mm long, stigmas fimbriate, white, sometimes grading to pale pink. Ovary obpyriform, 0.6 mm long, 0.5 mm wide, channelled opposite sepals, 4-locular. Fruit sessile or shortly pedunculate, red-purple, ovoid to obpyriform, 1.0-1.5 mm long, 1.1-1.2 mm wide, deeply channelled opposite sepals, smooth to weakly striated, not verrucose; sepals usually persistent, membranous, 0.2 by 0.2 mm; styles persistent, becoming reflexed. Fruit splitting at maturity into 4 single-seeded mericarps. Mericarps ± globular with attenuate neck formed from persistent style.

Planta valde similaris subsp. pedunculato, sed in fructu laevi et stigmatibus albis ad roseola differt.

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.

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.

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.

Dioecious perennial terrestrial herb, stems prostrate (rarely erect, forming tight cushions to 7 cm high), branching freely, rooting at nodes, forming clumps to 10 cm in diameter, glabrous. Leaves all opposite, entire, obovoid, 1.8-3.5 mm long, (0.5-)0.7-1.1 mm wide, ± planar or slightly concave on upper surface, convex below, spreading or slightly upward-curving, glabrous.. Hydathodes absent. Rarely, in inundated plants, submerged leaves are formed, which are linear, 2 mm long, 0.2 mm wide. Inflorescence a terminal spike with the unisexual flowers borne in the axils of opposite upper leaves indistinguishable from lower (sterile) leaves. Male and female flowers apparently borne on separate plants. Bracteoles linear, 0.5-0.7 mm long, 0.1-0.2 mm wide. Hydathodes 0. Male flowers 4-merous, sessile, usually in axils of the uppermost pair of leaves only. Sepals 0. Petals 4, green at first, becoming white with a dark reddish spot at tip (rarely, completely dark red), hooded, non-unguiculate, 1.5-1.8 mm long, 0.7-1.0 mm wide, reflexed at anthesis. Stamens 8; filaments 0.2-0.3 mm long, elongating to 0.8-11.0 mm at anthesis; anthers yellow, oblong, 1.2-1.3 mm long, 0.4 mm wide, 4-celled, nonapiculate or very weakly apiculate. Styles 0. Ovary vestigial. Female flowers 4-merous, sessile, in axils of upper 4-8 pairs of leaves. Sepals, petals and stamens 0. Styles 4, ± sessile, stigmas white, fimbriate, capitate. Ovary green, shortly cylindrical, 0.8 mm long, 0.6-0.8 mm wide, longitudinally channelled between styles, rounded opposite styles, otherwise smooth. Fruit black, depressed-globular, 0.9-1.0 mm long, 1.1-1.4 mm diameter, suture between mericarps buff coloured; mericarps separating at maturity. Mericarps planar on inner faces, outer face convex, smooth or very weakly punctate, sometimes with a faint median longitudinal ridge.