Perennial monoecious aquatic or marsh herb 30–40 cm tall. Leaves: submerged leaves in whorls of 4 or 5, crowded, pectinate, with 22–27 pinnae, lamina 18–25 mm long; emergent leaves in whorls of 3 or 4 (some irregular), linear to terete, entire, lamina 5–15 mm long; in terrestrial plants all leaves intermediate between submerged and emergent types. Bracteoles ovate; of male flowers 0.8–1.2 mm long, entire or erose; of female flowers smaller. Flowers 4-merous, sessile, solitary in axils. Male flowers: sepals linear to lanceolate, 0.9–1.1 mm long. acute to obtuse; petals 2.8–3 mm long, tardily caducous, reddish; stamens 8. Female flowers: sepals and petals absent; ovary 4-locular; styles elongate, becoming reflexed; stigmas red or pink, fading to white. Fruit ±cubic, deep reddish purple; mericarps ovoid, 0.8 mm long, tapering to an obliquely truncate apex, aculeate-tuberculate.
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