Aquatic or semiterrestrial. Stem branched, erect. Immersed leaves in whorls of (4-)5-6, 2-5 by 1½-4 cm in outline, with c. 12-24 pairs of c. 10-20 mm long, filiform, acute, brown-tipped segments. Lowermost aerial leaves mostly pinnate and rather stiffly spreading with short spreading lobes. Middle and upper aerial leaves mostly narrowly triangular in outline, 2-4 by ½-1 mm, with 6-12 pairs of erecto-patent, subulate, brown-tipped, very acute lobes, more rarely lanceolate to linear-lanceolate, 6-12 by 1-1½ long, with few, ± appressed subulate teeth, or almost entire, at first erecto-patent, in fruit recurved. Flowers solitary, ± sessile in the middle and upper leaf-axils, ☿, or the upper ♂. Bracteoles digitate or sometimes ± pinniform, 0.6-1 mm long, both with c. 5 subulate lobes. Sepals triangular, 0.1-0.3 by 0.1-0.3 mm, entire or finely serrate, acute, erect. Petals almost entire, c. 1(-1½) mm long, caducous after anthesis. Stamens 4; anthers ± oblong, 0.6-0.8 by 0.2-0.25 mm, obtuse. Fruit cruciform, c. 2 by 2 mm, mericarps ± ovate, with convex back and ± flattened sides, irregularly and finely tuberculate to smooth.
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Plants monoecious. Stem few branched, to 200 cm. Submerged leaves 5-whorled, pectinate, ovate to oblong in outline, ca. 4 × 10 cm; segments in 10-20 pairs, filiform, to 1.3 cm; emergent leaves narrower. Inflorescence a terminal spike of 5-whorled flowers; bracts entire at margin; bracteoles digitate at margin. Flowers 4-merous. Sepals triangular, ca. 1 mm, margin entire or finely serrate. Petals spatulate, ca. 1 mm, caducous. Fruit 4-loculed, subcylindric, ca. 3 mm; mericarps sparsely tuberculate to smooth. Fl. and fr. Mar-Sep.
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