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Stems 8-20 cm tall, 0.3-0.5 mm in diam. Leaves often in pseudowhorls of 5, ca. 2 cm × 0.3-0.5 mm; sheath 1-2 mm; auricles orbicular to slightly obcordate, short, minutely serrulate with 6 or 7 teeth, upper margin on each side minutely serrulate with 7-11 teeth. Plants monoecious; flowers 1-4 per axil, male flowers in upper axils and female ones throughout. Male flowers elliptic, 1-1.5 mm; spathe with a short neck, with brownish spine cells at apex; anther 1-thecous. Female flowers conspicuous, 2-3 mm; style 1-2 mm; stigmas 2-lobed. Fruit linear-ellipsoid, 2-3 × ca. 0.5 mm. Seeds narrowly ellipsoid, with more than 20 rows of pits; areoles oblong, longitudinally elongated. Fl. and fr. Jun-Aug. 2n = 12, 24.
Monoecious; light green; stems 0.5–5 dm, only 0.2–0.7 mm thick, sparingly branched; lvs lax, 0.5–3 cm × 0.1–0.5 mm, spreading to ascending, minutely serrulate with 13–17 unicellular teeth per side, the base abruptly expanded and minutely fringe-toothed across the subtruncate summit (but not down its sides); anthers monothecal and with a single microsporangium; style offset from the apex of the fr; seeds 2.0–3.2 mm, light brown, fusiform-cylindric, pitted with 20–45 longitudinal rows of minute areolae a little higher than wide. Oligotrophic, soft-water lakes; N.S. to Ala., w. to Minn. and Io., intolerant of pollution and now become rare.
