Plants mostly submerged. Stems ca. 25 cm. Leaves filiform, 3--4 cm × ca. 0.2 mm, 1-veined. Scapes 5--15, to 11 cm, 6-ribbed; sheath 2--2.5 cm; receptacle subglabrous or villous; heads dusky, subglobose, ca. 2.5 × 3 mm; involucral bracts reflexed, straw-colored, ovate, ca. 1 mm, membranous, glabrous; floral bracts orbicular to oblanceolate, abaxially white clavate hairy toward apex or glabrous. Male flowers: sepals spathelike, abaxially shortly hairy toward (2 or)3-lobed apex, lobes truncate at apex; petals 3, elliptic to subulate, small, anterior one slightly larger than others, adaxially white clavate hairy, abaxially white clavate hairy toward apex, gland black, sometimes absent; anthers black. Female flowers: sepals 3, free, boatlike to obovate, abaxially white clavate hairy toward margin; petals 3, free, oblanceolate-linear, unequal, membranous, gland black, sometimes absent; ovary 3-loculed; style 3-cleft. Seeds narrowly ovoid, 0.3--0.4 mm; testa hexagonally reticulate, prickles 1 or 2 per cell, linear, slimy coated. Fl. and fr. Aug--Oct.
Capitula blackish, small, seldom exceeding 4 mm. diam.
Stems elongate, clothed with numerous filiform leaves
Scapes up to 30 cm. high, 5–7-ribbed; sheaths shorter than the leaves, loose, parallel-sided, shortly 1–3-fid at the mouth.
Leaves filiform, yellow-green, flaccid, 3–10 cm. long, 0.1–0.3 mm. wide, 1-nerved, fenestrate, not sheathing at the base.
Aquatic; stem floating below the surface, slender, elongate, branching, up to 50 cm. or more long, densely clothed in numerous leaves, terminating in a stiff umbel of 1–20 scapes emergent above the water surface.
Seeds 0.4–0.5 mm. long, transversely white-papillose, mid-brown but often appearing blackish due to the adherent ovary-wall.
Capsule only very tardily dehiscent.
Female flowers: sepals subequal or the median smaller, deeply concave, the back gibbous with broadly rounded tip, spongy along the midline, infrequently lightly keeled and winged with a few white hairs on the upper keel and an acute tip, upper margins smooth or denticulate; petals subequal, linear-spathulate, rather spongy, glands small or absent, glabrous or a few white papillae at the tip; ovary black.
Male flowers: sepals oblong, concave, unequally connate into a spathe or almost free, glabrous or white papillose near the rounded tip; petals small, glandular; anthers 6, black.
Capitulum depressed-globose, (2–)3–5(–6) mm. diameter, blackish or white-pubescent; involucral bracts shorter than the capitulum width, blackish, scarious, obovate-oblong to suborbicular, broadly rounded, weakly reflexing at maturity; floral bracts cuneate-oblong, blackish, concave, acute, the inner sometimes white-papillose on the back; receptacle glabrous or thinly pilose; flowers trimerous, 0.9–1.4 mm. long, pedicellate, black.