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Stems usually erect, rather closely set on a coarse rhizome, 1–6 dm, with chiefly cauline lvs, these 2–4, the blade mostly 2–15 cm × 0.7–1.5 mm, terete, aerenchymatously hollow, septate-nodulose; infl broadly ovoid to depressed, with divergent to widely ascending branches, up to nearly twice as long as wide, with few to many obpyramidal to subhemispheric, 3–10-ld glomerules; fls eprophyllate; tep lanceolate to lance-subulate, (2–)2.5–3 mm, subequal (or the pet a bit longer than the sep), the sep acute to subacuminate, the pet more often obtuse; stamens 6; fr unilocular, chestnut-brown to purple-brown, exsert, sharply ovoid-trigonous, tapering in the distal half, 2.5–4 mm, acute or seldom obtuse below the mucronate tip; seeds 0.5 mm; 2n=80. Bogs, wet meadows, and shores; circumboreal, s. in our range to R.I., W.Va., s. O., n. Ind., and Minn. A form with creeping stems, rooting at the nodes is rarely found.
Plants perennial, tufted or rhizome creeping, (10--)15--40(--60) cm tall. Stems terete, 0.8--1.5 mm in diam. Cataphylls absent to 2. Cauline leaves 2--6; leaf sheath auricles well developed, acute, free part ca. 2 mm, membranous; leaf blade terete or somewhat laterally compressed, 2.5--7.5(--10) cm × 0.8--1.4 mm, perfectly septate. Inflorescences usually broad; branches rigid, spreading; involucral bract erect to ascending, 0.7--3 cm; heads 5--30, hemispheric to toplike, 5--10(--15)-flowered. Perianth segments reddish brown with greenish midvein, lanceolate, 2.5--3 mm, subequal, apex acute. Stamens 6, 1/2--3/4 as long as perianth; filaments 0.7--0.9 mm; anthers oblong, 0.5--1 mm. Capsule dark brown, shiny, trigonous ovoid, 3--3.5 mm, apex sharply acute and tapered to a conspicuous tip. Seeds ovoid, 0.5--0.7 mm, reticulate. Fl. Jun--Jul, fr. Aug--Sep. 2 n = 80.
Extremely variable, 6-60 cm high, dark green, commonly reddish-tinged, loosely or densely tufted or forming open colonies, rarely erect, more often decumbent or prostrate and rooting, or occasionally floating; rhizomes usually short with short internodes or occasionally long and giving rise to mat-like colonies. Leaves terete to ± compressed with transverse septa ± visible externally. Inflorescence very variable, 1-15-(40) cm long. Flowers in small dark brown clusters of 4-8 at tips of branchlets. Tepals 2.5-3 mm long, all ± equal, acute. Stamens 6. Capsule 2.5-3.5-(4) mm long, > tepals, ovate-oblong, trigonous, acute, upper third tapering to a distinct mucro, glossy, dark brown to black.
