Annual of perennial, without stolons. Stems tufted, erect or oblique, slender, often capillary, striate or somewhat angular-ribbed, smooth, bright green, 5-40 cm by 1/5-1 mm. Sheaths appressed, often purplish at the base, muticous or with a minute mucro at the oblique or very oblique mouth. Spikelet ovoid or lanceolate, acutish, terete, densely several-to many-flowered, 3-9 by 1½-3 mm, often pro-liferous at the base. Glumes membranous, appressed, oblong-ovate to lanceolate, obtuse, scarcely keeled, 1-nerved, pale ferrugineous, often tinged with red, with green midnerve and whitish, scarious margins, 1½-2 mm long. Bristles 6, slender, retrorsely scab-rous, slightly to distinctly longer than the nut, at first whitish, finally ferrugineous. Stamens 2(-3); anthers linear, shortly apiculate, 3/10-½ mm long. Style 3-fid. Nut trigonous, obovoid, smooth or obsoletely longitudinally striolate, shining, yellowish green to olivaceous, 4/5-1 by ½-3/5 mm; epidermal cells longitudinally oblong. Style-base shortly pyra-midal, triangular, as long as or slightly longer than wide, ⅓-½ as broad as the nut.
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Perennials. Rhizomes decumbent. Culms tufted, 10-40 cm tall, 1-1.5 mm thick, slightly firm, subterete, with several longitudinal thin ribs. Leaf sheaths 2; cauline sheaths blood-red but later becoming pale blood-red apically, 3-8 cm, mouth truncate, apex mucronate to aristate. Spikelet oblong to narrowly oblong-ovoid, 8-11 × 3-4 mm, densely many flowered, sometimes proliferous at spikelet base, apex obtuse to acute. Basalmost glume empty, amplexicaul for whole spikelet base; fertile glumes blood-red but green medially, tightly imbricate, oblong, ca. 2.2 × 0.9 mm, margin very narrowly hyaline, apex subrounded. Perianth bristles 6, longer than nutlet, slightly densely and shortly retrorsely spinulose. Stigmas 3. Nutlet yellowish to olivaceous, obovoid to ellipsoid, ca. 1 × 0.7 mm, 3-sided, sides convex, angles narrowly costate; persistent style base white, half-oblong, 1/4-1/3 as long and ca. 1/2 as wide as nutlet, spongy initially but hardened when dry, base cap-shaped, apex rounded but becoming pyramidal and acute when dry. Fl. and fr. May-Aug.
In shallow water, swampy places, ditches, wet rice-fields, 50-2800 m.
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