1Inflorescence a single terminal globose or semiglobose head. Stems leafy only towards the base. Lowest flower ♀ (exceptionally in R. rubra with a single stamen), upper one(s) ♂. Involucral bracts sheathless.
1'Inflorescence paniculate or consisting of 2-6 dense globose heads, in the latter case racemiform or spiciform. Stems leafy throughout. Lowest flower bisexual, the upper one(s) ♂ or ♀
2Nut laterally compressed (its edge towards the axis of the spikelet), turgidly biconvex, obovate or broadly obovate. Bristles sometimes as long as the nut, usually distinctly shorter, sometimes obsolete. Infructes- cence globose, brown. Stamens usually 3. Leaves and longest bracts almost keeled by the strong midrib.
2'Nut strongly dorsiventrally compressed (its face towards the axis of the spikelet), with concave to slightly convex sides, oblong-obovate to oblong. Bristles at least partly distinctly longer than the nut. Inflorescence usually semiglobose, pale brown. Stamens usually (1-)2. Midrib of leaves and bracts not prominent.
3Nut usually with obtuse edges.
3'Nut with sharp edges.
4Spikelets linear-lanceolate, 11-14 mm long. Bristles much exceeding the top of the style-base, the outer 3 densely long-plumose at the base, the longest (8-)11-12 mm long. Style-base about as broad as the nut.
4'Spikelets lanceolate, (6-)8-10mm long. Bristles about reaching the top of the style-base, at most slightly exceeding it, with glabrous or ciliate base, the longest 6-8 mm long. Style-base much narrower (about ½ as wide) as the nut
5Nut very small, obovate, 1½-1¾ mm long, shining brown when ripe, hispidulous at the top only. Longest bristles 3-4 times as long as the nut. Style-base distinctly constricted at the base, not decurrent on the shoulders of the nut
5'Nut oblong-obovate, 2-2¾(-3) mm long, blackish brown when ripe, papillose or setulose all over, sometimes almost glabrous. Longest bristles slightly longer than to about twice as long as the nut, rarely longer. Style-base not or hardly constricted at the base, often slightly decurrent on the shoulders of the nut.
6Nut papillose-tuberculate. Nut-bearing (5th) glume distinctly longer (c. 2 mm) than the next lower one. Ripe spikelets somewhat gaping, the bristles exserted (hence visible from the outside). Upper flower with 2(-3) stamens and 0-4 bristles. Very slender, with leaves 1-2 mm wide and stems ½-1 mm thick
6'Nut setulose to almost glabrous. Nut-bearing glume about as long as the next lower one. Spikelets not gaping at maturity, the bristles completely hidden by the glumes. Upper flower with 1-2 stamens, without bristles. Less slender, with leaves up to 4 mm wide and stems 1-2 mm thick.
7Inflorescence consisting of 2-6 dense globose heads, with stellately spreading spikelets, one head to each primary sheathless bract. Spikelets 1-flowered. Bristles capillary, flexuous, smooth or almost so.
7'Inflorescence paniculate, consisting of 1-4 superposed corymbiform anthelas each subtended by a long- sheathing bract. Spikelets 2 several-flowered. Bristles rigid, straight, scabrous, or absent.
8Apex of the nut suddenly narrowed ir’o a distinct neck. Nut 3½-5 mm long, smooth.
8'Apex of the nut obtuse or truncate, without neck. Nut 1¼-4 mm long, transversely wavy-ridged or irregularly plicate-rugose.
9Bristles present. Style-base compressed-conical to almost subulate, from slightly broader than long to much longer than broad. Leaves flat or canaliculate, not setaceous.
9'Bristles absent. Style-base saddle-shaped, i.e. much depressed and much broader than high. Nut 1¼-1½ mm long, coarsely transversely wrinkled. Leaves setaceous.
10Stems slender. Leaves 2-3(-5) mm wide, the sheaths of the cauline ones truncate at the orifice (without appendage). Spikelets with 2-4 ♀ flowers (upper ones often tabescent). Style-base ½ as long to as long as the nut. Glumes spirally arranged. Style deeply bifid. Nut 1½-2 mm long, finely transversely wrinkled
10'Stems rather stout to stout. Leaves (3-)4-20 mm wide, the sheaths of the cauline ones at the orifice with a membranous appendage opposite the blade. Only the lowest flower ♀. Style-base longer than the nut. Glumes distichous or almost so. Style shortly bilobed. Nut 2½-4 mm long.
11Style-base about as wide as the nut, deeply grooved on both sides. Nut irregularly plicate-rugose. Leaves 8-20 mm wide
11'Style-base much narrower than the nut, almost subulate, not grooved. Nut regularly transversely wrinkled. Leaves 4-6 mm wide