[+][+][+] Bristles long, denticulate downward, or both ways in n. 11.
[++] Spikelets white or whitish, becoming tawny with age, perfecting only a single flower; stamens usually 2; bristles 9–12, or even 20.
8. R. álba, Vahl. Culm slender (1–2° high), triangular above; leaves narrowly linear or almost bristle-form; spikelets lanceolate, densely crowded in a head-like terminal corymb and usually one or two lateral ones; achene oblong-obovate with a narrowed base, scarcely longer than the flattened-awl-shaped tubercle, shorter than the bristles.—Bogs, Newf. to Fla., west to N. Ind., Minn., and Oregon. (Eu.)
[++][++] Spikelets chestnut-colored or darker in n. 10 and 11, few–several-flowered; stamens 3; bristles usually 6.
9. R. capillàcea, Torr. Culm 6–9´ high, slender; leaves bristle-form; spikelets 3–6 in a terminal cluster, and commonly 1 or 2 on a remote axillary peduncle, oblong-lanceolate (pale chestnut-color, 3´´ long); achene oblong-ovoid, stipitate, very obscurely wrinkled, about half the length of the (6, rarely 12) stout bristles, and twice the length of the lanceolate-beaked tubercle.—Bogs and rocky river-banks, N. Vt. to Penn., west to western N. Y. and Minn.—Var. levisèta, Hill. Bristles perfectly smooth. N. W. Ind.
10. R. Knieskérnii, Carey. Culm 6–18° high, slender; leaves narrowly linear, short; spikelets numerous, crowded in 4–6 distant clusters, oblong-ovate (scarcely 1´´ long); achene obovate, narrowed at base, equalling the bristles, twice the length of the triangular flattened tubercle.—Pine-barrens of N. J. (on bog iron ore exclusively) to Va.; rare.
11. R. glomeràta, Vahl. Culm 1–3° high; leaves linear, flat; spikelets numerous in distant clusters or heads (often in pairs from the same sheath), ovoid-oblong; achene obovate, margined, narrowed at base, as long as the lance-awl-shaped flattened tubercle, which equals the always downwardly barbed bristles.—Low grounds, N. Eng. to Fla., west to Mich. and N. Ind.
12. R. cephalántha, Gray. Culm stout (2–3° high); leaves narrowly linear, flat, keeled; spikelets very numerous, crowded in 2 or 3 or more dense globular heads which are distant (and often in pairs), oblong-lanceolate, dark brown; achene orbicular-obovate, margined, narrowed at base, about as long as the awl-shaped beak; bristles twice longer, stout, barbed downward and sometimes also upward.—Sandy swamps, Long Island to N. J. and Fla.
§ 2. CERATOSCHŒ̀NUS. Spikelets lanceolate, acuminate, in fruit flattish, cymose-panicled, of only one perfect and 1–4 staminate flowers; scales few; bristles rigid, minutely scabrous upward; style simple or barely 2-toothed, filiform and gradually thickened downward, in fruit persistent as an exserted, slender-awl-shaped, upwardly roughened beak, several times longer than the smooth flat obovate achene; coarse perennials; spikelets in flower 4´´, in fruit including the projecting beak about 1´ long.
13. R. corniculàta, Gray. (Horned Rush.) Culm 3–6° high; leaves about 6´´ wide; cymes decompound, diffuse; bristles awl-shaped, stout, unequal, shorter than the achene.—Wet places, Penn. to Fla., west to S. Ind. and Mo.