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Scirpus
See list of 12 species in this genusHybridization is not uncommon in this genus. Hybrid plants can be recognized by low fruit set and frequently by their elongated spikelets (it has been hypothesized that the spiklet axis continues to elongate when resources are not devoted to fruit formation). The perianth bristles are usually either retrorsely barbellate (at least near the tip) or highly bent and contorted. These features help to separate the bristles from the filaments, which may be persistent on the fruits. References: Schuyler (1967), Whittemore and Schuyler (2002).
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1a. Perianth bristles at most 1.5 times the length of the achene, retrorsely barbed (often smooth in S. georgianus), straight or merely curved or perianth bristles entirely absent
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2a. Leaves 14–22 (–26) on each stem; floral scales red-brown, orbicular or nearly so; spikelets mostly broad-ovoid
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2b. Leaves 3–12 on each stem; floral scales green-brown or brown to black, elliptic to broad-elliptic; spikelets cylindrical or narrow-ellipsoid to narrow-ovoid or ovoid (rarely broad-ovoid, as sometimes in S. cyperinus)
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3a. Perianth bristles barbed in the apical 60–90% with sharp, stout, thick-walled teeth; achenes 0.6–1 mm wide
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4a. Lower sheaths green or white-brown to brown; branches of inflorescence arching to drooping; plants cespitose, with short, brown rhizomes; achenes 0.6–0.8 mm wide
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4b. Lower sheaths conspicuously anthocyanic; branches of inflorescence ascending to divaricately spreading; plants spreading by long, anthocyanic rhizomes; achenes (0.6–) 0.8–1 mm wide
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5a. Styles bifid; achenes lenticular to plano-convex, with firmly attached, persistent perianth bristles; branches of inflorescence scabrous only in the distal portion
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5b. Styles trifid; achenes compressed-trigonous, with weakly attached, sometimes caducous, perianth bristles; branches of inflorescence antrorsely scabrous their entire length
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3b. Perianth bristles smooth or barbed only in the apical 10–60% with rounded, slender teeth; achenes 0.3–0.6 mm wide
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6a. Perianth bristles numbering 0–3 per flower, up to 0.7 times as long as the achene, smooth or with barbs only near the very apex
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6b. Perianth bristles numbering mostly 5 or 6 per flower, more than 0.6 times as long as the achene, barbed in the apical half
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7a. Longer perianth bristles shorter than to equaling the length of the 0.8–1.1-mm- long achenes; floral scales appearing black under low magnification; spikelets 2–3.5 mm long; leaf blades 5–9 (–10) mm wide; basal leaves and sheaths usually with few and inconspicuous cross-septa
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7b. Longer perianth bristles exceeding the length of the 1–1.3-mm-long achenes; floral scales appearing brown under low magnification; spikelets mostly 3.5–5 mm long; leaf blades 7–17 (–18) mm wide; basal leaves and sheaths with many prominent cross-septa
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1b. Perianth bristles greatly exceeding the achenes when extended, smooth, bent and curled
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8a. Perianth bristles largely contained within the floral scales, even in fruit, and therefore not obviously visible; achenes 1–1.2 mm long; floral scales with evident green midribs, even at maturity of the fruit
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8b. Perianth bristles exserted beyond the floral scales, in fruit providing a “woolly” aspect to the inflorescence; achenes 0.6–1 mm long; floral scales with pale, brown, or relatively inconspicuously colored midribs
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9a. Achenes red-brown; floral scales 2–3.1 mm long; plants spreading by means of elongate rhizomes, often forming large circular colonies; base of involucral bracts glutinous
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9b. Achenes white to pale brown; floral scales 1.1–2.2 mm long; plants with short, branching rhizomes, forming dense tussocks; base of involucral bracts not glutinous
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10a. Spikelets mostly or all in glomerules of 2 or more; floral scales mostly shorter than 1.5 mm; achenes maturing in August and September
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10b. Spikelets mostly or entirely solitary, sessile or pedicellate; floral scales 1.4–1.8 mm long; achenes maturing in late June through July
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11a. Involucels and base of involucral bracts very dark brown to black; stems mostly 1–3 mm thick below the inflorescence; leaf blades 2–5 mm wide; floral scales rounded at apex or with a very tiny mucro to 0.05 mm long; achenes maturing late June through early July
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11b. Involucels and base of involucral bracts brown, sometimes with a dark edge; stems mostly 2–5 mm thick below the inflorescence; leaf blades 3–10 mm wide; floral scales rounded at the apex to, more commonly, mucronate, the mucro to 0.1 mm long; achenes maturing in mid- through late July
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Each photo represents one species in this genus.