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Click the number at the start of a key lead to highlight both that lead and its corresponding lead. Click again to show only the two highlighted leads. Click a third time to return to the full key with the selected leads still highlighted.

Key to Cyperus, Key E: Key to subgenus Cyperus – stigmas 3; achenes trigonous; spikelets borne in spikes on a well-developed rachis;
rachilla continuous, or articulate only at the base

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1 Leaf blades absent; culms 15-50 mm thick, nearly terete; inflorescence open and diffuse, primary rays 10-30 cm long, bearing numerous 2nd and 3rd order rays up to 20 cm long and bracts to 25 cm long, often drooping; anther connectives subulate, projected beyond the anther 0.2-0.5 mm; [exotics, commonly cultivated, rarely naturalizing]; [subgenus Cyperus, section Papyrus]
..2 Rays stiff, ascending to erect; primary bracts (8-) 20-45 cm long
..2 Rays slender, drooping or arching; primary bracts 3-8 cm long
1 Leaf blades well-developed (except C. articulatus); culms < 20mm thick, trigonous if broader than 5 mm; inflorescence smaller, more congested (except C. prolixus); anther connectives not prolonged.
....3 Spikelets compressed-quadrangular or compressed-subterete, mostly 2× or more as wide as thick (less compressed in C. erythrorhizos and some forms of C. esculentus and C. rotundus); scales 1-many, deciduous or persistent; rachilla persistent or deciduous after the scales.
......4 Spikes long-cylindric, with (25-) 35-80+ spikelets, and well-developed branches or 2nd order rays; scales deciduous, minute, (1.1-) 1.3-1.8 mm long, (6-) 12-20 (-30) per spikelet; spikelets quadrangular, slightly compressed (subterete in C. alopecuroides); achenes (0.4) 0.7-1 (-1.2) mm long, often moderately dorsiventrally compressed or plano-convex; plants coarse perennials, culms 5-25 dm tall, 5-50 mm in diameter (C. erythrorhizos keyed under both leads).
........5 Stigmas 2, or 2 and 3 on a plant; spikes narrowly cylindric, the rachis obscured by the densely packed, ascending, imbricate spikelets; spikelets subterete, somewhat compressed, ovoid to oblong; rachilla wings absent to 0.1 mm wide; achene lenticular, its face against the rachilla [rare waif, FL]; [section Alopecuroides]
........5 Stigmas 3; spikes less dense, spikelets spreading to divaricate; spikelets quadrangular, linear; rachilla winged, wings 0.3 mm wide; achenes not as above [collectively widespread]; [section Fastigiati].
..........6 Plants perennial, with indurate rhizomes (sometimes inconspicuous); roots not reddish; floral scales 1.6-1.8 mm long; anthers 0.4-0.5 mm long; culms robust, trigonous, 5-25 dm tall, 5-15 mm wide
..........6 Plants annual; roots reddish, fibrous; floral scales 1.3-1.5 mm long; anthers 0.2-0.3 mm long; culms shorter, mostly rounded, (0.05-) 0.5-2.5 (-10.5) dm tall, 1-7.5 mm wide
......4 Spikes not as above; scales persistent or deciduous, mostly > 1.8 mm long (except sections Iriodei and Aristati); achenes > 1 mm long, trigonous (dorsiventrally compressed in C. nipponicus and C. serotinus); culms mostly < 10 dm tall, < 7.5 mm in diameter (except C. prolixus).
............ 7 Spikelets 1.2-1.5 (-1.6)mm wide, only compressed proximally, filiform distally; adjacent scales spaced apart on the same side of the rachilla; rachilla wings absent or very reduced; [subgenus Cyperus, section Distantes]
............ 7 Spikelets mostly broader, not filiform distally; scales apices reaching or exceeding the base of the next scale on the same side of the rachilla; rachilla wings present or absent.
............ ..8 Scales strongly 5-nerved, obovate-orbiculate, 1-1.5 mm long, divaricate, scarcely overlapping, apex emarginate and mucronulate; style < 0.1 mm long; inflorescence diffuse, secondary rays present, spikelets widely spaced on elongate rachises; [weedy exotics]; [section Iriodei]
............ ....9 Mucro ≤ 0.12 mm long; spikelets appressed to erect-ascending
............ ..8 Scales variously nerved, but at least 1.8 mm long (if shorter, with an excurved cusp > 0.5 mm long), not emarginate; style at least 0.5 mm long; inflorescence generally more congested.
............ ......10 Scales cuspidate to awned, midrib excurrent at least 0.3 mm, conspicuously excurved (straight in C. compressus and C. nipponicus); plants either diminutive fibrous-rooted annuals, culms often decumbent, or cespitose perennials (section Elegantes) with involute leaves and vegetative parts viscid (sticky).
............ ......10 Scales acute, midrib not notably projecting, apices not excurved; plants erect, rhizomatous, stoloniferous, cormose-based, or cespitose perennials, vegetative parts not viscid, leaves plane or V-shaped (cuspidate perennial species also keyed above).
............ ........11 Stigmas 2; achenes dorsiventrally compressed; spikelets subterete, elliptic in ×-section; [subgenus Juncellus, section Stoloniferi]
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