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Keyed in multiple places:

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Key to Centaurea

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1 Phyllaries evidently spine-tipped.
..2 Leaf bases not decurrent on the stem, the stem merely angled; pappus absent; corollas pink-purple.
....3 Central spines of the principal phyllaries 10-25 mm long
....3 Central spines of the principal phyllaries 1-3 mm long
..2 Leaf bases decurrent on the stem as wings (only shortly so in C. benedicta); pappus present in at least the central flowers in the head; corollas yellow.
......4 Heads sessile, closely subtended and partially concealed by large foliar bracts
......4 Heads obviously pedunculate, lacking large foliar bracts subtending the head.
........5 Larger spines of the middle and outer phyllaries 5-9 mm long; marginal and central flowers of the head with pappus
........5 Larger spines of the middle and outer phyllaries 11-22 mm long; marginal flowers of the head lacking pappus
1 Phyllaries not spine-tipped.
..........6 Plant an annual; flowers pale to medium blue, flowering Apr-Jun
..........6 Plant a perennial; flowers pink to purple, flowering Jun-Oct.
............ 7 Phyllary appendages tapering to long, often recurved, pectinately dissected, filiform tips
............ 7 Phyllary appendages obtuse to acute, erect or ascending.
............ ..8 Involucres 15-25 mm high.
............ ....9 Phyllary appendages evidently decurrent along phyllary margins
............ ....9 Phyllary appendages not or only slightly decurrent along phyllary margins.
............ ......10 Phyllary appendages roundish (seldom triangular), scarious, light to dark brown, undivided to irregularly lacerate
............ ......10 Phyllary appendages more-or-less triangular, brown to black, more-or-less wholly pectinate-margined.
............ ........11 Heads discoid (the peripheral florets not expanded and showy); pappus blackish, < 1 mm long; green parts of phyllaries nearly or completely covered by black appendages, the involucres thus appearing totally black
............ ........11 Heads radiate (the peripheral florets expanded and showy); pappus absent or rudimentary (when present usually not black); green part of phyllaries sometimes evident, or the appendages light to dark brown.
............ ..........12 Heads relatively broad, the pressed involucres usually as wide as or wider than long; green parts of phyllaries usually covered by brown, variously pectinate fimbriate appendages, the involucres thus light to dark brown
............ ..........12 Heads relatively narrow, the pressed involucres usually longer then wide; green parts of phyllaries not fully covered by black appendages, the involucres black and green

Key to Asteraceae, Key E: herbaceous composites with leaves spiny, leaves alternate or basal, and heads discoid

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1 Heads appearing spherical (the ‘visual head’ actually a secondary head aggregated of numerous 1-flowered heads); disk flowers blue (rarely almost white)
1 Heads not spherical; disk flowers pink (rarely yellow).
..2 Stem winged, the wings armed with spines.
....3 Pappus of plumose bristles
....3 Pappus of barbellulate bristles.
......4 Cypselas borne on the surface of the receptacle, intermixed with setiform scales; foliage glabrous or more loosely and sparsely pubescent
......4 Cypselas borne in the pits of the naked honeycombed receptacle; foliage densely white-tomentose
..2 Stem not winged.
........5 Leaves green with white mottles
........5 Leaves green.
..........6 Disk flowers pink (rarely white)
..........6 Disk flowers yellow.
............ 7 Pappus of numerous plumose bristles
............ 7 Pappus absent or of scales or barbellulate bristles.
............ ..8 Cypselas 4-angled; receptacles bearing subulate scales
............ ..8 Cypselas terete, 20-ribbed; receptacles bearing flattened bristles
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