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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 10-13 mm high
               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 F: 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 (or relatively uniformly whitened by hairs, especially on the lower surface).
          6 Disc flowers pink (rarely white)
          6 Disc flowers yellow to maroon .
             7 Pappus absent or of scales or barbellulate bristles.
               8 Inner phyllaries tipped with small, simple spines; cypselas 4-angled; receptacles epaleate but bearing subulate scales; pappus absent or of narrow scales 1-10 mm long
               8 Inner phyllaries tipped by pinnately divided spines > 5 mm long; cypselas terete, 20-ribbed; receptacles epaleate but bearing flattened bristles; pappus of 2 series of smooth or roughened awns, the outer series 9-10 mm long, the inner 2-5 mm long
             7 Pappus of numerous, basally connate, plumose bristles
                 9 Heads radiate, the laminae broad and yellow-colored; leaves toothed with small spines present; [tribe Astereae].
                   10 Phyllaries spine-tipped with accompanying white margins (if lacking spine tips as in X. texanum, then inner phyllaries abruptly enlarged into ovate or sometimes elliptic blades 2-5 mm wide)
                   10 Phyllaries not spine-tipped and lacking conspicuous white margins, 0.9-1.7 mm wide.
                 9 Heads discoid (Carlina with inner phyllary tips stramineous, and reminiscent of ray laminae); leaves with prominent, sharp spines; [tribe Cardueae].
                     11 Receptacle epaleate but scaly, each flower surrounded by connatemembranous scales, dissected upwards into linear lobes
                     11 Receptacle epaleate but covered with tawny to white bristles or setiform scales
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