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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
               8 Involucres 15-25 mm high.
                 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
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