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Keyed in multiple places:

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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.
      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 .
               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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