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Key to Pycnanthemum

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1 Leaves 1-15 mm wide (to 30 mm wide in P. setosum), mostly > 3× as long as wide (except in P. nudum); calyx lobes not tipped with a tuft of long, jointed bristles (except P. clinopodioides).
  2 Longer calyx lobes 1.5-5 mm long, attenuate-aristate, stiff, whitened; [Coastal Plain pinelands, rarely in Mountain bogs with Coastal Plain affinities].
    3 Principal stem leaves 5-15 mm wide
    3 Principal stem leaves 10-30 mm wide
  2 Longer calyx lobes 0.5-1.6 mm long, deltoid to narrowly triangular, not notably stiff (except in P. tenuifolium) or whitened; [widespread in our area, but mainly of the Piedmont and Mountains].
      4 Leaves 10-15 mm wide (or more often even wider, to 25 mm wide, in P. clinopodioides); longer calyx lobes 0.7-1.6 mm long, tipped with a few long (1-3 mm) jointed bristles (P. clinopodioides) or not tipped (P. nudum).
        5 Leaves 1-2.5× as long as wide, coriaceous; stems and leaves glabrous; [Coastal Plain, se. SC southward and westward to FL and AL]
        5 Leaves 3-6× as long as wide, herbaceous; stems and leaves pubescent; [inland provinces and Coastal Plain, NC northward]
          6 Calyx zygomorphic, the lower 2 teeth about 1.5× as long as the upper 3; [mainly Coastal Plain, disjunct in mountains; SC and NC northwards]
          6 Calyx actinomorphic, the teeth similar in shape and size, all 0.5-1.0 mm in length; [primarily inland]
      4 Leaves 1-12 (-15) mm wide; longer calyx lobes 0.5-1.5 mm long, variously pubescent but not tipped with a tuft of long jointed bristles.
             7 Leaves glabrous on the lower and upper surface, with 2-3 pairs of lateral veins; stems glabrous on the faces and angles (rarely with a few small upwardly-curled hairs on the angles).
               8 Leaves 5-15 mm wide, 1-2.5× as long as wide; calyx lobes and inner bracts of the inflorescence herbaceous
               8 Leaves 1-4 (-5.5) mm wide, 8-15× as long as wide; calyx lobes and inner bracts of the inflorescence semi-spinose, their tips subulate, thickened, and stiff
             7 Leaves pubescent at least on the lower surface along the midrib and main veins; leaves with 4-5 pairs of lateral veins; stems glabrous or pubescent on the faces, pubescent on the angles.
                 9 Stems pubescent on the angles only (or distinctly less pubescent on the faces); leaves 3-10 mm wide
                 9 Stems pubescent on the faces and angles, the hairs distributed more-or-less evenly; leaves 8-12 (-15) mm wide.
                   10 Longer calyx teeth 1.0-1.5 mm long, lanceolate and attenuate; bracts of the inflorescence and leaves glabrous or very sparsely pubescent on the upper surface
                   10 Longer calyx teeth 0.5-1.0 mm long, narrowly deltoid; bracts of the inflorescence (and usually also the leaves) canescent on the upper surface
1 Leaves broad, 15-40 mm wide, mostly 1.5-3× as long as wide; calyx lobes usually tipped with a tuft of long, jointed bristles (except P. curvipes, P. muticum, P. setosum).
                     11 Bracts of the inflorescence glabrous (or very sparsely pubescent) on the upper surface, the margins long-ciliate; calyx lobes and upper part (at least) of the tube with long spreading hairs (independent of the apical tufts)
                     11 Bracts of the inflorescence puberulent on the upper surface, the margins not ciliate; calyx lobes and tube variously glabrous or puberulent (independent of the apical tufts).
                       12 Calyx lobes not tipped with a tuft of long, jointed bristles.
                          13 Calyx lobes 1.5-3 mm long, attenuated into a subulate tip; [Coastal Plain]
                          13 Calyx lobes 0.5-1.2 (-1.5) mm long, triangular to narrowly triangular, acute to acuminate, but not subulate; [collectively widespread].
                            14 Calyx distinctly bilabiate, the lower 2 lobes 1.5-2.5× longer than the upper 3 lobes, and separated from each other and the upper 3 lobes by deeper sinuses
                            14 Calyx not distinctly bilabiate, all of the calyx lobes about the same length, the sinuses about the same depth.
                              15 Petioles (2-) 3-15 mm long; inflorescence corymbose, loose, the branches apparent; [dry, sandy or rocky habitats].
                                16 Leaves 6-10 cm long; terminal capitula 2.5-5 cm wide in fruit; [dry rocky woodlands; sw. NC, w. SC, and southward]
                                16 Leaves 4-6.5 cm long; terminal capitula 1.5-2.5 cm wide in fruit; [longleaf pine sandhills, se. GA and FL]
                              15 Petioles 0-2 (-3) mm long; inflorescence capitate, tight, the branches within the clusters not apparent; [moist habitats, widespread].
                                  17 Calyx lobes attenuate, with stiff hairs usually extending beyond the apex; anthers nonfunctional, included; stems with long hairs (0.1-0.6 mm long), clearly hairy to the naked eye
                                  17 Calyx lobes deltoid, with flexuous hairs not or barely extending beyond the apex; anthers functional, exserted; stems glabrate or with short puberulence (0.1 mm or less long), appearing glabrous to the naked eye
                       12 Calyx lobes usually tipped with a tuft of long, jointed bristles.
                                    18 Calyx not distinctly bilabiate, all of the calyx lobes about the same length, the sinuses about the same depth.
                                       19 Longer calyx lobes 1-2 mm long; [Mountains]
                                       19 Longer calyx lobes 2.5-3 mm long; [Coastal Plain]
                                    18 Calyx distinctly bilabiate, the lower 2 lobes 1.5-2.5× longer than the upper 3 lobes, and separated from each other and the upper 3 lobes by deeper sinuses.
                                         20 Leaves lanceolate, (10-) 15-25 mm wide, > 3× as long as wide
                                         20 Leaves ovate, 13-50 mm wide, < 3× as long as wide.
                                           21 Leaves of the lower and middle stem with lower surfaces glabrate, glandular-punctate, similar in color to the dark green upper surface; calyx 5-7 mm long
                                           21 Leaves of the lower and middle stems with lower surface canescent, distinctly paler than the dark green upper surface; calyx 3-6.5 mm long.
                                             22 Calyx lobes broadly triangular, their apices obtuse, acute, or somewhat acuminate; calyx tube > 2× as long as the longest (lower) calyx lobes.
                                               23 Pubescence of the stem of dense, very small downwardly-curved hairs, usually mixed with scattered longer and spreading hairs
                                               23 Pubescence of the stem of sparse, coarse, downwardly-curved hairs
                                             22 Calyx lobes narrowly triangular, their apices acuminate to attenuate; calyx tube 1-2× as long as the longest (lower) calyx lobes.
                                                 24 Mericarps 0.5-1.3 mm long, with a smooth surface, glabrous or with a few short hairs at the tip
                                                 24 Mericarps 1.2-1.5 mm long, with a rugose or pitted surface, densely pubescent at the tip.
                                                    25 Stems and lower leaf surfaces canescent, the short hairs often intermixed with longer, spreading ones
                                                    25 Stems and lower leaf surfaces with coarse, spreading hairs only
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