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Key to Bidens

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1 Plant aquatic, the leaves finely divided into segments < 0.5 mm wide; pappus awns 13-25 (-40) mm long
1 Plant terrestrial or wetland, but not aquatic, the leaf segments > 0.5 mm wide; pappus awns lacking or present, if present < 10 mm long.
  2 Inner cypselas more-or-less equally 4-angled, thickest near the middle and equally tapered to both ends; ray florets white, pink, or pale yellowish (or absent).
    3 Leaves 2-3× dissected, primary lobes > 20, the ultimate segments rounded to acute, 2-10 mm wide; ray florets pale yellowish
    3 Leaves mostly once-pinnate, primary lobes 3-7, the ultimate segments serrate and acute, 8-50 mm wide; ray florets white or absent.
      4 Ray florets 5-8, the ligule 5-16 mm long; cypselas 2-awned, the awns 1-2 mm long; outer phyllaries (8-) 12 (-16)
      4 Ray florets 0 (or if a few present, the ligule < 3 mm long); cypselas 3 (-5)-awned, the awns 1-3 mm long; outer phyllaries 7-10
  2 Inner cypselas flattened (if 4-angled, the alternating angles acute and obtuse), thickest toward the tip; ray florets yellow or orange (or absent).
        5 Most leaves simple, the margins dentate to serrate or incised (with 3-7 lobes).
          6 Leaves (except sometimes the lower) sessile; heads usually nodding, at least in age.
             7 Rays absent, or present and 2-15 (-18) mm long; pales (receptacular bracts) with tan or yellow tips; outer cypselas (3-) 5-6+ mm long, inner cypselas 4-8 mm long (the margins ± thickened or winged); pappus of (2-) 4 awns (1-) 2-4 mm long
             7 Rays present, (10-) 15-25 (-30) mm long; pales (receptacular bracts) with orange or red tips; outer cypselas 6-8 mm, inner cypselas 8-10 mm (margins not notably thickened or winged); pappus of 2-4 awns 3-5 mm long
          6 Leaves with a distinct petiole 1-4 cm long (this sometimes winged); heads erect.
               8 Rays 12-25+ mm long; cypselas 2.5-4.5 mm long, the margins not barbed or ciliate
               8 Rays absent or 2-5 (-12) mm long; cypselas (3-) 6-13 mm long, the margins sometimes barbed or ciliate.
                 9 Involucres usually campanulate to cylindric, sometimes ± hemispheric; disc florets (6-) 10-25 (-60); cypsela faces usually ± striate, sometimes tuberculate; [MD northward, estuarine].
                   10 Cypselas usually antrorsely strigillose throughout; cypsela angles weak or absent
                   10 Cypselas glabrous or sparsely setulose; cypselas angles strong to weak or rarely absent
                 9 Involucres campanulate to hemispheric or broader; disc florets (5-) 20-60 (-150+); cypsela faces usually smooth or tuberculate (not notably striate); [collectively widely distributed, in a diversity of wetland types].
                     11 Cypselas ± flattened, sometimes weakly 3 (-4)-angled and 3 (-4)-awned, the faces usually smooth, seldom notably tuberculate; disc corollas 4-lobed, light yellow; anthers usually pale
                     11 Cypselas (at least inner) usually ± 4-angled and 4-awned, the faces usually strigose or tuberculate; disc corollas 5-lobed, orange-yellow; anthers usually blackish
        5 Most leaves either 1-pinnately compound, the 3-5 (-7) leaflets petiolulate, or -1-2× pinnately lobed.
                       12 Ray florets 0, or rays 1-3, the laminae 2-3.5 mm long.
                          13 Calyculus bractlets (2-) 4 (-5), seldom ciliate; disc florets usually 10-20
                          13 Calyculus bractlets (5-) 8-16 (-21), usually ciliate; disc florets 20-150.
                            14 Calyculus bractlets (5-) 8 (-10); leaves usually 3 (-5)-foliolate
                       12 Ray florets (5-) 8-13, the laminae 10-30 mm long.
                              15 Cypselas 1.5-2 (-2.5)× as long as wide.
                                16 Cypselas (4-) 5-8 mm long, the margins usually barbed or ciliate, and often also corky-winged.
                                  17 Calyculus bractlets 8-12 (-16), these (4-) 5-7 (-12) mm long; calyculus bractlets glabrous or fine-pubescent on the lower surface; achenes 2.5-5 mm long
                                  17 Calyculus bractlets 12-21, these (6-) 8-12 (20) mm long; calyculus bractlets usually coarsely short-pubescent on the lower surface; achenes 5-7 mm long
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