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Key to Mentzelia

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1 Stamen filaments all filiform (or the outer 5 narrowly spatulate, but not petaloid); petals yellow to orange; [section Mentzelia].
  2 Flowers and fruits sessile, or the pedicels to 2.5 mm long and about 2 mm in diameter (the pedicel thus not visually differentiated from the capsule base); [IL, MO, AR, and e. TX westwards]
  2 Flowers and fruits on pedicels 0.6-5 mm long and < 1 mm in diameter, well-differentiated from the capsule; [either peninsular and n FL or c. and s. TX westwards].
    3 Outer 5 stamen filaments narrowly spatulate; [FL]
1 Stamen filaments dimorphic, the outer 5 stamens dilated and petaloid; petals white or yellow; [section Bartonia].
      4 Petals white.
        5 Petals 47-75 mm long, 13-23 mm wide; stamens white to yellow
        5 Petals 22.6-49 mm long, 3.6-10.3 mm wide; stamens white
      4 Petals yellow.
          6 Upper leaf bases clasping; petals 5.7-9.2 mm long
          6 Upper leaf bases sessile; petals (9.6-) 11.1-14.1 (-22) mm long