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Keyed in multiple places:

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Key to Hydrangea

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1 Woody vine, climbing by adventitious roots; petals 7-10; [section Decumaria of clade Hydrangea I; or genus Decumaria]
1 Shrub; petals 4-5.
  2 Leaves pinnately lobed, the lobes toothed; inflorescence a panicle; large sterile flowers many (> 20 per inflorescence), borne throughout the inflorescence; [section undetermined (incertae sedis) of clade Hydrangea I; or genus Hydrangea s.s.]
  2 Leaves unlobed, merely toothed; inflorescence a corymb (except H. paniculata); large sterile flowers absent to relatively few (0-15 per inflorescence), borne around the periphery of the corymb (except H. paniculata).
    3 Inflorescence a panicle; large sterile flowers many (> 20 per inflorescence), borne throughout the inflorescence; [large shrub to small tree, to 8 m tall and 10 cm trunk diameter]; [exotic, cultivated and sometimes persistent]; [section Heteromallae of clade Hydrangea I; or genus Heteromalla]
    3 Inflorescence a corymb; large sterile flowers absent to relatively few (0-15 per inflorescence), borne around the periphery of the corymb; [small to medium shrub, to 3 m tall and 2 cm trunk diameter]; [native]; [section Hydrangea of clade Hydrangea I; or genus Hydrangea s.s.].
      4 Lower leaf surface glabrous or inconspicuously puberulent, appearing green; trichomes of the lower leaf surface restricted to the midrib and major veins; sterile flowers absent, or, if present, usually < 1 cm in diameter
      4 Lower leaf surface variously pubescent, appearing white or gray; trichomes of the lower leaf surface on veins and interveinal areas; sterile flowers usually present, large and showy, usually greater than 1 cm in diameter.
        5 Lower leaf surface velutinous, pilose, or tomentose, appearing gray; trichomes usually not dense enough to entirely mask the green leaf surface; trichomes with prominent tubercles (as seen at 40× magnification); sterile flowers generally very few (0-3 per inflorescence)
        5 Lower leaf surface densely floccose-velutinous, felt-like, appearing bright white or silver; trichomes dense enough to entirely mask the green leaf surface; trichomes without tubercles, or with small and inconspicuous tubercles (as seen at 40× magnification); sterile flowers generally fairly many (2-15 per inflorescence)

Key J1: woody plants with opposite, simple, palmately or pinnately lobed leaves

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  2 Leaves harshly scabrous on the upper surface; leaves typically a mix of alternate, opposite, and whorled
  2 Leaves glabrous or glabrescent on the upper surface; leaves strictly opposite
    3 Plants climbing by twining; stems with retrorse prickles; foliage scabrous
    3 Plants erect trees or shrubs; stems not prickly; foliage smooth or pubescent, but not scabrous.
      4 Leaves 3-9-lobed, the margins generally serrate or sublobed; fruit either a drupe or a schizocarp of 2 samaroid mericarps (maple “keys”).
        5 Fruit a schizocarp of 2 samaroid mericarps (maple “keys”); stamens (4-) 8 (-12); small to large trees; petioles >1× as long as the leaf blade
        5 Fruit a drupe; stamens 5; shrubs; petioles < ¾× as long as the leaf blade
      4 Leaves 3-lobed, the margins entire; fruit a capsule.
          6 Flowers white to yellow; capsules linear, >10× as long as wide; leaf undersurface with curly simple hairs; nectar glands present in the main vein axils on the undersurface of the leaf (visible from the underside or the upperside in fresh leaves and herbarium specimens)
          6 Flowers lavender; pods ellipsoid, < 2× as long as wide; leaf undersurface with branched (dendritic) stellate hairs; nectar glands absent
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