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Key to Populus

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1 Winter buds not viscid; stamens 5-20.
  2 Stamens 12-20; scales of the catkins deeply fimbriate; petioles terete; [section Leucoides]
  2 Stamens 5-12; scales of the catkins dentate or with only 3-7 linear-triangular lobes; petioles strongly flattened laterally (90 degrees to the plane of the leaf blade), especially near the junction with the blade; [section Populus].
    3 Petioles strongly flattened laterally; leaves glabrous when mature (pubescent when young in P. grandidentata); [native trees].
      4 Leaf margins coarsely crenate- or undulate-toothed, with fewer than 12 (-15) teeth per side, the sinuses of the larger teeth 1.5-6 mm deep; leaves puberulent beneath when young (glabrate in age); buds gray-pubescent
      4 Leaf margins finely crenulate-serrulate, with (12-) 15-35 (-45) teeth per side, the sinuses 0.5-1.0 mm deep; leaves glabrous; buds glossy brown
    3 Petioles terete or nearly so; leaves densely pubescent (P. alba) or glabrescent (P. ×canescens); [exotic trees].
        5 Leaves of vigorous shoots palmately 3-5 (-7)-lobed (and also toothed); leaves densely white-tomentose beneath when young and mature
        5 Leaves of vigorous shoots merely toothed; leaves glabrescent when mature
1 Winter buds viscid (sticky and shiny as if recently varnished); stamens (15-) 20-80.
          6 Petiole terete or dorsally flattened (in the plane of the leaf blade), and often also channeled above; leaf blades dark green above, glaucous white beneath; leaf margin not translucent, finely serrate with teeth < 1 mm deep; [section Tacamahaca].
          6 Petiole laterally flattened (90 degrees to the plane of the leaf blade), especially near the junction with the blade; leaf blades light green above, often paler beneath but not distinctly whitened; leaf margin translucent, finely to coarsely serrate with teeth > 1 mm deep.
             7 First-year branches reddish-brown; leaves noticeably paler beneath; flattened portion of petiole < 2× as deep as wide; early leaves with > 20 teeth per side, the largest < 2.5 mm deep; [probable intersectional hybrid of section Aegeiros and section Tacamahaca]
             7 First-year branches yellow- to orange- brown; leaves nearly the same color above and below; flattened portion of petiole > 2× as deep as wide; early leaves usually with < 20 teeth per side, the largest < 2.5 mm deep; [section Aegeiros].
               8 Stigmas 3-4; stamens (30-) 40-80; [native tree, common]
                 9 Glands at leaf blade base 3-6; leaf apices short-acuminate; fruiting pedicels grading from short at the base of the catkin to longer near the apex, 1-13 (-17) mm long; [widespread in our area]
                 9 Glands at leaf blade base 2; leaf apices long-acuminate; fruiting pedicels relatively uniform in length, 1-6 (-8) mm long; [PA, OH, IN, IL, MO, OK, and TX and northwards and westwards]
               8 Stigmas 2-3; stamens (15-) 20-30; [exotic trees, rare out of cultivation].
                   10 Floral disk 2-4 mm wide; stigmas 2-3; ovules and seeds 6-14 (-20) per placenta
                   10 Floral disk 1-2 mm wide; stigmas 2; ovules and seeds 4-8 per placenta