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Key to Prunus, Key B: CHERRIES, subgenera Cerasus and Lithocerasus

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1 Plants shrubs, to 1.5 (-3) m tall; [subgenus Lithocerasus; section Microcerasus].
  2 Inflorescences of 1-2 flowers; drupes reddish, 10-15 mm long; [exotic, persistent from cultivation]
  2 Inflorescences of 2-5 flowers; [native].
    3 Youngest twigs minutely and densely puberulent (use 10× magnification); leaf blades ca 2.6× as long as wide; plants erect, stones mostly orbicular, (5-) 6-8 (-9) mm long, 5-6 mm wide; leaf apices usually obtuse, sometimes acute or rounded; [plants of acid sandy or rocky upland barrens, savannas, or woodlands]
    3 Youngest twigs usually glabrous, sometimes sparsely puberulent; leaf blades ca. 3.3-3.7× as long as wide; plants decumbent or erect; stones fusiform, ovoid, or subglobose, 6-9.5 mm long, 4.5-5 mm wide (P. pumila var. depressa) or 6-7 mm wide (P. pumila var. pumila); leaf apices short-acuminate, acute, or obtuse; [usually in open, sandy or gravelly habitats].
      4 Stems prostrate to decumbent; leaf blades oblanceolate, averaging 3.7× as long as wide; stones ovoid to fusiform, 4.5-5 mm wide; [s. PA northward through New England]
      4 Stems usually erect-ascending, sometimes decumbent or sprawling; leaf blades elliptic, obovate, or oblanceolate, lengths ca. 2.9-3.3× as long as wide; stones subglobose to ovoid, 5-7 mm wide; [endemic to sandy coastal areas of the Great Lakes; known in our area only from Presque Isle, PA]
1 Plants trees, well over 3 m tall when mature; [subgenus Cerasus].
        5 Leaf serrations single to double, the tips of the serrations acuminate to attenuate; leaf blades strongly acuminate to attenuate-caudate.
          6 Leaf serrations attenuate; tree with upright form; leaf apices attenuate-caudate; twigs, petioles, and leaf blades glabrous; [subgenus Cerasus, section Sargentiella]
          6 Leaf serrations acuminate; tree with weeping or upright (spreading) form; twigs, petioles, and leaf blades hairy (the blades at least along the veins on the lower surface); [subgenus Cerasus, section Microcalymma]
             7 Petioles 10-20 mm long.
               8 Leaf blade base obtuse to rounded; leaf blades < 1.7× as long as wide
               8 Leaf blade base cuneate; leaf blades > 1.8× as long as wide
        5 Leaf serrations small and obscure or well-developed but rounded to acute; leaf blades acute to acuminate.
                 9 Petals 4-7.5 mm long; fruit < 1 cm in diameter; [subgenus Cerasus, section Phyllomahaleb].
                   10 Inflorescence with a central axis, thus nearly or actually racemose; fruit blackish; leaves 1-1.5× as long as wide; [exotic tree]
                   10 Inflorescence umbellate to corymbose, the central axis absent or poorly developed; fruit red; leaves 2-5× as long as wide; [native tree]
                 9 Petals 9-15 mm long; fruit 1.3-2.5 cm in diameter; [subgenus Cerasus, section Eucerasus].
                     11 Leaves 7-15 cm long, persistently hairy beneath, at least along the midrib and veins; pair of petiolar glands on the petiole near the blade; fruit sweet when ripe
                     11 Leaves 4-8 cm long, glabrous beneath once fully-expanded; pair of petiolar glands on the base of the leaf blade; fruit sour when ripe
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