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Key to Prunus, Key C: PLUMS, subgenus Prunus

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1 Flowers and fruits sessile or the pedicel < 3 mm long; flowers solitary; petals ca. 2 mm long; [endemic in c. FL peninsula]
1 Flowers and fruits on pedicels 0.5-20 mm long; flowers solitary or 2-5 per inflorescence; petals 3-14 mm long; [collectively widespread]
  2 Flowers 1-2 (-3) per inflorescence; stone somewhat sculptured; [exotic]; [section Prunus].
    3 First-year twigs green, shiny, glabrous; fruits 2-3 cm long, yellow to red when ripe; inflorescence of a solitary flower
    3 First-year twigs brown to gray, dull, often pubescent; fruits either 3-7 cm or 1-2 cm long, blue-black, yellow, or greenish when ripe; inflorescence of 1-3 flowers.
      4 Fruits 3-7 cm long, blue-black, yellow, or greenish when ripe; leaves 3-8 cm long; twigs somewhat to not at all thorny
      4 Fruits 1-2 cm long, blue-black when ripe; leaves 1-3 (-4) cm long; twigs very thorny
  2 Flowers (3-) 4-6 per inflorescence; stone nearly smooth; [native, though some species also cultivated and/or weedy]; [section Prunocerasus].
        5 Leaf teeth gland-tipped (or with a scar where the gland has fallen); sepals with marginal glands (except P. angustifolia); ripe fruit yellow, orange, or red.
          6 Leaves 3-6 cm long, often folded longitudinally; sepals lacking marginal glands
          6 Leaves (4-) 5-13 cm long, not folded (folded in P. munsoniana and P. rivularis); sepals with marginal glands.
             7 Leaves < 2× as long as wide; petals 10-15 mm long; [w. VA, c. IN, and c. IL northwards]
             7 Leaves > 2× as long as wide; petals 4-7 mm long; [mainly west of the Mississippi River and north of the Ohio River, but scattered southeastwards as both natives and introductions].
               8 Leaf teeth triangular, ascending, the gland terminal; flowers opening with the leaves; leaves not folded along the midrib
               8 Leaf teeth depressed, the gland near the sinus; flowers opening before the leaves
                 9 Leaf blades mostly (6-) 7-10 cm long; calyx lobes as long as the calyx tube; shrub or tree to 10 m tall
                 9 Leaf blades mostly 5-7 cm long; calyx lobes shorter than the calyx tube; shrub to 3 m tall
        5 Leaf teeth glandless (or if glandular, then sharp-tipped); sepals without marginal glands; ripe fruit yellow, orange red, purple-red, purple, or black.
                   10 Petals 10-15 mm long; leaves 6-10 cm long, acuminate; fruit 2-2.5 cm long, red or yellow.
                     11 Leaves narrowly to broadly cuneate at the base; petiole usually lacking glands near its junction with the leaf blade; sepals glabrous on the lower side
                     11 Leaves broadly rounded at the base; petiole usually with glands near its junction with the leaf blade; sepals pubescent on the lower side
                   10 Petals 4-9 mm long; leaves 2-8 cm long, obtuse, acute, or slightly acuminate; fruit 0.9-1.5 cm long, dark purple, black, yellow, orange, or red.
                       12 Twigs and pedicels pubescent, often densely so; leaf apices acute to obtuse, rarely rounded; leaf blades mostly < 2× as long as wide; shrubs to 2.5 m tall; [Coastal Plain of e. VA, and northward]
                          13 Upper leaf surfaces finely hairy; leaf blades 1.2.5 (-3.6) cm wide, > 2× as long as wide; [KS, w. AR, w. LA, TX, westwards]
                          13 Upper leaf surfaces glabrous; leaf blades 2-4 cm wide, < 2× as long as wide; [e. VA, e. MD, DE, e. PA, NJ northwards]
                       12 Twigs and pedicels usually glabrous, but occasionally pubescent; leaf apices acute, sometimes acuminate; leaf blades mostly > 2× as long as wide; shrubs or small trees, 2-6 m tall; [either inland in the Mountains and Piedmont from nw. NC northward, or Coastal Plain, Piedmont, and Mountains of s. NC, SC, GA, FL, AL, MS, LA, and westward].
                            14 Plants often suckering and forming thickets, less often single plants, shrubs or less often trees; leaf apices short acuminate (to merely acute); pedicels usually < 10 mm long; [Mountains and Piedmont from nw. NC and e. TN northward]
                            14 Plants usually not suckering, often single plants but sometimes in small colonies, generally trees; leaf apices mostly acute; pedicels usually > 10 mm long; [Coastal Plain, Piedmont, and Mountains of s. NC, SC, GA, FL, AL, MS, LA, and westward]