Ulmus

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  U. parvifolia

  1a. Flowers serotinous, appearing in late summer; samaras not winged; mature leaf blades 1.5–2.5 cm wide, with (4–) 5 or more lateral veins forking well before reaching margin per half of leaf
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  1b. Flowers precocious, appearing in early spring; samaras winged; mature leaf blades 2–8 (–10) cm wide, when less than 3 cm wide then no more than 3 lateral veins forking well before reaching margin per half of leaf
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    U. pumila

    2a. Mature leaf blades 2–3.5 cm wide, ± single serrate on the margin; samaras glabrous 
and eciliate
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    2b. Mature leaf blades 2.5–8 (–10) cm wide, double serrate (at least in the distal half) 
 [Fig. 920]; samaras pubescent and/or ciliate (though the pubescence sometimes limited to a small region of the fruit)
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      3a. Lateral veins of leaf blade not forking until near the margin (rarely some leaf blades with 1 vein forking well before reaching margin) [Fig. 919]; samara marginally ciliate through most or all of its margin
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        U. americana

        4a. Inflorescence a dense fascicle usually shorter than 2.5 cm; samaras glabrous over the surface of body and wing; branches lacking irregular corky wings
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        U. thomasii

        4b. Inflorescence a raceme-like cyme, up to 5 cm long; samaras pubescent over the surface of body and wing; branches often irregularly winged with 2 or more corky plates
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      3b. Usually 2 or more lateral veins of leaf blade forking well before reaching the margin [Figs. 920,921]; samaras eciliate or ciliate only near the apex (though pubescent on surface of body or wing)
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        U. rubra

        5a. Leaf blades with marginal cilia; winter bud scales red, pubescent with red tomentum near the margins; samara pubescent on only the body
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        5b. Leaf blades without marginal cilia; winter bud scales red-brown to brown or dark brown, with white or pale cilia near the margins; samara pubescent on only the central vein of wing or near apical margin
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          U. glabra

          6a. Leaf blade with a strongly inequilateral base, with one side partially concealing the petiole, the apex sometimes with 3 acuminate lobes [Fig. 920]; branches lacking corky wings; calyx with prominent lobes, the sinuses between them extending ca. half the distance to the base; samara elliptic to obovate, pubescent on central vein of wing, with the seed positioned near the middle
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          U. procera

          6b. Leaf blade with a weakly to moderately inequilateral base, neither side concealing the petiole, the apex without additional lobes; branches often irregularly winged with 2 or more corky plates; calyx with short lobes, the sinuses between them extending much less than half the distance to the base; samara ± orbicular at maturity (narrower during development), pubescent near apical margin, with the seed positioned distal to middle

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