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Betula

Measurements of carpellate aments provided in the identification key are from infructescences that are fully mature and beginning to disarticulate. Measurements (especially width) taken from before this stage of maturation will be too narrow and cause problems in the key. Betula platyphylla Sukaczev has been attributed to MA on the basis of a collection from along the banks of the Neponset River— Svenson 11746 ( GH!). However, the specimen is clearly misidentified and has leaf blades that are of the wrong shape, are too large, and have too many pairs of lateral veins (among other discrepancies). Though the identity of the specimen is still in question, it is superficially similar to B. papyrifera.

See list of 2 species in 1a
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1a. Leaf blades crenate to crenate-dentate with rounded or bluntly pointed teeth, obovate to orbicular, rounded to obtuse at the apex [Fig. 484]
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  B. glandulosa
  2a. Leaf blades 0.5–3 cm long; branchlets with abundant, large resin glands; scales of carpellate ament with upturned lateral lobes; plants of alpine ridges and plateaus
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  B. pumila
  2b. Leaf blades mostly 2.5–5 (–7) cm long; branchlets without resin glands or with few, small resin glands, these most abundant near the nodes; scales of carpellate ament with widely divergent lateral lobes; plants of fens and swamps
See list of 9 species in 1b
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1b. Leaf blades obscurely to evidently doubly serrate with pointed teeth (the teeth often  blunt in B. minor), ovate, rhombic, or triangular to narrow-ovate or oblong-ovate, acute to long-acuminate at the apex
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  3a. Leaf blades ovate to narrow-ovate or oblong-ovate, the larger with 12–18 pairs of lateral veins; fresh branchlets with wintergreen odor; carpellate aments ± sessile; body of samara wider than the individual wings
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    B. alleghaniensis
    4a. Bark exfoliating, yellow to silver; scales of carpellate aments pubescent and/or ciliate, 6–13 mm long; leaf blades somewhat coarsely, and often irregularly, serrate, usually with fewer than 6 teeth per cm; branchlets often pubescent
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    B. lenta
    4b. Bark smooth, not exfoliating, brown; scales of carpellate aments glabrous, 5–7 mm long; leaf blades finely and regularly serrate, usually with 6 or more teeth per cm; branchlets often glabrous
- See list of 7 species in 3b
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  3b. Leaf blades ovate or narrow-ovate to rhombic or triangular, the larger with 2–12 pairs of lateral veins; fresh branchlets lacking wintergreen odor; carpellate aments borne on peduncles 5–15 mm long; body of samara as wide as or narrower than the individual wings (wider than wings in B. nigra and B. pubescens)
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    5a. Leaf blades triangular or rhombic-triangular to rhombic-ovate, acuminate to long-acuminate at apex [Fig. 486], glabrous to sparsely pubescent along and in the axils of major veins; central lobe of carpellate scales much shorter than the lateral lobes
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      B. pendula
      6a. Branches on mature trees usually pendulous; staminate aments mostly in pairs (sometimes solitary or in trios); carpellate aments (19–) 23–40 × (7–) 8–11 mm
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      B. populifolia
      6b. Branches spreading to ascending; staminate aments mostly solitary (sometimes paired); carpellate aments 10–25 (–30) ×6–8 mm
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    5b. Leaf blades ovate to narrow-ovate or rhombic-ovate (to rhombic in B. nigra), obtuse to acute or short-acuminate at the apex, usually sparsely to moderately pubescent along and in the axils of major veins; central lobe of carpellate scales as long as or longer than the lateral lobes
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      B. nigra
      7a. Leaves with rhombic-ovate blades and tomentose petioles; carpellate aments 10–25 mm thick in fruit; body of samara wider than the individual wings
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      7b. Leaves with broad-ovate to narrow-ovate blades (sometimes rhombic-ovate  in B. pubescens) and glabrous to pubescent, but not tomentose, petioles; carpellate aments 5–12 mm thick in fruit; body of samara as wide as or narrower than the individual wings
      - See list of 2 species in 8a
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        8a. Leaf blades 1.5–5.5 (–6) cm long, obscurely double serrate; mature carpellate aments 10–30 mm long
        
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        B. minor
        9a. Native shrubs to 3 m tall of alpine ravines and plateaus; bark dark brown; branchlets glabrous to sparsely pubescent with relatively soft hairs; lateral lobes of carpellate scales ascending
        
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        B. pubescens
        9b. Planted trees to 20 m tall, typically near areas of civilization; bark gray-white; branchlets pubescent with short, stiff, erect hairs; lateral lobes of carpellate scales divergent
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        8b. Leaf blades 5–10 (–14) cm long (sometimes shorter in high-elevation individuals), double serrate; mature carpellate aments 25–55 mm long
        
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        B. papyrifera
        10a. Leaf blades cuneate to truncate at the base, with 7–9 pairs of lateral  veins; scales of carpellate aments 3.9–6.2 mm long, with divergent, lateral lobes   [Fig. 485]; bark of mature trees white (rarely light brown to dark brown)
        
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        B. cordifolia
        10b. Leaf blades cordate at the base, with (8–) 9–12 pairs of lateral veins (with fewer pairs in dwarfed, high-elevation individuals); scales of carpellate aments 5.6–8.7 mm long, with upturned, lateral lobes [Fig. 483]; bark of mature trees pink-white to brown-white or red-brown-white

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