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Keyed in multiple places:

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Key to Crataegus, Key A: hawthorns with leaf blades widest below midpoint;
blade bases subcordate, truncate, rounded, or abruptly contracted

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1 Primary lateral veins of lobed leaves run to sinuses between lobes as well as to points of lobes, in some or most leaves [see also Crataegus viridis].
  2 Leaves usually pubescent or villous abaxially or on petiole, especially when young; inflorescence hairy (villous, pubescent or tomentose).
    3 Styles usually 2; fruit < 9 mm broad, usually oval or elliptic in shape; [widespread species]
    3 Styles 3 to 5; fruit > 9 mm broad, subglobose; [rare Arkansas endemic nothospecies]
      4 Short-shoot leaf blades 2-3 cm long, often broad as long
      4 Short-shoot leaf blades 3-4 cm long, longer than broad
  2 Leaves and inflorescence glabrous [commonly cultivated species]
        5 Leaf lobes with few serrations; pyrene 1 per fruit; thorns usually 1-2 cm long; [exotic]
        5 Leaf lobes with numerous serrations; pyrenes 5 per fruit; thorns usually 3-5 cm long; [native]
1 Primary lateral veins of lobed leaves run only to lobe points, excepting on vigorous sprouts.
          6 Sepals foliaceous, equal or exceeding petal length in flower and persistent on fruit
          6 Sepals not foliaceous, much shorter than petals.
             7 Leaves abaxially with whitish hair tufts in proximal main vein axils, especially in spring; [Virides series and hybrids].
               8 Leaves often ovate to rhomboid or suborbicular, bases cuneate or rounded.
                     11 Leaves pubescent, particularly when young; leaves ovate or rhomboid, often lobed, bases cuneate; [interserial hybrids].
                       12 Leaves most pubescent abaxially; teeth and lobes subacute; sepals subentire.
                              15 Terminal shoot leaves often suborbicular, lobed 20% or less to midrib.
             7 Leaves lacking hair tufts; hairs, if present, dispersed over veins or surface of leaves.
                                       19 Anthers 1.5-3 mm long; fruit usually soft-fleshed when mature; [widespread in southern Midwest]

Key to Crataegus, Key B: hawthorns with leaf blades widest at midpoint or beyond midpoint; blade bases acute or cuneate;
leaves conspicuously glandular on petiole and teeth; twigs and branchlets geniculate

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1 Leaf blades usually < 12 mm wide and < 2 cm long (on floreal shoots).
  2 Leaf blades often nearly wide as long, blade broadly obovate or suborbicular.
    3 Leaves denticulate or remotely toothed, rarely lobed; petiole glabrate; shrub usually < 1m tall
    3 Leaves sharply toothed, often lobed; petiole hairy; shrub or tree > 1 m tall
  2 Leaf blades 1.5-2× long as broad, mostly spatulate or oblanceolate.
      4 Leaves and flower pedicels glabrate
      4 Leaves and flower pedicels pubescent to tomentose.
          6 Thorns 1-3 cm long; plant usually < 1 m tall
          6 Thorns 3-6 cm long; plant usually 1-2 m tall
1 Leaf blades mostly > 12 mm wide and > 2 cm long.
             7 Leaf margin mostly subentire; may have glandular-denticulate teeth or 3 lobelike points on distal part of blade.
               8 Leaves often with 3 lobelike points near apex.
               8 Leaves denticulate or subentire to apex, rarely with any lobelike points.
                   10 Thorns abundant, present at nearly every node and > 3 cm long
                   10 Thorns less numerous, most < 3 cm long.
             7 Leaf margin toothed along at least half of margin, the teeth fine, acute, obtuse or dentate.
                       12 Floreal leaves often rhomboid or broadly elliptical; twigs and branches usually not drooping.
                          13 Leaves distinctly sharply toothed and/or lobed; blades broadly ovate on terminal shoots and > 3.5 cm long
                          13 Leaves with small or blunt teeth, mostly unlobed; blades rhombic, obovate or orbicular on terminal shoots, < 3.5 cm long.
                            14 Stamens 10 per flower; terminal shoot leaves broadly rhombic to suborbicular
                            14 Stamens 20 per flower; terminal shoot leaves usually orbicular or broadly obovate
                       12 Floreal leaves mostly obovate or spatulate; leading twigs or branches drooping or pendulous.
                              15 Leaf apex rounded or with an abrupt short point, margins finely or sharply toothed beyond middle of blade; terminal shoot leaves unlobed to very obscurely lobed.
                              15 Leaf apex 3-lobed or with 3-5 distinct points or several larger teeth; terminal shoot leaves moderately to deeply lobed.
                                               23 Leaves obovate or sub-rhombic, with 3-5 pairs of main lateral veins; fruit 12-15 mm, flesh soft.

Key to Crataegus, Key C: hawthorns with leaf blades widest at midpoint or beyond midpoint; blade bases acute or cuneate;
leaves eglandular, or if glandular then twigs and branchlets not geniculate

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1 Leaves abaxially canescent, margins mostly entire; fruit calyx open, exposing pyrenes
1 Leaves abaxially pubescent, villous or glabrous, margins toothed; fruit calyx not fully open.
  2 Leaves spatulate or oblanceolate, < 13 mm wide; fruit < 6 mm diameter; pyrenes about 3 mm long
  2 Leaves variously shaped but most > 13 mm wide; fruit > 6 mm; pyrenes 4 mm long or more.
    3 Leaves abaxially with dense hair tufts in proximal main vein axils, especially in spring; [plants typically of wet or floodplain habitats].
      4 Inflorescence simple, 1-5-flowered; fruit 10-15 mm diameter, lustrous red, mature in late spring.
        5 Leaf blades mostly 5-7 cm long, elliptic, margin wavy or with uneven, blunt teeth
        5 Leaf blades usually < 5 cm long, most obovate, margin finely toothed.
          6 Leaf adaxial surface usually glossy; whitish hair tufts in main vein axils abaxially
          6 Leaf adaxial surface matte; reddish hair tufts in main vein axils abaxially
      4 Inflorescence compound, 5-20-flowered; fruit 5-15 mm diameter, dull red or orange-red, mature in autumn.
             7 Petiole 5-10 mm long; terminal shoot leaves < 25 mm broad
             7 Petiole > 10 mm long; terminal shoot leaves > 25 mm broad; [Virides series and hybrids].
               8 Leaves glabrous, except for hair tufts.
                 9 Leaves mostly ovate; terminal shoot leaves suborbicular, unlobed or barely lobed; bases rounded
                 9 Leaves predominately lanceolate, elliptical, rhomboid or obovate; terminal shoot leaves often ovate; bases usually cuneate.
                   10 Leaves rarely lanceolate, mostly elliptical or rhomboid.
                     11 Leaf serrations small or crenate; terminal shoot leaves often incised on each side nearly to midrib
               8 Leaves and/or inflorescences hairy, at least in spring.
    3 Leaves glabrous or with hairs scattered, not in tufts; [plants typically of upland habitats].
                                16 Leaves persistently pubescent, at least on petiole or main veins abaxially.
                                               23 Sepals foliaceous, equaling or exceeding petal length in flower and persistent on fruit; flowers usually 1-5 per inflorescence.
                                                    25 Thorns short (< 2 cm), or spinose spur shoots present; main lateral leaf veins run to sinuses and lobe tips in lobed leaves.
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