Pinus

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  P. strobus

  1a. Leaves in fascicles of 5, with 1 fibrovascular bundle in cross-section; bud scales with entire margins; seed cones 8–20 cm long; umbo terminal
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  1b. Leaves in fascicles of 2 or 3, with 2 fibrovascular bundles in cross-section; bud scales fringed on the margin; seed cones 3–10 cm long; umbo dorsal
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    P. rigida

    2a. Leaves in fascicles of 3; apophyses of mature seed cones red-brown (fading on older, fallen cones)
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    2b. Leaves in fascicles of 2; apophyses of mature seed cones cream to light brown or gray, or red-brown in P. resinosa
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      3a. Leaves (7–) 9–16 cm long, straight or only slightly twisted; seed cones symmetric, with a straight axis and without unequal development of apophyses
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        P. resinosa

        4a. Leaves brittle, tending to break when bent; winter buds red-brown; smaller branches orange-brown; seed cones, when falling, leaving some of the basal scales on the branch (i.e., the seed cone hollow-based); umbo unarmed
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        4b. Leaves pliable, tending not to break when bent; winter buds gray; smaller branches brown to gray-brown; seed cones falling with all the scales intact; umbo armed with a slender prickle (though the prickle often deciduous)
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          P. nigra

          5a. Winter buds resinous; growth habit relatively uniform and often with an unbranched main stem; seed cones sessile, with a ± rounded base
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          P. thunbergii

          5b. Winter buds not resinous; growth habit irregular, the main stem and branches often forked; seed cones stalked, with a ± truncate base
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      3b. Leaves 2–7 (–8) cm long, noticeably twisted (relatively straight in the planted P. mugo); seed cones ± asymmetric, with a curved axis and/or obviously unequal development of apophyses on one side
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        P. mugo

        6a. Umbo encircled by a thin, dark gray to nearly black ring; low, multi-stemmed shrubs; leaves relatively straight
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        6b. Umbo without a dark ring; upright, single-stemmed, short to tall trees; leaves twisted
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          P. sylvestris

          7a. Leaves 3–7 cm long, glaucous; larger branches orange-brown; seed cones usually falling after second winter; apophyses elongating toward base of seed cone on one side
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          P. banksiana

          7b. Leaves 2–3.5 (–5) cm long, not glaucous; larger branches gray-brown; seed cones serotinous, persisting on plant for many years; apophyses all ± of equal size [Fig. 48]

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