Flora of the Southeastern United States
2022 Edition

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Key to Acer

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1 Leaves pinnately compound, divided into 3-7 (-9) leaflets; [section Negundo]
..2 Twigs glabrous (or pubescent when young); samaras glabrous
..2 Twigs persistently hairy; samaras pubescent (or rarely glabrous)
1 Leaves simple, generally shallowly to deeply 3-5 (-7) lobed.
....3 Leaves not toothed, or often with a few rounded, coarse, and irregular teeth on the principal lobes, these teeth 0-5 per principal lobe; sinuses between the principal leaf lobes generally broadly rounded, the sinus broader than deep.
......4 Petioles and young twigs exuding milky sap when broken; inflorescence peduncled, the flowers on ascending, moderately stout pedicels; paired samaras held at >135° from one another; [section Platanoidea]; [alien].
........5 Leaves 3-5-lobed, 4-8 (-10) cm wide
........5 Leaves 5-7-lobed, 10-18 cm wide
......4 Petioles and young twigs exuding clear sap when broken; inflorescence sessile, the flowers on drooping, filiform pedicels; paired samaras held at <110° from one another; [section Acer, series Saccharodendron]; [native, also widely planted].
..........6 {morphological characters to be added}; [c. TX westwards]
..........6 {morphological characters to be added}; [sc. OK and e. TX eastwards].
............7 Leaves green beneath, moderately to densely pubescent across the surface; leaf sinuses on either side of the terminal lobe shallow, the two sides of each sinus forming an angle of > 90 degrees (the terminal lobe typically broadly triangular); leaves sometimes planar, more usually with drooping lobe tips.
..............8 Leaves small, (3-) avg. 6 (-11) cm broad; small trees, often multi-trunked and crooked; bark whitish (in part because of dense growth of crustose lichens), becoming cracked and blackened on larger stems; [primarily of the Piedmont, extending into the lower Mountains in w. SC and n. GA]
..............8 Leaves large, (8-) avg. 15 (-20) cm broad; large trees, single-trunked; bark dark brown or blackish, becoming furrowed in large individuals; [primarily of the Mountains and westward]
............7 Leaves pale, grayish, silvery-gray, or strongly glaucous beneath, glabrous, pubescent on the veins, or pubescent across the surface; leaf sinuses on either side of the terminal lobe deep, the two sides of each sinus forming an angle of < 70 degrees (the terminal lobe typically with parallel margins, or even narrower toward the base than toward the tip); leaves usually planar, but sometimes with drooping lobe tips, especially in sun-exposed individuals.
................9 Leaves small, (3.5-) avg. 8 (-11) cm broad; leaf undersurface usually pubescent; fruits 20-25 mm long; medium to large trees; bark gray, smooth and beech-like, becoming irregularly furrowed or plated in large individuals; [primarily of the Coastal Plain and Piedmont, extending into the lower Mountains]
................9 Leaves large, (8-) avg. 15 (-20) cm broad; leaf undersurface glabrous, pubescent only on the veins, or pubescent across the surface; fruits 25-30 mm long; large trees; bark grayish-brown, with loose-edged plates; [primarily of the Mountains and upper Piedmont].
..................10 Leaves glabrous or pubescent only along the veins, the hairs appressed and straight; leaves strongly lobed and sublobed, the lobes with more attenuated (but ultimately rounded) tips
..................10 Leaves pubescent across the surface, often densely so, the hairs typically upright or tangled; leaves sparingly lobed and with more rounded lobe tips
....3 Leaves finely to coarsely toothed, the toothing often regular, the teeth 8-50 per principal lobe; sinuses between the principal leaf lobes generally sharp, forming a definite angle (or if rounded, then the sinus much deeper than broad).
....................11 Leaves deeply lobed, the two sinuses on either side of the central lobe deep and narrow, approaching the midrib, the terminal lobe thus narrower at its base than at its middle; flowers either with petals (A. palmatum) or without petals (A. saccharinum).
......................12 Leaves green beneath (or purple in many forms); main leaf lobes 5-9, these main lobes merely toothed or in some cultivars variously further divided; [small exotic tree, commonly planted and weakly naturalizing]; [section Palmata]
......................12 Leaves silvery white beneath; main leaf lobes 3-5, these main lobes with coarse teeth and smaller lateral lobes; [large native tree (also extensively planted)]; [section Rubra]
....................11 Leaves shallowly lobed, the two sinuses on either side of the central lobe broadly wedge-shaped, not approaching the midrib, the terminal lobe thus broadest at its base and progressively (though often irregularly) narrowing toward the tip; flowers with petals.
........................13 Winter buds stalked, with 2-4 valvate scales; inflorescence an elongate drooping raceme or erect panicle; petals green to bright yellow, 2-10 mm long; fruits maturing in midsummer to autumn; leaves green beneath; shrub, small tree, or medium tree (to 35 cm DBH).
..........................14 Bark with narrow white stripes on a green background (best seen on stems 3-10 cm in diameter); leaf blades 12-20 (-30) cm long and wide, finely serrate (5-10 teeth per cm), pubescent beneath with yellow to orange hairs 0.1-0.3 mm long (as seen at 10× magnification); inflorescence a drooping raceme; [section Macrantha]
..........................14 Bark brownish, never conspicuously striped; leaf blades 8-12 (-14) cm long and wide, coarsely serrate (2-3 teeth per cm), pubescent beneath with whitish hairs 0.3-1.0 mm long (as seen at 10× magnification); inflorescence an erect panicle; [section Parviflora]
........................13 Winter buds sessile, with 4-10 imbricate scales; inflorescence either a drooping panicle (A. pseudoplatanus), or a sessile or subsessile cluster or fascicle, or a terminal panicle; fruits maturing either in midsummer to autumn (A. pseudoplatanus) or in spring; leaves either lightly to strongly glaucous-whitened beneath or green beneath (A. ginnala); medium to large tree (to 100 cm DBH).
............................15 Leaves green beneath; inflorescence a terminal panicle; fruit segments nearly parallel, diverging from one another at < 35 degrees
..............................16 Leaves 3-lobed; leaf upper surface dark-green and shiny
..............................16 Leaves unlobed (or with small side lobes); leaf upper surface light green and dull
............................15 Leaves lightly to strongly glaucous-whitened beneath; inflorescence a drooping panicle (A. pseudoplatanus) or a sessile or subsessile cluster or panicle; fruit segments diverging at an angle of 30-90 degrees
................................17 Inflorescence a drooping panicle, flowering in May-Jun, fruiting Aug-Sep (and persisting overwinter); petals yellowish-green; leaf blades 8-17 cm long; [section Acer, series Acer]
................................17 Inflorescence a sessile or subsessile cluster or fascicle, flowering in Jan-Mar, fruiting Apr-Jul (and dropping); petals red (rarely yellowish), 1-3 mm long; leaf blades < 10 cm long; [section Rubra].
..................................18 Mature leaves densely white tomentose (felty-pubescent) beneath; petioles usually with white tomentum; mature samaras 2.7-5 cm long
..................................18 Mature leaves glabrous to densely pubescent (but not white-tomentose) beneath; petioles usually glabrous; mature samaras 1.5-3 cm long.
....................................19 Leaves (3-) 5 (-9)-lobed, the central lobe 4-8 cm long, the 2 upper lateral lobes 2-5 cm long; leaf base generally cordate (rarely rounded); leaves 7-18 cm wide; [widespread, in nearly all habitats]
....................................19 Leaves unlobed or 3 (-5)-lobed, the central lobe 1-5 cm long, the lateral lobes (if present) 0.5-2 (-3) cm long; leaf base broadly cuneate to rounded or subcordate; leaves 2-10 cm wide; [primarily of wetlands, especially in the Coastal Plain]