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Acer
See list of 13 species in this genus-
1a. Leaves pinnately compound with 3–9 leaflets; nectary disk wanting; branchlets often glaucous (not glaucous in var. negundo), with a line from each leaf scar meeting in a raised point between the leaf scars
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1b. Leaves simple, palmately lobed [Figs. 900,901]; nectary disk present (absent or vestigial in A. rubrum and A. saccharinum);
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2a. Leaf blades ± evenly toothed or double-toothed through the margin, the sinuses angled (i.e., V-shaped) [Fig. 901]
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3a. Sinuses of leaf blades extending ⅔ or more the distance to the base of the blade; sepals red to purple, ca. 3 mm long
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3b. Sinuses of leaf blades extending less than ½ the distance to the base of the blade [Fig. 901]; sepals not red or purple, except in A. rubrum and then 1–3 mm long
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4a. Winter buds stalked, covered by 2 valvate scales
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5a. Inflorescence an upright panicle, pubescent; petals 2–3 mm long; branchlets and bud scales pubescent; bark brown to gray-brown; leaf blades 6–12.5 cm long
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5b. Inflorescence a drooping raceme, ± glabrous [Fig. 898]; petals 5–10 mm long; branchlets and bud scales glabrous; bark green to green-brown, with slender, longitudinal, pale stripes; leaf blades 12–18 (–20) cm long
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4b. Winter buds sessile, covered by 3–10 imbricate scales
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6a. Inflorescence an upright panicle; mericarps nearly parallel, diverging at an angle of less than 30 degrees; leaf blades green on the abaxial surface
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7a. Leaf blades dark green and lustrous on the adaxial suface, 3-lobed
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7b. Leaf blades light green and dull on the adaxial surface, unlobed or sometimes with 3 obscure lobes
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6b. Inflorescence either a dense, umbel-like fascicle or a drooping panicle; mericarps diverging at an angle of 30–90 degrees; leaf blades thinly glaucous on the abaxial surface
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8a. Flowers coetaneous or serotinous, in drooping panicles; sepals green, 4–5 mm long; leaf blades crenate-serrate, the secondary veins prominent on the adaxial surface; collateral winter buds absent
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8b. Flowers precocious, in dense, umbel-like fascicles [Fig. 899]; sepals red, 1–3 mm long; leaf blades serrate, the secondary veins visible, but not prominent, on the adaxial surface; collateral winter buds often present on some branchlets on the plant
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2b. Leaf blades entire through much of the margin, rarely with more than a few pairs of points on each major lobe ( A. saccharinum often with more teeth, but these concentrated near the tips of the lobes), the sinues rounded (i.e., U-shaped; sometimes angled in A. campestre and A. saccharinum) [Fig. 900]
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9a. Flowers with petals; mericarps usually diverging at an angle of 170–180 degrees; broken petioles yielding a milky latex; leaf blades green on the abaxial surface
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10a. Leaf blades mostly 4–7 cm long, ciliate, abaxially pubescent, with short, blunt lobes; corymb pubescent; mericarps usually pubescent
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10b. Leaf blades (5–) 10–15 cm long, eciliate, abaxially glabrous or with hairs in the junctions of veins, with prominent, pointed lobes; corymb ± glabrous; mericarps glabrous
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9b. Flowers without petals (i.e., with only sepals); mericarps usually diverging at an angle of 60–120 degrees; broken petioles yielding a watery latex; leaf blades glaucous on the abaxial surface (green on the abaxial surface in A. nigrum)
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11a. Flowers precocious, borne in dense clusters on very short pedicels that barely exceed the associated bud scales (pedicels later elongating as fruits mature); ovary and fruit pubescent; nectary disk absent or vestigial; winter buds red, blunt at the apex; branchlets often with collateral buds; central lobe of the leaf more than 50% as long as the blade and narrowed near the base; bark on mature trees with long, curling plates
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11b. Flowers ± coetaneous, evidently pedicellate; ovary and fruit glabrous; extrastaminal nectary disk present; winter buds brown, pointed at the apex; branchlets without collateral buds; central lobe of the leaf up to 50% as long as the blade and not narrowed near the base [Fig. 900]; bark on mature trees with furrows
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12a. Leaf blades abaxially glaucous and glabrous (rarely green) at maturity (sometimes pubescent on the veins or rarely across the surface with appressed to ascending hairs), relatively plane (i.e., the sides not drooping); bark gray to gray-brown, with loose-edged plates on mature trees; middle lobe of leaf blades often with ± parallel basal margins [Fig. 900]; petioles without foliaceous outgrowths, not pubescent through their length
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12b. Leaf blades green and abaxially pubescent across the surface with spreading to erect hairs, the sides often drooping; bark dark brown and furrowed; middle lobe of leaf blades usually ± triangular and tapering from base to apex; petioles of vigorous shoots sometimes with foliaceous outgrowths that ± conceal the axillary buds, sometimes pubescent through their length
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Each photo represents one species in this genus.