In most species, both bisexual and unisexual are produced, the plants polygamo-dioecious or sometimes polygamo-monoecious. The characteristic fruits of maples readily identify them. These are samaras and grow in pairs, each with a seed at the base and a long broad wing beyond.
1. Leaves compound; leaf scars joined around the stem.
1. Leaves simple; leaf scars not quite meeting around the stem.
2. Sinuses between the principal leaf lobes entire, ± acute to obtuse or even broadly rounded.
3. Primary leaf lobes all rounded-obtuse at apex; rarely escaped shrub or small tree.
3. Primary leaf lobes acuminate; full sized trees.
4. Flowers with conspicuous petals (ca. 4–7 mm long), in glabrous erect or ascending corymbs; samaras (3.5–) 4–5 cm long, the wings of the 2 in each pair often diverging almost 180°; petioles with milky sap.
4. Flowers without petals, on pubescent soon drooping pedicels; samaras ca. 2–3.5 cm long, the wings diverging at an acute angle (often becoming nearly parallel); petioles with watery sap.
5. Leaf blades glabrous or nearly so beneath, except sometimes in the axils of the main veins; petioles glabrous.
5. Leaf blades with ± dense puberulence beneath; petioles usually puberulent.
2. Sinuses between the principal leaf lobes sparsely to densely toothed at last part way to the base, often acute.
6. Flowers (or at least the bud scales subtending them) red, in sessile umbel-like inflorescences (compact when first blooming, the pedicels later elongating), opening well before the leaves; terminal main lobe of leaf with sides parallel or angled inward near the base (a subordinate lobe on each side).
7. Terminal main lobe of leaf ca. 1/2 the length of the blade, the margins parallel toward the base or nearly so, the adjacent sinuses much wider at their mouth than elsewhere; sepals separate; petals present, nearly or quite as long as the sepals; ovary and fruit glabrous; mature samaras 1.8–2.5 cm long.
7. Terminal main lobe of leaf ca. 2/3 the length of the blade, the margins flaring from a narrow base, so that each adjacent sinus is usually narrower at its mouth than it is closer to the base; sepals mostly connate part or all their length; petals none; ovary and at least young fruit pubescent; mature samaras 3.5–5.5 (–6) cm long.
6. Flowers yellowish or greenish, in peduncled, elongate racemes or panicles, opening with or after the leaves; terminal main lobe of leaf (except in A. pseudoplatanus) broadest at base, tapered ± uniformly to apex (with no subordinate lobes).
8. Leaf blades whitened beneath, mostly 5-lobed (including a pair of smaller basal lobes), the terminal lobe ± parallel-sided (or even broader above the middle); petals about equaling the sepals or slightly shorter; trees.
8. Leaf blades not whitened beneath, 3-lobed, the terminal lobe ± triangular (broadest at base); petals at maturity exceeding the sepals; often shrubby.
9. Leaves finely doubly serrate; inflorescence a pendent raceme; petals obovate; fruit nearly or quite smooth over the seed; bark of young trunks and branches green with pale stripes.
9. Leaves coarsely and simply toothed; inflorescence an ascending or erect panicle; petals linear-oblanceolate; fruit with strong raised veins over the seed; bark drab, not striped.
10. Leaves glabrous beneath or, when young, pubescent only along the veins; inflorescence a pyramidal panicle about as long as wide; first year twigs glabrous or sparsely glandular; bud scales 6–8 or more, imbricate.
10. Leaves finely pubescent beneath; inflorescence an elongate panicle 2–3 times as long as wide; first year twigs pubescent; bud scales 2, valvate.