1. Acer L. (maple)

Plants shrubs or more commonly small or large trees, lacking tendrils. Bark and twigs various. Winter buds ovate to elliptic-ovate, bluntly to sharply pointed at the tip, mostly with 4–12 overlapping scales. Leaves opposite, petiolate. Leaf blades variously shaped, usually palmately lobed, less commonly pinnately compound. Inflorescences terminal or lateral toward the branch tips, sometimes appearing axillary, ranging from small clusters to racemes or small panicles. Flowers actinomorphic, hypogynous, the staminate ones often perigynous. Calyces of 4 or 5(6) sepals, these sometimes fused, often colored. Corollas absent or of 4 or 5(6) free petals. Stamens 3–8 (except in pistillate flowers), usually strongly exserted, the anthers red, purple, yellow, or yellowish green. Pistil of 2 fused carpels (except in staminate flowers), usually with 2 locules, flattened at right angles to the septum. Styles 2 per flower or sometimes 1 and deeply 2-lobed, the stigmas club-shaped. Ovules usually 2 per locule. Fruits consisting of 2 samaras that are initially fused at the base but break apart at maturity and are dispersed independantly, each with a single basal seed and an obliquely terminal wing. About 115 species, widespread in temperate portions of the northern hemisphere and in mountains in the tropics.

For the treatment of the 5 species of Acer present in Missouri, please refer to Volume 2 (pp. 7–16) of the present work (Yatskievych, 2006).