This family includes only 2 genera, one of which (Dipternia) contains a single Chinese species. The genus Acer comprises about 70 species distributed principally in the northern hemisphere. There are 13 species native to the United States, 6 of which are found in West Virginia.

The maples not only produce much valuable wood but are used more extensively than any other group for ornamental purposes. The principal exotic species are Norway Maple (Acer platanoides, L.), and Sycamore Maple (Acer Pseudo-Platanus, L.).

The following key will be of use in distinguishing the species:

KEY TO THE SPECIES OF ACER

• a. Leaves simple.
• b. Leaf sinuses acute at base.
• Leaf-lobes long and narrow, leaves silvery white beneath; fruit in pairs, each key 1-2 inches long, falling in May
• A. saccharinum, p. 187.
• Leaf-lobes short and broad, leaves white-downy beneath, 3-lobed; fruit small, several, persistent till fall, in long drooping clusters; a small tree or shrub
• A. spicatum, p. 181.
• Leaves whitish and nearly glabrous beneath, 3-5 lobed, lobes broad and short; fruit in small clusters, falling in early summer
• A. rubrum, p. 189.
• b. Leaf sinuses rounded at base, leaves 3-lobed, finely and evenly toothed; fruit several in drooping racemes; a small tree or shrub with striped bark
• A. pennsylvanicum, p. 179.
• Leaves usually 5-lobed (or 3-lobed in variety nigrum, p. 185), the lobes sparingly wavy-toothed; fruit in small clusters, persisting until fall; a large tree
• A. saccharum, p. 183.
• a. Leaves compound; twigs greenish; fruit in long drooping racemes
• A. negundo, p. 191.