Acer L. (1753) Maple, in the family Aceraceae, contains around 120 species throughout the northern hemisphere.

Description Trees to 40m, or shrubs. The leaves are opposite, deciduous or evergreen, simple, 3- to 5-lobed, palmate or pinnate, with 3–5 or rarely 9 leaflets. The flowers are small, unisexual, bisexual or functionally unisexual, in short spikes or rounded or flat heads. Sepals 5 or rarely 4. Petals 5, rarely 4, 6, or absent, usually green or reddish. Stamens 8, rarely 4 or 5, or 10–12. Pollination is by insects, often bees or small, black gnats, or rarely by wind. Ovary usually superior, with 2 cells and 2 styles, each cell with 2 ovules. The fruits are winged on 1 side and usually paired, with 1 seed developing in each fruit.

Key Recognition Features The opposite leaves and paired fruits with spreading wings.

Evolution and Relationships Aceraceae is close to Sapindaceae, and consists of only 2 genera, Acer and Dipteronia. Fossil remains of Acer have been found in deposits in western North America ranging from the Paleocene to the Pliocene, and have shown that Aceraceae probably evolved from Sapindaceae. Acer has evolved great diversity of floral arrangement and leaf shape.

Ecology and Geography On dry hills and cliffs, in woods, and in swamps from western Europe, across Asia to Japan, and throughout North America; the greatest diversity is found in China.

Comment Many species are grown for ornament, especially for their red and orange autumn colour. Maple syrup is produced by bleeding the bark of A. saccharum Marshall in the early spring. The sycamore, A. pseudoplatanus L. is often regarded as a weed, but its wood is good for veneers, furniture, and making violins. The Japanese maples are particularly variable in leaf shape and colour, and many selected forms with coloured or finely dissected leaves are cultivated in Japan and in other parts of the world. Many of the cultivated species of Acer fall into natural groups (the main ones detailed opposite), others are isolated with no close relatives.

Japanese maples These have leaves with 5–13 lobes, often deeply dissected and coloured. The flowers are usually in hanging umbels. Most of those grown are selections from the species A. palmatum Thunb., A. japonicum Thunb., A. sieboldianum Miq., and A. pseudosieboldianum (Pax) Kom., all from northeastern Asia, and A. circinnatum Pursh, the vine maple, from the Pacific northwest from British Columbia to California. Acer pentaphyllum Diels, a rare and isolated species from western China, has leaves like a divided Japanese maple, A. palmatum ‘Linearilobum’, but the leaflets fall separately.

Snakebark maples These have green and often beautifully striped bark on the young branches, and flowers in hanging catkins. Cultivated species include A. davidii Franch., A. capillipes Maxim. and A. pectinatum Wall. all from China and Japan, and A. pensylvanicum L. the moosewood, from northeastern North America, often grown in its red-twigged form ‘Erythrocladum’. Acer micranthum Sieb. & Zucc. from Japan, has attractive, deeply lobed leaves with long points.

Species with almost unlobed leaves These have leaves rather unlike maples, but can be identified by the leaves being opposite and by their pairs of winged fruit. Examples include A. tataricum L. from Asia, A. carpinifolium Sieb. & Zucc. from Japan with hornbeam-like leaves and A. distylum Sieb. & Zucc., with leaves like a lime or a Davidia. Acer buergerianum Miq. has unusual, 3-pointed leaves. Some forms of A. davidii are also unlobed.

Large tree maples Many of the species that form large trees are familiar in parks and large gardens. European species include the Norway maple A. platanoides L. and the sycamore A. pseudoplatanus. The Asiatic A. cappadocicum Gled. has leaves with pointed lobes. Species in America include the red maple, A. rubrum L., its Japanese equivalent A. pycnanthum K. Koch, and the silver maple A. saccharinum L., which have red flowers before the leaves open, and the sugar maple A. saccharum L. with green, hanging flowers.

Species with compound leaves Maples with 3 leaflets include A. griseum (Franch.) Pax, grown for its reddish-brown, peeling bark, and A. maximowiczianum Miq. from China and Japan, formerly called A. nikoense (Maxim.) Miq., grown mainly for its autumn colour. Acer henryi Pax. from China is related to A. negundo L. the box elder, from North America. This can have pinnate leaves with 9 leaflets, similar to those of Dipteronia; its variegated form is often grown.