Acer ( ) is a genus of trees or shrubs commonly known as Maple. Maples are variously classified in a family of their own, the Aceraceae, or (together with the Hippocastanaceae) included in the family Sapindaceae. Modern classifications, including the Angiosperm Phylogeny Group classification, favour inclusion in Sapindaceae. There are approximately 125 species, most of which are native to Asia,Fact: date=May 2008 but several species also occur in Europe, northern Africa, and North America.

The word Acer is derived from a Latin word meaning "sharp" (referring to the characteristic points on the leaves) and was first applied to the genus by the French botanist Joseph Pitton de Tournefort in 1700. The type species of the genus is Acer pseudoplatanus (Sycamore Maple).van Gelderen, C. J. & van Gelderen, D. M. (1999). Maples for Gardens: A Color Encyclopedia

Since April 25, 1996, the maple tree has been the national arboreal emblem of Canada.

Morphology

The distinctive fruit are called samaras or "maple keys". These seeds occur in distinctive pairs each containing one seed enclosed in a "nutlet" attached to a flattened wing of fibrous, papery tissue. They are shaped to spin as they fall and to carry the seeds a considerable distance on the wind. Children often call them "helicopters" due to the way that they spin as they fall. Seed maturation is usually in a few weeks to six months after flowering, with seed dispersal shortly after maturity. However, one tree can release hundreds of the seeds at a time. Depending on the species, the seeds can be small and green to yellow and big with thicker seed pods. The green seeds are released in pairs, sometimes with the stems still connected. The yellow seeds are released individually and almost always without the stems. Most species require stratification in order to germinate, and some seeds can remain dormant in the soil for several years before germinating.

The genus is subdivided by its morphology into a multitude of sections and subsections.

Pests and diseases

Maples are affected by a number of fungal diseases. Several are susceptible to Verticillium wilt caused by Verticillium species, which can cause significant local mortality. Sooty bark disease, caused by Cryptostroma species, can kill trees which are under stress due to drought. Death of maples can rarely be caused by Phytophthora root rot and Ganoderma root decay. Maple leaves in late summer and autumn are commonly disfigured by "tar spot" caused by Rhytisma species and mildew caused by Uncinula species, though these diseases do not usually have an adverse effect on the trees' long-term health.Phillips, D. H. & Burdekin, D. A. (1992). Diseases of Forest and Ornamental Trees. Macmillan. ISBN 0-333-49493-8.