Maple

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 system, favour inclusion in Sapindaceae. 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
There are approximately 129 species, most of which are native to Asia,http://www.efloras.org/florataxon.aspx?flora_id=2&taxon_id=100167 with a number also appearing in Europe, northern Africa, and North America. Only one species, the poorly studied Acer laurinum, is native to the Southern Hemisphere.Gibbs, D. & Chen, Y. (2009) The Red List of Maples Botanic Gardens Conservation International (BGCI) ISBN: 978-1-905164-31-8 Fifty four species of maples meet the International Union for Conservation of Nature criteria for being under threat of extinction in their native habitat.Gibbs, D. & Chen, Y. (2009) The Red List of Maples Botanic Gardens Conservation International (BGCI) ISBN: 978-1-905164-31-8
The word Acer is derived from a Latin word meaning "sharp" (compare "acerbic"), referring to the characteristic points on maple leaves. It was first applied to the genus by the French botanist Joseph Pitton de Tournefort in 1700.
A red sugar maple leaf is the prominent feature of the flag of Canada.
==Morphology==
Most maples are trees growing to 10 – 45 metres (30 – 145 ft) in height. Others are shrubs less than 10 metres tall with a number of small trunks originating at ground level. Most species are deciduous, but a few in southern Asia and the Mediterranean region are evergreen. Most are shade-tolerant when young, and are often late-successional in ecology; many of the root systems are typically dense and fibrous. A few species, notably Acer cappadocicum, frequently produce root sprouts, which can develop into clonal colonies.
Maples are distinguished by opposite leaf arrangement. The leaves in most species are palmate veined and lobed, with 3 to 9 (rarely to 13) veins each leading to a lobe, one of which is central or apical. A small number of species differ in having palmate compound, pinnate compound, pinnate veined or unlobed leaves. Several species, including Acer griseum (Paperbark maple); Acer mandshuricum (Manchurian maple); Acer maximowiczianum (Nikko maple); and Acer triflorum (Three-flowered maple), have trifoliate leaves. One species, Acer negundo (Box-elder), has pinnately compound leaves that may be simply trifoliate or may have five, seven, or rarely nine leaflets. A few, such as Acer laevigatum (Nepal maple) and Acer carpinifolium (Hornbeam maple), have pinnately-veined simple leaves.
 
The flowers are regular, pentamerous, and borne in racemes, corymbs, or umbels. They have four or five sepals, four or five petals about 1 – 6 mm long (absent in some species), four to ten stamens about 6 – 10 mm long, and two pistils or a pistil with two styles. The ovary is superior and has two carpels, whose wings elongate the flowers, making it easy to tell which flowers are female. Maples flower in late winter or early spring, in most species with or just after the appearance of the leaves, but in some before the trees leaf out.Huxley, A., ed. (1992). New RHS Dictionary of Gardening. Macmillan ISBN 0-333-47494-5.
 
Maple flowers are green, yellow, orange or red. Though individually small, the effect of an entire tree in flower can be striking in several species. Some maples are an early spring source of pollen and nectar for bees.
The distinctive fruit are called samaras or "maple keys". These seeds, or 'whirlybirds,' 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 thousands of seeds at a time. Depending on the species, the seeds can be small and green to orange 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.Classification of maples
== Pests and diseases == 
The leaves are used as a food plant for the larvae of a number of Lepidoptera species (see List of Lepidoptera that feed on maples). Aphids are also very common sap-feeders on maples. In horticultural applications a dimethoate spray will solve this.
In the United States and Canada, all maple species are threatened by the Asian Longhorned Beetle (Anoplophora glabripennis). Infestations have resulted in the destruction of thousands of maples and other tree species in Illinois, Massachusetts, New Jersey and New York.http://www.beetlebusters.info/; http://www.umassgreeninfo.org/fact_sheets/wood_attackers/asian_longhorned_beetle_MA.html#226
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.
==Uses==
===Horticulture=== 
Maples are planted as ornamental trees by homeowners, businesses and municipalities. Acer platanoides (Norway maple) is especially popular as it is fast-growing and extremely cold-resistant, though it is also an invasive species in some regions. Other maples, especially smaller or more unusual species, are popular as specimen trees.
====Cultivars====
Numerous maple cultivars which have been selected for particular characteristics can be propagated only by asexual reproduction such as cuttings, tissue culture, budding or grafting. Acer palmatum (Japanese maple) alone has over 1,000 cultivars, most selected in Japan, and many of them no longer propagated or not in cultivation in the Western world. Some delicate cultivars are usually grown in pots and rarely reach heights of more than 50–100 cm.
====Bonsai==== (Trident maple), Acer ginnala (Amur maple), Acer campestre (Field maple) and Montpellier maple (A. monspessulanum) are popular choices and respond well to techniques that encourage leaf reduction and ramification, but most species can be used.
====Collections==== 
Maple collections, sometimes called aceretums, occupy space in many gardens and arboreta around the world including the "five great W's" in England: Wakehurst Place Garden, Westonbirt Arboretum, Windsor Great Park, Winkworth Arboretum and Wisley Garden. In the United States, the aceretum at the Harvard-owned Arnold Arboretum in Boston is especially notable. In the number of species and cultivars, the Esveld Aceretum in Boskoop, Netherlands is the largest in the world.
====Agriculture====
As they are a major source of pollen in early spring before many other plants have flowered, maples are important to the survival of honeybees that play a commercially-important role later in the spring and summer.
 
==References==