Maple
Acer is a genus of trees and shrubs commonly known as maples. The genus is placed in the soapberry family Sapindaceae. There are approximately 132 species, most of which are native to East Asia, with a number also appearing in Europe, northern Africa, and North America. Only one species, Acer laurinum, extends to the Southern Hemisphere. The type species of the genus is the sycamore maple Acer pseudoplatanus, one of the most common maple species in Europe. Most maples usually have easily identifiable palmate leaves and all share distinctive winged fruits. The closest relative of the maples is the small east Asian genus Dipteronia, followed by the more widespread genus Aesculus. Maple syrup is made from the sap of some maple species. It is one of the most common genera of trees in Asia. Many maple species are grown in gardens where they are valued for their autumn colour and often decorative foliage, some also for their attractive flowers, fruit, or bark.
Evolutionary history
The closest relative of Acer is Dipteronia, which only has two living species in China, but has a fossil record extending back to the middle Paleocene in North America. The oldest known fossils of Acer are from the late Paleocene of Northeast Asia and northern North America, around 60 million years old. The oldest fossils of Acer in Europe are from Svalbard, dating to the late Eocene.Morphology
Most maples or acers are trees growing to a height of. Others are shrubs less than 10 meters tall with a number of small trunks originating at about ground level. Most species are deciduous, and many are renowned for their autumn leaf colours, but a few in southern Asia and the Mediterranean region are mostly evergreen. Most are shade-tolerant when young and are often riparian, understory, or pioneer species rather than climax overstory trees. There are a few exceptions such as sugar maple, which may form monodominant forests. Many of the root systems are typically dense and fibrous, inhibiting the growth of other vegetation underneath them. 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 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, Acer mandshuricum, Acer maximowiczianum and Acer triflorum, have trifoliate leaves. One species, Acer negundo, has pinnately compound leaves that may be simply trifoliate or may have five, seven, or rarely nine leaflets. A few, such as Acer laevigatum and Acer carpinifolium, have pinnately veined simple leaves.
Maple species, such as Acer rubrum, may be monoecious, dioecious or polygamodioecious. 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, 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.
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.
File:3D rendering of a µCT scan of a maple seed.ogv|thumb|right|3D rendering of a μCT scan of a samara. Resolution is about 45 μm/voxel.
The distinctive fruits are called samaras, "maple keys", "helicopters", "whirlybirds" or "polynoses". 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. People often call them "helicopters" due to the way that they spin as they fall. During World War II, the US Army developed a special airdrop supply carrier that could carry up to of supplies and was based on the maple seed. 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 Acer, together with genus Dipteronia, were formerly often classified in a family of their own, the Aceraceae, but recent botanical consensus, including the Angiosperm Phylogeny Group system, includes them in the family Sapindaceae; their exclusion from Sapindaceae would leave that family paraphyletic. Within Sapindaceae, Acer is placed in the subfamily Hippocastanoideae. The genus is subdivided by its morphology into a multitude of sections and subsections. Molecular studies incorporating DNA sequence data from both chloroplast and nuclear genomes, aiming to resolve the internal relationships and reconstruct the evolutionary history of the group, suggest a Late Paleocene origin for the group, appearing first in the northeastern Palearctic. Rapid lineage divergence was followed by several independent dispersals to the Nearctic and Western Palearctic regions. Fifty-four species of maples meet the International Union for Conservation of Nature criteria for being under threat of extinction in their native habitat.
Pests and diseases
The leaves are used as a food plant for the larvae of a number of the order Lepidoptera. In high concentrations, caterpillars, like the greenstriped mapleworm, can feed on the leaves so much that they cause temporary defoliation of host maple trees. Aphids are also very common sap-feeders on maples. In horticultural applications a dimethoate spray will solve this.Infestations of the Asian long-horned beetle have resulted in the destruction of thousands of maples and other tree species in Illinois, Massachusetts, New Jersey, New York, and Ohio in the United States and Ontario, Canada.
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 that 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.
Cultural significance
A maple leaf appears on the coat of arms of Canada, and is on the Canadian flag. The maple is a common symbol of strength and endurance and has been chosen as the national tree of Canada. Maple leaves are traditionally an important part of Canadian Forces military regalia, for example, the military rank insignia for generals use maple leaf symbols.There are 10 species naturally growing in the country, with at least one in each province. Although the idea of the tree as a national symbol originally hailed from the province of Quebec where the sugar maple is significant, today's arboreal emblem of Canada rather refers to a generic maple. The design on the flag is an eleven-point stylization modeled after a sugar maple leaf.
It is also in the name of the Canadian ice hockey team, the Toronto Maple Leafs.
The first attested use of the word was in 1260 as "mapole", and it also appears a century later in Geoffrey Chaucer's Canterbury Tales, spelled as "mapul". The maple is also a symbol of Hiroshima, ubiquitous in the local meibutsu.
A maple leaf, along with samaras, appears in the coat of arms of Sammatti, a former municipality of Uusimaa, Finland.
Uses
Horticulture
Some species of maple are extensively planted as ornamental trees by homeowners, businesses, and municipalities due to their fall colour, relatively fast growth, ease of transplanting, and lack of hard seeds that would pose a problem for mowing lawns. Particularly popular are Norway maple, silver maple, Japanese maple, and red maple. The vine maple is also occasionally used as an ornamental tree. Other maples, especially smaller or more unusual species, are popular as specimen trees.Cultivars
Numerous maple cultivars that have been selected for particular characteristics can be propagated only by asexual reproduction such as cuttings, tissue culture, budding or grafting. Acer palmatum 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.File:Japanese Maple 'Kiyohime'.jpg|thumb|Acer palmatum var. 'Kiyohime' as bonsai during fall abscission. This dwarf hybrid cultivar is prized for its small leaves and bright red fall colors.