||No other tree genus has as many species valued for their ornamental qualities in gardens as does Acer, at least in temperate climates. It consists of around 120 species, in the wild being virtually confined to the Northern Hemisphere and absent from all but the far northwest of Africa. In fact it is mainland east Asia that has by far the largest number of native species (86), followed by Japan (24), western Asia (13), Europe and North Africa (10) and North America (9). Some species extend through more than one of these regions of Europe and Asia. The North American species, although few in number, include some very widespread and abundant maples such as Acer saccharum and A. negundo, each so variable as to have a number of subspecies recognized. Most maples are forest or woodland trees of moist climates, but some grow in more stunted scrublands of drier regions such as the eastern Mediterranean and Central Asia.
Maples all grow to tree size (except for dwarf cultivars), and all but a very few are deciduous. The evergreen species are mostly rainforest trees of tropical southeast Asia and south China; it is thought that these are closer to the ancestral forms of Acer than the deciduous species, an evolutionary tendency echoed in most deciduous tree genera (for example Magnolia). The majority of maples have simple leaves, mostly toothed or lobed, borne on slender leaf stalks attached to the twigs in opposite pairs. A small number of species, all east Asian except for the American A. negundo, have compound leaves with 3, 5 or 7 leaflets. Flowers are small, mostly green, cream or reddish, in clusters or dense spikes that appear with or before the new leaves. The fruits that follow show the feature by which the genus can instantly be recognized: each consists of two small nuts (samaras), joined where they are attached to the flower stalk, and each terminating in an elongated wing. The wings may spread apart in a more or less straight line, or be bent upward to give the fruit a V or U form. The two samaras finally fall, together or separately, their descent slowed by the ?helicopter? action of the wings which allows them to be carried further from the parent tree.
It is the elegant foliage and growth habit of many maples that makes them so attractive to gardeners, combined with the beautiful colors the foliage takes on in autumn. Owners of large gardens often become so enthusiastic about maples that they build up large collections. A few species have been the subject of intensive horticultural selection, none more so than the Japanese maple (A. palmatum), of which several hundred named cultivars are recognized, including some known for centuries in Japan. This and several other species are favorite bonsai subjects. The maples include some significant timber trees, most important being the North American sugar maple (A. saccharum). This happens to be the main source of maple syrup, which is tapped from the bark.
||CULTIVATION: Most maples thrive best in cooler temperate climates with adequate rainfall. Optimum growth and autumn color is aided by warm, humid summers and sharply demarcated winters. While not too particular about soils, like most trees they grow best in deep, well-drained soil with permanent subsoil moisture. Some of the most beautiful species need the shelter of woodland glades to preserve their foliage from summer scorching, but there are other maples that tolerate exposure to drying winds. The standard means of propagation of maple species is from seed, as most are difficult to grow from cuttings. Seeds (actually the fruitlets) are collected on the point of falling and sown immediately in late summer or autumn. Keeping them moist through the winter cold allows embryos to develop fully, and germination usually takes place in spring. A few species have very low percentages of fertile seed, the solution for these being to sow very large numbers. Cultivars are normally propagated by grafting onto a seedling rootstock; in the case of A. palmatum cultivars, grafting requires a degree of expertise and several years for production of nursery plants, so that plants are fairly expensive.