A. Most maples are trees growing to 10–45 m (33–148 ft) 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 riparian, understory, or pioneer species rather than climax over story trees with a few exceptions such as Sugar Maple. 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.
B. 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 Paperbark maple; Manchurian maple; Nikko maple; and Three-flowered maple, have trifoliate leaves. One species, Box-elder, has innately compound leaves that may be simply trifoliate or may have five, seven, or rarely nine leaflets. A few, such as Nepal maple and Hornbeam maple, have pinnately veined simple leaves.
C. 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 carpals, 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.
D. The distinctive fruit are called samaras, "maple keys", "helicopters", "whirlybirds" or "polynoses". These seeds occur in distinctive pairs each containing one seed enclosed in a "nut let" 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. During World War II, the US Army developed a special air drop supply carrier that could carry up to 65 pounds 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.
E. Not given in any of the above paragraphs.
