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Trees or shrubs, belonging to the natural order Aceraceæ, which is often considered to be a division of Sapindaceæ. Leaves usually deciduous, rarely evergreen, opposite, without stipules—simple, in which case they are undivided or palmately lobed—or compound with three to five leaflets. Buds covered by several scales arranged in decussate pairs, or protected by two valvate scales, sessile or occasionally stalked. Twigs with epidermis persisting for more than one year and remaining green in the second year; or becoming corky on the surface and changing colour in the first season. Inflorescence terminal on two- to four-leaved branchlets, or arising out of lateral buds without leaves, in racemes, corymbs, or fascicles. Flowers appearing at the same time as the leaves or earlier ; regular, dicecious, or with male and perfect flowers on the same tree, or with male flowers on one tree and perfect flowers on another tree. Parts of the flowers in fours or fives or multiples of those numbers. Calyx with four, five, to twelve sepals, usually free, occasionally connate. Petals equal in number to the sepals, absent in some species. Disc secreting honey usually present, absent in a few species, annular, lobed, or reduced to small teeth. Stamens four to ten, usually eight, inserted either outside the disc, inside it, or upon it. Ovary, two-lobed, two- celled, each cell containing two ovules. Styles or stigmas two, free or connate at the base. Fruit of two samara, attached by their bases, with long and diverging wings. Seeds one or two in each samara, without albumen; cotyledons appearing above the ground on germination.

About 110 species of maple are known, occurring usually in mountainous regions; in Europe, south of lat. 62°, in Algeria, Asia Minor, the Caucasus, Persia, Turkestan, the Himalayas, China, Manchuria, Japan, Formosa, the Philippines, Java, Sumatra, Celebes, and in North America from Southern Canada and Oregon to Mexico and Guatemala. A large number have been introduced into cultivation, fifty-seven species being enumerated in the Kew Hand-List; but many of these are shrubs or small trees, the detailed treatment of which does not come within the scope of our work.

The genus is divided by natural characters into thirteen sections by Pax, whose monograph and that of Schneider should be consulted by cultivators of the