This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol2", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Trees or shrubs, with watery often saccharine sap, opposite simple and palmately lobed (rarely entire) or pinnate leaves, and axillary or terminal cymose or racemose regular polygamous or dioecious flowers. Calyx generally 5-parted, the segments imbricated. Petals of the same number, or none. Disk thick, annular, lobed, sometimes obsolete. Stamens 4-12, often 8; filaments filiform. Ovary 2-lobed, 2-celled; styles 2, inserted between the lobes. Fruit of 2 winged samaras, joined at the base and I-seeded (rarely 2-seeded). Seeds compressed, ascending; cotyledons thin, folded.
The family consists of the following genus and Dipteronia Oliver, of central Asia, which differs from Acer in the samara being winged all around. There are more than 100 species of Maples.