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 (a few genera almost herbaceous) with opposite or rarely alternate simple or pinnate exstipulate entire or dentate leaves and regular perfect polygamous or dioecious, 2-4-parted flowers in terminal or axillary panicles, cymes or fascicles. Calyx inferior, free from the ovary, usually small, sometimes none. Corolla gamopetalous, polypetalous, or none. Stamens 2-4, usually 2, inserted on the corolla; filaments usually short, separate; anthers mostly large, ovate, oblong or linear, 2-celled, the sacs longitudinally dehiscent. Ovary superior, 2-celled; ovules few in each cavity, anatropous or amphitropous; style usually short or none, rarely elongated. Fruit a capsule, samara, berry or drupe. Seeds erect or pendulous; endosperm fleshy, horny or wanting; embryo straight, rather large; cotyledons flat, or plano-convex; radicle usually short.
About 21 genera and 525 species, of wide distribution in temperate and tropical regions.
Fruit a loculicidal capsule; leaves simple; flowers complete.
Fruit a samara; leaves pinnate; flowers dioecious or polygamous.
Fruit a drupe or berry; leaves simple.
Flowers dioecious, apetalous, from catkin-like scaly buds.
Flowers complete, polypetalous, paniculate; petals linear.
Flowers complete, gamopetalous, paniculate.