This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol1", 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 milky sap, alternate petioled stipulate leaves, the stipules fugacious, and small monoecious or dioecious axillary clustered flowers, or the pistillate flowers solitary in some exotic genera. Calyx mostly 4-5-parted, becoming fleshy in fruit, inferior. Petals none. Staminate flowers, spicate or capitate, the stamens as many as the calyx-segments. Filaments inflexed in the bud. Pistillate flowers capitate or spicate. Ovary superior, 1-celled in our genera. Ovule solitary, pendulous, anatropous. Styles 1 or 2. Fruit mostly aggregate. Embryo curved or spiral.
Staminate and pistillate flowers spiked; leaves dentate or lobed.
Staminate flowers racemose or spiked; pistillate capitate.
Pistillate perianth deeply 4-cleft; leaves entire.
Pistillate perianth 3-4-toothed; leaves various.