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..
Herbs (some tropical species shrubs or trees), with watery sap, alternate or opposite mostly stipulate simple leaves, and small greenish dioecious, monoecious or polygamous flowers, variously clustered. Calyx 2-5-cleft, or of distinct sepals. Petals none. Stamens in the staminate flowers as many as the lobes or segments of the calyx (sepals) and opposite them, the filaments inflexed and anthers reversed in the bud, straightening at anthesis. Ovary superior, 1-celled; style simple; stigma capitate and penicillate, or filiform; ovule solitary, erect or ascending, orthotropous, or in some genera partly amphitropous. Fruit an achene. Endosperm oily, usually not copious; embryo straight.
About 40 genera and 550 species of wide geographic distribution.
Herbs with stinging hairs.
Leaves opposite; both kinds of flowers 4-parted; achene straight.
Leaves alternate; staminate flowers 5-parted; achene oblique.
Herbs without stinging hairs.
Flower-clusters panicled or spiked, not involucrate; leaves mostly opposite.
Pistillate calyx 3-parted or of 3 sepals.
Pistillate calyx 2-4-toothed or entire.
Flower-clusters involucrate by leafy bracts; leaves alternate.