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..
Monoecious or dioecious herbs, shrubs or trees, with acrid often milky sap. Leaves opposite, alternate or verticillate, entire or toothed, sessile or petioled, sometimes with glands at the base; stipules present, obsolete or wanting. Inflorescence various. Flowers apetalous or petaliferous, sometimes much reduced and subtended by an involucre which resembles a calyx (genera 12-17), the number of parts in the floral whorls often different in the staminate and pistillate flowers. Stamens few, or numerous, in one series or many; filaments separate or united. Ovary usually 3-celled; ovules 1 or 2 in each cavity, pendulous; styles as many as the cavities of the ovary, simple, didvided, or many-cleft. Fruit a mostly 3-lobed capsule, separating, often elastically, into 3 2-valved carpels from a persistent axis at maturity. Seeds anatropous; embryo straight, or slightly curved, in fleshy or oily endosperm, the broad cotyledons almost filling the seed-coats.
About 250 genera and over 4000 species, of wide geographic distribution.
Flowers not in an involucre, with a true calyx.
Ovules 2 in each cavity of the ovary.
Petals none; stamens usually 3.
Petals present at least in the staminate flowers; stamens 5 or 6.
Ovule 1 in each cavity of the ovary.
Flowers spicate, racemose or axillary; calyx not corolla-like.
Corolla present in either the staminate or pistillate flowers, or in both
Stamens 5 or 6; filaments distinct; pubescence stellate.
Ovary, and dehiscent capsule, 2-4-celled, mostly 3-celled.
Ovary, and achene-like capsule, 1-celled.
Stamens 10; filaments monodelphous; pubescence not stellate.
Corolla none; pubescence not stellate.
Styles many-cleft; pistillate flowers with cleft or laciniate bracts.
Styles not cleft; bracts neither cleft nor laciniate.
Pistillate flowers with foliaceous or scale-like bracts; stamens 8 or more.
Flowers in simple spikes or racemes; leaf-blades not peltate
Styles 3; ovary 3-celled; capsule 3-lobed.
Styles 2; ovary 2-celled; capsule 2-lobed.
Flowers in panicled racemes; leaf-blades peltate.
Pistillate flowers with glandular saucer-shaped bracts; stamens 2 or 3.
Flowers in cymes; calyx corolla-like, saberform.
Flowers in an involucre, the calyx represented by a minute scale at the base of the filament-like
Glands of the involucres with petal-like appendages, these sometimes much reduced.
Leaves all opposite.
Leaf-blades inequilateral, oblique at the base.
Leaf-blades equilateral, not oblique at the base.
Leaves alternate or scattered at least below the inflorescence.
Annual or biennial; stipules narrow; bracts petal-like.
Perennial; stipules none; bracts not petal-like.
* Revised by Dr. J. K. Small.
Glands of the involucres without petal-like appendages, entirely naked, sometimes with crescentlike horns.
Stem topped by an umbel; stipules none; involucres in open cymes, each with 4 glands and
entire or toothed lobes.
Stem not topped by an umbel; stipules gland-like; involucres in cluster-like cymes, each with
a single gland or rarely 4 glands and fimbriate lobes.