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..
Annual or perennial herbs or shrubby plants, with simple or branched stems, which are topped by several-rayed umbel-like cymes. Leaves below the umbel usually scattered or alternate, without stipules, often broadened upward. Bracts of the umbel quite different from the stem-leaves; blades entire or toothed. Involucres sessile or peduncled, axillary, disposed in open or close cymes; lobes often toothed. Glands 4, transversely oblong, reni-form or crescent-shaped by the horn-like appendages, the missing one represented by a thin often ciliate lobe. Capsule exserted, smooth or tuberculate; lobes rounded, sharp or keeled. Seeds variously pitted, often with caruncles. [Greek, referring to the milky juice of these plants.]
About 250 species of wide geographic distribution. Besides the following, several others occur in western North America. Type species: Euphorbia dendroides L.
Leaves mainly opposite and decussate; capsules 5" wide or more.
Leaves alternate or scattered; capsules less than 5" wide.
Leaves serrulate; glands of the involucre oblong, oval or orbicular.
Seeds smooth or faintly reticulated.
Glands stalked; warts on the capsules elongated; seeds faintly reticulated.
Glands sessile; warts on the capsules depressed; seeds smooth.
Seeds strongly and prominently reticulated.
Glands nearly sessile; capsules warty.
Upper stem-leaves with small basal lobes; bracts of the umbel about as broad as long
Upper stem-leaves merely sessile; bracts of the umbel manifestly longer than broad.
Glands stalked; capsules smooth.
Leaves entire; glands of the involucres crescent-shaped or reniform.
Plants perennial; seeds smooth.
Glands crescent-shaped, horned.
Stem-leaves 2"-6" broad; capsules smooth.
Leaves subtending, the umbel lanceolate or oblanceolate.
Leaves subtending, the umbel ovate, oval or obovate.
Stem-leaves 1/4"-1 1/2" broad; capsules granular on the lobes.
Glands reniform, not horned.
Plants annual or biennial; seeds pitted or rugose-pitted.
Leaves thin, petioled.
Capsules with 2-crested lobes.
Capsules with rounded lobes.
Leaves thick, sessile.