This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol3", 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 (two Pacific Island genera trees), almost always with milky, acrid or bitter juice, alternate or basal leaves, and yellow, rarely pink, blue purple or white flowers in involucrate heads (anthodia). Bracts of the involucre in 1 to several series. Receptacle of the head flat or flattish, naked, scaly (paleaceous), smooth, pitted, or honeycombed. Flowers all alike (heads homogamous), perfect. Calyx-tube completely adnate to the ovary, its limb (pappus) of scales, or simple or plumose bristles, or both, or wanting. Corolla gamopetalous, with a short or long tube, and a strap-shaped (ligulate) usually 5-toothed limb (ray). Anthers connate into a tube around the style, the sacs sagittate or auricled at the base, not tailed, usually appendaged at the summit, the simple pollen-grains usually 12-sided. Ovary i-celled; ovule 1, anatropous; style very slender, 2-cleft, or 2-lobed, the lobes minutely papillose. Fruit an achene. Seed erect; endosperm none; radicle narrower than the cotyledons.
About 70 genera and 1500 species, of wide geographic distribution. The family is also known as Liguliflorae, and is often regarded as a tribe of the Compositae.
* Pappus of scales, or of scales and bristles, or none. Flowers blue or white; pappus of blunt scales.
Bracts of the involucre membranous, or herbaceous. Pappus none: achenes 20-30-nerved.
Pappus none; achenes 8-10-ribbed.
3. Serine a.
Pappus of rounded scales, with or without an inner series of bristles.
Bracts of the involucre 9-18, reflexed in fruit, not keeled; pappus-bristles usually more numerous than the scales. Annuals; pappus-scales 5, obovate or rounded.
Perennials; pappus-scales 10-15, linear or oblong, minute.
Bracts of the involucre 5-8, erect in fruit, keeled; pappus-bristles and scales 5.
Bracts of the involucre thickened and keeled after flowering; pappus none.
** Pappus, at least some of it, of plumose bristles. Receptacle chaffy.
Plume-branches of the pappus not interwebbed. Flowers yellow; plants scapose, the leaves basal.
Flowers yellow; plants leafy-stemmed.
Plume-branches of the pappus interwebbed.
*** Pappus of simple bristles or of some soft white scales.
† Receptacle with a few deciduous bristles; flowers yellow †† Receptacle naked.
1. Achenes spinulose, or with short processes near the summit. Heads few- (6-15-) flowered, yellow; stem branching.
Heads many-flowered, yellow, solitary on scapes.
2. Achenes smooth, or papillose, not spinulose toward the summit, (a) Achenes flattened.
Achenes truncate, not beaked; flowers yellow.
Achenes narrowed at the summit, or beaked; flowers blue or yellow.
(b) Achenes cylindric, or prismatic.
Achenes terete, not narrowed either at the base or summit; flowers pink or purple.
Achenes narrowed at the base, narrowed or beaked at the summit; flowers yellow. Achenes slender-beaked.
Pappus-bristles not surrounded by a villous ring at base.
Pappus-bristles surrounded by a villous white ring at base.
Achenes merely narrowed above, not beaked.
Achenes narrowed at the base, otherwise columnar, truncate (except in 2 species of Hieraciutn). Flowers yellow, orange, or red.
Flowers white, cream-color, or purple.