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, rarely shrubs (some tropical forms trees), with watery or resinous (rarely milky) sap, and opposite alternate or basal exstipulate leaves. Flowers perfect, pistillate, or neutral, or sometimes monoecious or dioecious, borne on a common receptacle, forming heads, subtended by an involucre of few to many bracts arranged in one or more series. Receptacle naked, or with chaffy scales subtending the flowers, smooth, or variously pitted or honeycombed. Calyx-tube completely adnate to the ovary, the limb (pappus) of bristles, awns, teeth, scales, or crown-like, or cup-like, or wanting. Corolla tubular, usually 5-lobed or 5-cleft, the lobes valvate, or that of the marginal flowers of the head expanded into a ligule (ray); when the ray-flowers are absent the head is said to be discoid; when present, radiate; the tubular flowers form the disk. Stamens usually 5, borne on the corolla and alternate with its lobes, their anthers united into a tube (syngenesious), often appendaged at the apex, sometimes sagittate or tailed at the base; pollen-grains globose, often rough or prickly. Ovary i-celled; ovule 1, anatropous; style of fertile flowers 2-cleft, its branches variously appendaged, or unappendaged; stigmas marginal; style of sterile flowers commonly undivided. Fruit an achene. Seed erect; endosperm none; embryo straight; hypocotyl inferior.
About 800 genera and not less than 10,000 species, of wide geographic distribution. The family is also known as Carduaceae, Aggregatae, and by the English name of Asterworts. In Kuhnia, the anthers are distinct, or nearly so.
* Anthers not tailed at the base; stigmatic lines of the style-branches only at the base, or not extending beyond the middle; flowers all tubular and perfect, never yellow; rays none.
Style-branches filiform or subulate, hispidulous; receptacle naked.
Tribe 1. Vernonieae.
Style-branches thickened upward, obtuse, papillose.
Tribe 2. Eupatorieae.
** Anthers tailed at the base, unappendaged at the tip; heads small; rays none (except in Inula where they are large, yellow).
Tribe 4. Inuleae.
*** Anthers not tailed at the base; stigmatic lines of the style-branches in the perfect flowers extending to the summit; flowers tubular only, or tubular and radiate, often yellow.
1. Receptacle naked (see also Nos. 85-88 of Tribe 7). a. Bracts of the involucre usually well imbricated. Style-branches of the perfect flowers flattened, with terminal appendages.
Tribe 3. Astereae.
Style-branches truncate, or with hairy tips.
Tribe 6. Helenieae.
b. Bracts little imbricated, or not at all; pappus of soft bristles.
Tribe 8. Senecioneae.
2. Receptacle chaffy (except in Nos. 85-88). Bracts of the involucre herbaceous or foliaceous; not scarious.
Tribe 5;. Heliantheae.
Bracts of the involucre dry, scarious.
Tribe 7. Anthemideae.
**** Anthers long-tailed at the base, with elongated appendages at the tip; heads large; rays none (in Centaurea often with enlarged marginal flowers); bracts imbricated.
Tribe 9. Cynareae.
Tribe 1. Vernonieae.
Pappus double, the inner of rough capillary bristles, the outer of scales, or short bristles; heads not glomerate.
Pappus a single series of bristles; heads glomerate, subtended by sessile bracts.
Tribe 2. Eupatorieae.
* Achenes 3-5-angled, not ribbed. Pappus of 5 broad obtuse scales; aquatic herb with linear whorled leaves.
Pappus of numerous capillary bristles.
Involucre of more than 4 bracts; erect herbs.
Involucre of 4 bracts; our species twining herbaceous vines.
** Achenes 8-10-ribbed, or 8-10-striate. 1. Bracts of the involucre in several series, the outer successively shorter. Bracts of the involucre strongly striate-nerved; heads panicled or corymbed in our species. Pappus-bristles rough or serrate; involucral bracts numerous.
Pappus-bristles plumose; involucral bracts few.
Bracts of the involucre faintly striate, if at all; heads spiked or racemed.
2. Bracts of the involucre in only 2 or 3 series; all nearly equal.
Tribe 3. Astereae. A. Ray-flowers yellow (white in one species of Solidago), or wanting; plants not dioecious.
* Pappus of scales, or awns, or wanting, never of numerous capillary bristles.
Heads small, not over 2" high; leaves narrowly linear, entire. Perennial herbs, or shrubs; all the flowers fertile.
Annual herbs: disk-flowers sterile.
Heads large, showy; leaves oblong to lanceolate, spinulose-dentate.
** Pappus of either the radiate or tubular flowers, or both, of numerous capillary bristles, with or without an outer series of shorter ones, or of scales.
† Pappus, at least of the disk-flowers, double, an inner series of capillary bristles, and an outer one of scales or short bristles; heads large.
Achenes of the ray-flowers thick, the pappus obsolete, or of a few caducous bristles; achenes of the disk-flowers flat.
Achenes of both ray-flowers and disk-flowers flattened.
†† Pappus wholly of capillary bristles.
1. Heads wholly of disk-flowers (in our species); rays none; leaves narrowly linear.
a. Perennial herbs; style-tips not exserted; eastern.
b. Shrubs; style-tips mostly exserted; western. Involucral bracts gradually narrowed to the tip, keeled, chartaceous.
Involucral bracts cuspidate, flat, herbaceous.