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..
Erect, branching or simple, prickly herbs, some species acaulescent, with alternate or basal, sinuate-dentate, lobed or pinnatifid, usually very spiny leaves, sometimes decurrent, and large, many-flowered, solitary or clustered, discoid heads of purple, yellow or white, tubular, perfect and fertile, or rarely dioecious flowers. Involucre ovoid or globose, its bracts prickle-tipped or unarmed, imbricated in many series. Receptacle flat or convex, bristly. Corolla-tube slender, the limb deeply 5-cleft. Filaments pilose, or rarely glabrous. Anthers sagittate at the base. Style-branches short or elongated, obtuse. Achenes obovate or oblong, compressed or obtusely 4-angled, glabrous, smooth or ribbed. Pappus of several series of slender, plumose bristles, connate at the base. [Greek, referring to the use of the thistle as a remedy for swollen veins.]
Over 200 species, widely distributed in the northern hemisphere. Besides the following, some 50 others occur in the southern and western parts of North America and many hybrids have been described. Type species: Carduus heterophyllus L.
† Outer involucral bracts, or all of them, strongly prickly-pointed. 1. Leaves glabrous or hispid above, tomentose beneath. All the bracts of the involucre tipped with prickles; naturalized weed.
1. C. lanceolatum.
Outer bracts prickle-tipped, the inner merely acuminate; native species. Branches leafy up to the heads; involucral bracts firm or rigid. Leaves undivided, lobed or dentate, rarely pinnatifid.
2. C. altissimum.
Leaves deeply pinnatifid into lanceolate or linear segments.
3. C. discolor.
Heads naked-peduncled, 1' high: involucral bracts thin.
4. C. virginianum.
2. Leaves tomentose on both sides, or becoming glabrous above; western. Leaves pinnately parted; segments linear, entire or lobed.
5. C. Pitcheri.
Leaves pinnatifid into triangular or lanceolate dentate segments. Outer bracts with spines less than one-half their length. Leaf-lobes triangular; flowers pink or purple.
6. C. undulatum.
Leaf-lobes linear-lanceolate to oblong. Flowers yellow or cream-color.
7. C. plattense.
8. C. Flodmant.
Outer bracts with spines of nearly or quite their length.
9. C. ochrocentrum.
Leaves entire or undulate: outer pappus-bristles barbellate.
3. Leaves green both sides, somewhat pubescent beneath. Leaf-lobes acute; bracts of the involucre faintly nerved; roots solid.
11. C. odoratum.
Leaf-lobes blunt: bracts with prominent glutinous midnerve; root hollow.
†† Bracts of the involucre not at all prickly-pointed, or scarcely so. Heads large, few, 1-4' broad; flowers all perfect and fertile.
Heads involucrate by the upper very spiny leaves; flowers usually yellow.
13. C. horridulum.
Heads peduncled, naked, or with 1 or 2 bracts at the base; flowers purple.
14. C. muticum.
Heads small, numerous, 1' or less broad.
Heads partly dioecious; leaves not decurrent.
15. C. arvense.
Heads not dioecious; leaf-bases decurrent.
16. C. palustre.