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..
Annual or perennial herbs (some tropical species shrubby or even arborescent), with alternate or basal leaves, and solitary corymbose or paniculate many-flowered heads, of both tubular and radiate, or only tubular flowers, in our species yellow. Involucre cylindric or campanulate, its principal bracts in 1 series, distinct, or united at the base, usually with some shorter outer ones. Receptacle flat or slightly convex, mostly naked, often honeycombed. Rays, when present, pistillate, fertile. Disk-flowers perfect, fertile, their corollas tubular, the limb 5-toothed or 5-lobed. Anthers obtuse and entire at the base, or rarely slightly sagittate. Style-branches of the disk-flowers usually recurving or spreading. Achenes terete, or those of the marginal flowers somewhat compressed, 5-10-ribbed, papillose or canescent, at least after wetting, and then usually emitting a pair of spiral threads. Pappus of numerous slender or capillary, smooth or rough, mostly white bristles. [Latin, senex, an old man, referring to the hoary character of some species, or to the white pappus.]
An immense genus of probably at least 1200 species, of very wide geographic distribution. In addition to the following, many others occur in the southern and western parts of North America. Our species known as Groundsel, Ragwort, or Squaw-weed. Type species: Senecio vulgaris L.
A. Annual or biennial species, with stems leafy throughout. Rays none, or very short and inconspicuous; introduced. The short outer involucral bracts black-tipped; rays none.
1. S. vulgaris.
The short outer involucral bracts not black-tipped, sometimes none; rays very short. Plant sparingly pubescent.
2. S. sylvaticus.
Plant densely viscid-pubescent.
3. S. viscosus.
Rays large and conspicuous; native species. Leaves pinnately divided; heads 2" - 3" high.
4. S. glabellus.
Leaves sinuate-dentate or the lower entire; heads 4"-s" high.
5. S. palustris.
B. Perennial species, mostly with rootstocks. a. Stems woody at base; leaves linear, or pinnatifid with linear lobes; western species. Leaves linear, entire or serrate, thin.
6. S. spartioides.
Leaves deeply pinnatifid into linear lobes, firm.
7. S. Riddellii.
b. Stems wholly herbaceous; leaves various. * Heads very large; involucre 7"- 10" high; boreal species.
Stem stout, leafy above; heads several, 1 1/2' - 2' broad.
8. S. Pseudo-Arnica.
Stem slender; upper leaves few and small; head mostly solitary, about 1' broad.
9. S. fngidus.
** Heads smaller; involucre 4"-7" high, † Leafy up to the inflorescence; stem leaves 2-3-pinnatifid; introduced species.
10. S. Jacobaea.
ft Leaves mostly borne on the lower part of the stem, the upper ones much smaller; native species.
‡ Leaves and stems more or less persistently woolly or tomentose. Low species, seldom over 1° high, with small oval-oblong to spatulate basal leaves. Basal leaves angulate-dentate, oval.
11. S. antennariifolius.
Basal leaves entire or sparingly toothed, oblong to spatulate.
12. S". canus.
Taller, up to 2 1/2 ° high; basal leaves ovate to oblong-lanceolate.
Densely persistently tomentose; stem-leaves mostly merely dentate.
13. S. tomentosus.
Loosely tomentose, bcoming glabrate; stem leaves mostly pinnatifid.
14. S. plattensis.
XX Plants glabrous, or nearly so, at least when mature, the stem sometimes tomentose at the base.
Basal leaves or some of them deeply cordate. Basal leaves lanceolate or oblong-lanceolate.
15. S. Robbinsii.
Basal leaves orbicular to ovate.
16. S. aureus.
None of the leaves cordate.
At least the stein-leaves lobed, laciniate, or pinnatifid; pubescence, if any, woolly; basal leaves dentate or crenate.
17. S. discoideus.
Head rayless; boreal species. Heads radiate.
Basal leaves obovate to suborbicular.
18. S. obovatus.
Basal leaves oblong to spatulate.
Basal leaves oblong to lanceolate, dentate or crenate. Involucre 4" high; basal leaves sharply serrate.
19. S. Crawfordii.
Involucre 2 1/2" - 3 1/2" high; basal leaves mostly crenate. Heads few; basal leaves mostly short.
20. S. pauperculus.
Heads very numerous; basal leaves long.
Basal leaves linear-cuneate, entire, or few-toothed at the apex.
22. S. densus.
All leaves entire or very nearly so; pubescence, if any, of crisp hairs.
23. S. integerrimus