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, annual or perennial, mostly branched herbs, with opposite or alternate, simple leaves, and large peduncled corymbose or solitary heads of both tubular and radiate flowers, the rays yellow, the disk yellow, brown, or purple. Involucre hemispheric, or depressed, its bracts imbricated in several series. Receptacle flat, convex or conic, chaffy, the chaff subtending the disk-flowers. Ray-flowers neutral (in our species), the rays spreading, mostly entire. Disk-flowers perfect, fertile, the corolla tubular, the tube short, the limb 5-lobed. Anthers entire, or minutely 2-toothed at the base. Style-branches tipped with hirsute appendages. Achenes thick, oblong or obovate, compressed, or somewhat 4-angled. Pappus of 2 scales or awns, or sometimes with 2-4 additional shorter ones, deciduous. [Greek, sunflower.]
About 70 species, natives of the New World. Besides the following, about 30 others occur in the southern and western parts of North America, and hybrids may exist. Type species: Helianthus annuus L.
A. Annual species; disk brown or purple; heads large. Leaves dentate; bracts ovate to ovate-lanceolate, strongly ciliate.
1. H. annuus.
Leaves mostly entire; bracts lanceolate to ovate-lanceolate, canescent, sometimes ciliate.
2. H. petiolaris.
B. Perennial species.
* Disk purple or purple-brown.
Leaves narrowly linear or the lower broader, many of them alternate. Stem rough; leaves linear or linear-lanceolate.
3. H. angustifolius.
Stem smooth; leaves elongated-lanceolate.
4. H. orgyalis.
Leaves lanceolate to ovate, mainly opposite.
Leaves hispid, rather thin, abruptly contracted into winged petioles.
5. H. atrorubens.
Leaves firm in texture, gradually narrowed into petioles. Leaves lanceolate: bracts acute or obtusish.
6. H. scaberrimus.
Leaves rhombic-ovate to rhombic-lanceolate; bracts acute or acuminate.
7. H. subrhomboideus.
** Disk yellow or yellowish; receptacle convex or conic, † Leaves nearly all basal or near the base; upper bract-like.
8. H. occidentalis.
†† Stem leafy; leaves alternate or opposite. 1. Leaves prevailingly lanceolate and 3-8 times as long as wide. a. Leaves glabrous on both sides; heads 1' - 1 1/2' broad.
9. H. laevigatus.
b. Leaves scabrous, at least on the upper surface. Heads 1'- 1 1/2' broad; leaves thin; stem glabrous.
10. H. microcephalus.
Heads 1 1/2' - 3 1/2' broad; leaves firm, many of them often alternate. Stem scabrous, scabrate or hispid; leaves sessile or nearly so. Leaves lanceolate, scabrous above, hirsute beneath, flat.
11. H. giganteus.
Leaves very scabrous on both sides.
Leaves elongated-lanceolate, conduplicate, pinnately-veined. Heads numerous; leaves mostly alternate.
12. H. Maximiliani.
Heads only 1 or 2; leaves, all but the upper, opposite.
13. H. Dalyi.
Leaves lanceolate to ovate-lanceolate, flat, 3-nerved.
14. H. subtuberosus.
Stem glabrous; leaves sessile by a truncate base.
17. H. divaricatus.
Stem glabrous; leaves petioled.
Leaves lanceolate, firm, canescent and pale beneath.
15- H. grosse-serratus.
Leaves linear-lanceolate, thin, green on both sides.
16. H. Kellermani.
2. Leaves prevailingly ovate, ovate-lanceolate, or oblong. a. Leaves sessile, or very nearly so. Stem glabrous; leaves divaricate.
17. H. divaricatus.
Stem hirsute or hispid; leaves ascending. Leaves cordate-clasping at the base.
18. H. mollis.
Leaves narrowed from below the middle.
19. H. doronicoides.
b. Leaves manifestly petioled. Stem puberulent or glabrous.
Leaves membranous or thin, slender-petioled, sharply serrate.
20. H. decapetalus.
Leaves firmer, shorter-petioled, less serrate or entire. Bracts of the involucre much longer than the disk.
21. H. tracheliifolius.
Bracts of the involucre about equalling the disk.
22. H. strumosus.
Stem hirsute, hispid, or scabrous.
Leaves rounded or truncate at the base, short-petioled.
23. H. hirsutus.
Leaves, at least the upper, narrowed at the base.
Bracts of the involucre ovate-lanceolate, appressed.
24. H. lae iflorus.
Bracts of the involucre lanceolate-acuminate, spreading. Leaves villous-pubescent beneath.
25. H. totmentosus.
Leaves scabrous or puberulent beneath.
26. H. tuberosus.