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..
28. A. nebraskensis.
Leaves linear to linear-lanceolate, 2"-2" wide.
29. A. amethystinus.
** Leaves, at least the lower, serrate. Stem usually pilose; bracts very glandular.
30. A. modestus.
Stem hispid-pubescent; bracts glabrous, or ciliate.
31. A. puniceus.
2. Stem glabrous, or only sparingly pubescent above. * Leaves sharply serrate. § Leaves tapering to the base. Leaves narrowed to the base, the lower into winged petioles.
32. A. tardiflorus.
Leaves scarcely or gradually narrowed to the base.
31. A. puniceus.
§8 Leaves abruptly contracted into margined petioles, often enlarged near the base.
33. A. prenanthoides.
§§§ Leaves usually strongly cordate-clasping; bracts green-tipped.
34. A. laevis.
** Leaves entire, or very nearly so. § Involucre campanulate, its bracts appressed, green-tipped. † Bracts of the involucre with rhomboid green tips. Stem leaves oblong, lanceolate, or oval-lanceolate.
34. A. laevis.
Stem leaves elongated-lanceolate.
35. A. concinnus.
† Bracts of the involucre linear, the tips narrower, lanceolate; stem leaves linear or narrowly lanceolate.
36. A. purpuratus.
§§ Involucre hemispheric, † Bracts of the involucre narrow, not foliaceous.
(a) Bracts in several series, unequal. Bracts linear-subulate; leaves narrowly linear.
37. A. junceus.
Bracts lanceolate, linear, or spatulate; leaves lanceolate to linear. Western dry soil plant, 1°-2° tall; leaves 1-3' long.
38. A. adscendens.
Eastern swamp plant, 2-5 tall; leaves 2-6 long.
39. A. novi-belgii.
(b) Bracts in only 1 or 2 series; leaves linear to lanceolate.
40. A. longifolius.
†† Bracts of the involucre green, foliaceous; western. Leaves lanceolate, firm; heads few.
Leaves oblong-lanceolate, thin; heads many.
42. A. phyllodes.
C. Leaves sessile or petioled, not at all clasping, or scarcely so. 1. Leaves silky, silvery or canescent on both sides, entire. Heads corymbose-paniculate; bracts oblong; achenes glabrous.
43. A. sericeus.
Heads in a narrow raceme; bracts linear; achenes silky.
44. A. concolor.
2. Leaves neither silvery, silky nor canescent, entire or toothed.
* Bracts of the involucre with herbaceous tips.
† Bracts thin or coriaceous, relatively small.
+ Leaves bristly-ciliate, linear, rigid; western species.
45. A. Fendleri.
++ Leaves not bristly-ciliate. ° Tips of the involucral bracts spreading (little spreading in A. Radula, erect or spreading in A. Herveyi); heads large and showy; rays violet to purple. Leaves linear to linear-oblong, rigid, obtuse, entire; heads 2' broad.
46. A. grandiflorus.
leaves lanceolate to oblong, the lower sparingly dentate. Basal leaves with margined petioles.
Involucre hemispheric to campanulate; heads 1' or more. Bracts of the involucre glandular.
47. A. spec abilis.
Bracts of the involucre ciliate, or glabrous.
48. A. surculosus.
Involucre turbinate; heads 6 -9 broad.
49. A. gracilis.
Basal leaves with unmargined petioles.
51. A. Herveyi.
Leaves oblong-lanceolate, sharply serrate, rugose, the basal usually wanting.
50. A. Radula.
' Involucral bracts all appressed (except in A. multiflorus and A. commutatus, small-headed species).
A Involucre top-shaped; rays violet.
52. A. turbinellus.
^^ Involucre hemispheric to campanulate; rays mostly white, sometimes purple. Heads unilaterally racemose.
Stem leaves oval, oblong, or lanceolate, serrate, or chiefly so. Stem pubescent or glabrate.
53. A. lateriflorus.
Stem villous; leaves narrowly lanceolate, thin.
54. A. hirsuticaulis.
Stem leaves linear-lanceolate to linear, nearly entire; stem glabrate.
55. A. vimineus.
Heads not unilaterally racemose, mostly paniculate.
Involucral bracts spatulate, mostly ciliate, somewhat spreading, at least the outer obtuse; plants roughish-puberulent. Heads 3"-4" broad, numerous; rays 10-20.
56. A. multiflorus.
Heads 5"-8" broad, relatively few; rays 20-30.
57. A. commutatus.
Involucral bracts appressed, acute.
Heads solitary at the ends of very small-leaved branchlets.
58. A. dumosus.
Stem leaves lanceolate, serrate or entire. Heads 8"-10" broad.
Plants glabrous, or sparingly pubescent above.
Leaves firm, roughish or rough; rays often purplish; involucral bracts acute.
59. A. salicifolius.
Leaves thin, smoothish; rays chiefly white; involucral bracts acuminate.
60. A. paniculatus.
Plant puberulent all over.
6t. A. missouriensis.
Heads 6"-8" broad; stem leaves narrowly lanceolate.
62. A. Tradescanti.
Stem leaves linear-lanceolate to subulate, mostly entire. Heads scattered, 6"-o" broad; upper leaves linear.
63. A. Faxoni.
64. A. ericoides.
Simple, or with slender ascending branches.
65. A. Pringlei.
Kays purple; heads 8-12 broad.
66. A. Prtceae.
Involucre top-shoped, 2 1/2" high or less. Plant pilose-pubescent, 1 1/2°-2° high.
6y. A. parviceps.
Plant glabrous, very slender, 1° high or less.
68. A. depauperatus.
†† Bracts stiff, relatively large; leaves narrow, rigid, entire. [Genus Heleastrum DC]
69. A. paludosus.
** Bracts of the involucre without herbaceous tips.
Bracts linear-subulate, acuminate.
Leaves firm, 3' long or less, entire or sparingly serrate.
70. A. nemoralis.
Leaves thin, 6' long or less, sharply serrate.
71. A. acutmnatus.
Bracts oblong or oblong-lanceolate, obtuse or acutish; leaves narrow, entire. [Genus Unamia Greene.]
72. A. ptarmicoides.
D. Leaves fleshy, narrow, entire; plants of salt marshes or saline soil (No. 74 sometimes in non-saline situations). Perennial; heads 6"-12" broad; involucral bracts lanceolate, acuminate.
73. A. tenuifolius.
74. A. exilis.
Involucre cylindraceous; disk-flowers fewer than the very short rays.
75. A. subulatus.