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..
2. Heads with both ray-flowers and disk-flowers (in our species). a. None of the leaves cordate; pappus mostly as long as the achene, or longer. Leaves bristly-serrate or pinnatifid.
Achenes glabrous; pappus-bristles deciduous.
Achenes white-tomentose: pappus-bristles persistent.
Leaves entire, or toothed, not bristly-serrate.
Leaves narrow, coriaceous, evergreen, entire; low western undershrubs.
Annual or perennial herbs; leaves not evergreen.
Pappus-bristles unequal; heads loosely panicled; annual.
Pappus-bristles equal; heads variously clustered; perennial herbs.
Receptacle alveolate; rays mostly not more numerous than the disk-flowers.
Receptacle fimbrillate; rays more numerous than the disk-flowers; heads corymbose-paniculate.
b. Lower and basal leaves cordate: pappus-bristles shorter than the achene.
B. Ray-flowers present, not yellow in any of our species. * Pappus a mere crown, or of a few awns or bristles, or wanting, never of numerous capillary bristles.
1. Receptacle conic. Outer bracts of the involucre shorter than the inner; achenes prismatic.
Bracts of the involucre all about equal; achenes flattened.
2. Receptacle flat, or somewhat convex.
Achenes of the disk-flowers compressed, of the ray-flowers 3-angled; perennial herbs with large heads.
Achenes fusiform, terete: annual herbs with small heads.
Achenes obovate, flattened, with thickened or winged margins; perennials.
** Pappus of numerous capillary bristles.
1. Pappus a single series of capillary bristles; sometimes with an outer series of shorter ones.
a. Bracts of the involucre in 2 to many series.
Bracts mostly in 2-5 series; teeth or lobes of the leaves not bristle-tipped. Involucre narrow, its bracts firm: rays few, white.
Disk-flowers permanently white; leaves subulate to spatulate, crowded, rigid.
Involucre turbinate; rays not exceeding the mature pappus, or none.
Bracts in many series; teeth or lobes of the leaves bristle-tipped in our species.
b. Bracts of the involucre mostly in 1 or 2 series, very narrow; heads mostly long-peduncled. Rays longer than the diameter of the disk.
Rays not longer than the diameter of the disk; heads very small, panicled.
2. Pappus distinctly double, the inner bristles long, the outer shorter. Leaves lanceolate, ovate, or obovate: rays white.
Leaves narrowly linear; rays violet.
C. Ray-flowers none; dioecious shrubs; pappus capillary.
Tribe 4. Inuleae.
* Heads small, rays none; flowers white, or whitish.
1. Receptacle chaffy. Receptacle convex; pappus none.
Receptacle subulate; pappus of the inner flowers of rough capillary bristles.
2. Receptacle naked. a. Pappus, at least that of pistillate flowers, of capillary bristles. Bracts of the involucre not scarious; plants pubescent or glabrous, not woolly.
Bracts of the involucre scarious, mostly white or pink; plants woolly. Plants dioecious, or polygamo-dioecious.
Pappus-bristles of staminate flowers thickened above.
Pappus-bristles not thickened; stem leafy.
Plants not dioecious; flowers all fertile.
b. Pappus none: leaves broad, alternate, woolly beneath.
** Heads large; ray-flowers yellow.
Key to Genera.
Tribe 5. Heliantheae. A. Disk-flowers perfect, but sterile.
Achenes thick, short, not flattened; pappus none.
Achenes merely subtended by the inner involucral bracts.
Achenes embraced and enclosed by the inner involucral bracts. Involucral bracts unarmed.
Involucral bracts armed with hooked prickles.
Ray-flowers in 2 or 3 series; achenes falling away free.
Ray-flowers in 1 series; achenes adnate to 2 or 3 scales of the receptacle, falling away with them. Rays large, yellow.
Leaves opposite and basal.
Pappus none, or of 2 caducous awns.
Pappus a persistent irregularly cleft crown.
Rays small, the head appearing discoid.
B. Disk-flowers fertile. * Ray-flowers persistent upon the achenes.
Achenes compressed, or 3-angled; leaves entire.
Achenes short, thick; leaves toothed.
** Ray-flowers deciduous, or none.
† Pappus a cup, or crown, or of a few teeth, awns, or bristles.
1. Achenes, at least those of disk-flowers, not compressed (except in Ratibida and Phaethusa); scales of the receptacle mostly concave, or clasping.
Scales of the receptacle small, awn-like or bristle-like; rays white, short.
Scales of the receptacle broad, larger.
Involucre of 4 large somewhat united bracts.
Involucre of several or numerous separate bracts. Receptacle conic, or columnar.
Ray-flowers fertile, or wanting; leaves opposite.
• 60. Spilanthes.
Ray-flowers sterile, or neutral; leaves mostly alternate. Rays yellow.
Achenes 4-angled or terete. Achenes 4-angled.
Achenes terete: leaves cordate-clasping.
Achenes compressed, winged.
Rays rose-purple or yellow.
Receptacle flat, or convex (low-conic in species of Nos. 66 and 68). Low fleshy sea-coast shrubs.
Tall herbs, not fleshy.
Achenes not much flattened, not winged, nor margined.
Achenes of disk-flowers flattened and margined, or winged. Involucre of a few deflexed bracts.
Involucre of 2 series or more of appressed or spreading bracts. Perennials: bracts erect or appressed.
Annuals; bracts spreading.
2. Achenes very flat; scales of the receptacle flat, or but slightly concave. a. Bracts of the involucre all separate. Pappus of 2 short teeth or awns, or a mere border, or none.
Pappus of 2-6 awns or teeth, upwardly or downwardly barbed or hispid. Achenes flat, or angled.
Achenes terete: aquatic, the submerged leaves filiform-dissected.
b. Inner bracts of the involucre united to about the middle.
†† Pappus of numerous scales. Leaves opposite, toothed; ray-flowers fertile; rays small.
Leaves alternate, entire.
Rays large, neutral; receptacle deeply honey-combed.
Rays none; scales of the receptacle narrow, rigid.
Tribe 6. Helenieae.
A. Ray-flowers persistent on the achenes, falling away with them, papery.
B. Ray-flowers deciduous, or wanting. * Plants not dotted with oil-glands.
a. Pappus none.
b. Pappus present (in all our species), of separate scales or bristles. 1. Bracts of the involucre petal-like, colored, their margins and apices scarious. Leaves, at least the lower, pinnately parted, or pinnatifid; rays none; corolla-lobes of disk-flowers ovate.
Leaves entire; rays present, or none: corolla-lobes of disk-flowers linear.
2. Bracts of the involucre herbaceous, not scarious-tipped, nor petal-like, appressed, or spreading. Receptacle naked.
Bracts of the involucre appressed. Achenes 4-angled, linear or oblong.
Achenes 5-10-ribbed or 5-10-angled, top-shaped. Involucral bracts separate to the base.
Outer involucral bracts connate.
Bracts of the involucre spreading, or reflexed at maturity.
Receptacle with bristle-like chaff.
** Plants dotted with oil-glands, especially the leaves and involucre.
Involucral bracts more or less united; style-branches of the disk-flowers elongated. Involucral bracts united at the base.
Involucral bracts united high up into a cup.
Involucral bracts separate; style-branches of the disk-flowers very short.
Tribe 7. Anthemideae.
* Receptacle chaffy.
Achenes flattened; involucre obovoid to campanulate; heads small.
Achenes terete; involucre hemispheric; heads large.
** Receptacle not chaffy, naked, or sometimes hairy. 1. Ray-flowers usually present, sometimes wanting. Receptacle flat to hemispheric; bracts of the involucre in several series.
Receptacle conic to ovoid; bracts in few series.
2. Ray-flowers none; heads small. Heads corymbed; pappus a short crown; flowers yellow.
Heads racemose, spicate or panicled; pappus none.
Tribe 8. Senecioneae.
Leaves all basal; heads on scapes. Heads solitary; flowers yellow.
Heads corymbed; flowers white or purple.
Leaves opposite; rays yellow.
Involucre of several thin herbaceous bracts.
Involucre of 4 or 5 broad fleshy bracts.
Flowers white, whitish or pinkish; rays none.
Marginal flowers pistillate: disk-flowers perfect.
Flowers all perfect.
Involucre of about 5 bracts; sap milky.
Involucre of about 12 bracts and several smaller outer ones.
Flowers yellow; ray-flowers mostly present.
Tribe 9. Cynareae.
* Achenes inserted on the receptacle by their bases, not oblique.
Receptacle densely bristly. Filaments separate.
Involucral bracts hooked at the tip; leaves not bristly.
Involucral bracts not hooked; leaves bristly. Pappus-bristles plumose.
Pappus-bristles not plumose.
Filaments united below.
Receptacle fleshy, not bristly.
** Achenes obliquely inserted on the receptacle. Heads not subtended by bristly leaves; involucral bracts often bristly.
Heads sessile, subtended by bristly leaves.