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, with opposite serrate lobed divided or dissected leaves, or the uppermost alternate, and mostly large heads of both tubular and radiate flowers, or the rays none, or rudimentary. Involucre campanulate or hemispheric, its bracts in 2 series, distinct, or slightly united at the base; the outer often foliaceous and much larger than the inner. Receptacle flat or nearly so, chaffy, the chaff subtending the disk-flowers. Rays, when present, neutral, mostly entire, yellow in our species. Disk-flowers perfect, fertile, their corollas tubular, 5-toothed. Anthers entire, or minutely sagittate at the base. Style-branches with short or subulate tips. Achenes flat, or quadrangular, cuneate, oblong or linear, the outer ones often shorter than the inner. Pappus of 2-6 teeth or subulate awns, upwardly or downwardly barbed or hispid. [Latin, two-toothed, referring to the achenes.]
About 75 species of wide geographic distribution. Besides the following, about 12 others occur in the southern and southwestern United States. Type species: Bidens tripartita L.
Leaves lanceolate, serrate, undivided, rarely 3-5-lobed or incised. Rays present, large and conspicuous.
Rays large, longer than the involucral bracts.
1. B. laevis.
Rays short, rarely as long as the involucral bracts.
2. B. cernua.
Rays rudimentary, or none.
Heads nodding after flowering.
2. B. cernua.
Heads persistently erect.
Pappus awns downwardly barbed.
3. B. connata.
Involucral bracts foliaceous; stem straw-color; flowers greenish yellow.
4. B. comosa.
Pappus awns upwardly barbed; involucre narrow.
5. B. bidentoides.
Leaves, some or all of them, pinnately 1-3-parted or dissected. Rays rudimentary, or none, or very short.
Achenes flat; leaves, some or all of them, 1-3-divided.
6. B. discoidea.
Leaves not membranous; heads 5" -7" high: awns long.
7. B. frondosa.
Outer involucral bracts 10-16; achenes brown.
8. B. vulgata.
Achenes linear; leaves dissected.
9. B. bipinnata.
Rays large and conspicuous.
Achenes sparingly pubescent, not ciliate; pappus of 2 short teeth.
10. B. coronata.
Achenes ciliate; pappus 2-4 subulate teeth or awns. Achenes cuneate, or linear-cuneate.
Achenes obovate, very flat.
Bracts of the involucre glabrous, or ciliate, short.
12. B. aristosa.
Outer bracts densely hispid, much longer than the inner.
13. B. involucrata.
Helianthus laevis L. Sp. P1. 906. 1753- Bidens chrysanthemoides Michx. Fl. Bor. Am. 2:
Annual; glabrous throughout; stems branched, erect or ascending, 1°-3° high. Leaves sessile, lanceolate, evenly serrate or serrulate, acuminate at the apex, narrowed to the sometimes connate-perfoliate base, 3'-8' long, 1/4'-1' wide; heads numerous, short-peduncled, erect in flower, often declined in fruit, 1'-21/2' broad; rays very showy, golden yellow; involucre hemispheric, its outer bracts linear-oblong or spatulate, equalling or exceeding the broader, ovate or oblong, membranous inner ones, shorter than the rays; rays 8-10, obovate-oblong, obtuse; achenes cuneate, truncate, 2"-4" long, retrorsely hispid on the margins; pappus of 2-4 (usually 2), rigid downwardly barbed awns, shorter than the achene.
In swamps and wet meadows, Massachusetts to Illinois, Kansas, Florida, Louisiana and Mexico. Northern records of this species apply to Bidens cernua. Aug.-Nov.
Bidens elegans Greene, of southeastern Virginia, appears to be a narrow-rayed race of this species.