13. C. aùrea, Ait. Nearly glabrous, 1–3° high; leaves variable, commonly 3–7-divided, or some or all undivided, the segments incisely serrate or lobed; achenes broadly cuneate, 1–2´´ long, with 2 very short blunt spreading teeth.—Wet ground, Va. to Fl.
14. C. trichospérma, Michx. (Tickseed Sunflower.) Smooth, branched; leaves short-petioled, nearly all 3–7-divided; leaflets lanceolate or linear, cut-toothed, or the upper leaves only 3–5-cleft and almost sessile; heads panicled-corymbose; achenes narrowly wedge-oblong or the inner ones wedge-linear, about 4´´ long, smooth or sparsely hairy, marginless, crowned with 2 erect triangular or awl-shaped stout teeth.—Swamps, Mass. to Va. near the coast. Also Buffalo, N. Y., to Ill., where is a var. tenuíloba, Gray, with shorter achenes, approaching the last. Aug.–Oct.
[++][++] Achenes obovate, very flat, with thin ciliate margins.
15. C. aristòsa, Michx. Somewhat pubescent; leaves 1–2-pinnately 5–7-divided, petioled; leaflets lanceolate, cut-toothed or pinnatifid; heads panicled-corymbose; outer scales 8–10, not exceeding the inner, barely ciliate; achenes with 2 (rarely 4) long and slender diverging awns as long as the achene itself.—Swamps, Ohio to Mich., Minn., and southwestward. Aug.–Oct.—Var. mùtica has two short divergent teeth or points in place of the awns.—W. Ill. and southwestward. Forms occur with the barbs of the awns spreading or retrorse, hybrids with Bidens frondosa or other species.
16. C. involucràta, Nutt. Heads rather larger, the outer scales 12–20, mostly exceeding the inner, slender and hispid; achenes with 2 short acute teeth.—W. Ill. to Kan. and Tex.
[+][+] Rays none, or rarely rudimentary; outer scales usually 3–5, loose, leafy, commonly surpassing the short-pedunculate heads; achenes narrowly cuneate; plants glabrous, 1–3° high; leaves petiolate.
17. C. bidentoìdes, Nutt. Paniculately branched; leaves undivided, lanceolate, coarsely toothed, tapering at both ends; heads 6–10´´ long; achenes nearly subulate, bearing a pair of very slender upwardly roughened awns surpassing the corolla (4´´ long), but shorter than the achene, often also 2 minute teeth alternate with the awns.—Shores of Delaware River, near Philad., and Delaware Bay, to Md. Hybridizes with Bidens frondosa.
18. C. discoídea, Torr. & Gray. Diffusely branched, 1–2° high; leaves ternately divided, slender-petioled; leaflets ovate-lanceolate, pointed, coarsely serrate; heads 2–3´´ long; achenes linear-wedge-shaped (2–3´´ long), bearing a pair of short and stout upwardly-barbed awns of the length of the corolla.—Wet banks and swamps, Conn. to Ohio, Ill., and southward. July.
56. BÌDENS, L. Bur-Marigold.
Heads many-flowered; the rays when present 3–8, neutral. Involucre double, the outer commonly large and foliaceous. Receptacle flattish; the chaff deciduous with the fruit. Achenes flattened parallel with the scales of the involucre, or slender and 4-sided, crowned with 2 or more rigid and persistent awns which are downwardly barbed.—Annual or perennial herbs, with opposite various leaves, and mostly yellow flowers. (Latin, bidens, two-toothed.)