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, simple or little branched herbs, with opposite leaves, or the upper rarely alternate, and large, long-peduncled heads of both tubular and radiate, yellow flowers, or rays wanting in some species. Involucre turbinate or campanulate, its bracts in 1 or 2 series, narrow, nearly equal. Receptacle flat, naked, fimbrillate or villous. Ray-flowers pistillate, fertile, the rays spreading, entire, or 2-3-toothed. Disk-flowers perfect, fertile, the corolla 5-lobed, the style with slender branches. Anthers entire or minutely 2-auriculate at the base. Achenes linear, 5-10-ribbed, more or less pubescent. Pappus a single series of rough or barbellate, rigid, slender bristles. [Derivation uncertain, perhaps from Ptarmica.]
About 45 species, natives of the northern hemisphere. Besides the following, many others occur in the western parts of North America. Type species: Arnica montana L.
Basal leaves ovate or oval, sessile; southern.
1. A. acaulis.
Basal leaves oblong, lanceolate, or cordate-ovate, petioled. Basal leaves cordate-ovate.
2. A. cordifolia.
Basal leaves not cordate, tapering to the petiole. Leaves dentate.
Pappus brownish, plumose.
3. A. mollis.
Pappus white, barbellate.
4. A. chionopappa.
Leaves entire or nearly so.
5. A. alpina.
Glandular-hirsute; stem 1°-3° high, bearing several slender-peduncled heads at the summit. Basal leaves tufted, ovate or oval, obtuse, narrowed to a sessile base, denticulate or entire, 2'-5' long, 1 1/2'-3' wide; stem leaves 1-3 pairs, and some alternate, very small ones above; heads 1'-1 1/2' broad; bracts of the involucre linear-lanceolate, acute or acutish; rays 12-15, commonly 3-toothed at the truncate apex; achenes pubescent when young, glabrous or nearly so when mature.
Arnica cordifolia Hook. Fl. Bor. Am. 1: 331. 1833- Villous or pubescent; stem simple or sparingly branched, glandular above, 1°-2° high. Basal and lower leaves ovate to nearly orbicular, obtuse or acute, deeply cordate at the base, dentate, 1-3' long, with slender sometimes margined petioles; stem leaves 1-3 pairs, ovate to oblong, sessile or short-petioled, much smaller; heads 1-8, 2'-3' broad; bracts of the involucre acute or acuminate, villous, 6"-10" long; rays 12-16, toothed at the apex; achenes hirsute-pubescent, or glabrous at the base; pappus barbellate, white.
Lake Superior to North Dakota, Yukon, Montana, New Mexico and California. Recorded from western Nebraska. May-July.