[+] Leaves white-woolly beneath, and sometimes also above; outer scales successively shorter, and tipped with short prickles.
2. C. Pítcheri, Torr. White-woolly throughout, low; stem very leafy; leaves all pinnately parted into rigid narrowly linear and elongated, sometimes again pinnatifid divisions, with revolute margins; flowers cream-color. (Cirsium, Torr. & Gray.)—Sandy shores of Lakes Michigan, Huron, and Superior.
3. C. undulàtus, Gray. White-woolly throughout, low and stout, leafy; leaves lanceolate-oblong, partly clasping, undivided, undulate-pinnatifid, or rarely pinnately parted, moderately prickly; flowers reddish-purple. (Cirsium, Spreng.)—Islands of L. Huron to Minn., Kan., and westward. The heads vary much in size.
4. C. altíssimus, Willd. Stem downy, branching (3–10° high), leafy quite to the heads; leaves roughish-hairy above, whitened with close wool beneath, oblong-ovate to narrowly lanceolate, undivided, sinuate-toothed, undulate-pinnatifid, or twice pinnatifid, the lobes or teeth weakly prickly; heads 1½–2´ high; flowers chiefly purple. (Cirsium, Spreng.)—Fields and copses, Mass. to Minn., and southward.
Var. discolor, Gray. Stem 2–6° high; leaves nearly all deeply pinnatifid into lanceolate or linear lobes. (Cirsium discolor, Spreng.)—Common; N. Eng. to Ill., and southward.
5. C. Virginiànus, Pursh. Stem woolly, slender, simple or sparingly branched (1–3° high), the branches or long peduncles naked; leaves lanceolate, green above, whitened with close wool beneath, ciliate with prickly bristles, entire or sparingly sinuate-lobed, sometimes the lower deeply sinuate-pinnatifid; heads small; outer scales scarcely prickly; flowers purple. (Cirsium, Michx.)—Woods and plains, Va., Ohio, and southward.
[+][+] Leaves green both sides, or only with loose cobwebby hairs underneath; heads large; scales scarcely prickly-pointed.
6. C. mùticus, Pursh. (Swamp Thistle.) Stem tall (3–8° high), angled, smoothish, panicled at the summit; branches sparingly leafy, bearing single or few rather large heads; leaves somewhat hairy above, whitened with loose webby hairs beneath when young, deeply pinnatifid, the divisions lanceolate, acute, cut-lobed, prickly-pointed; scales of the webby and glutinous (sometimes glabrate) involucre closely appressed, pointless or barely mucronate; flowers purple. (Cirsium, Michx.)—Swamps and low woods; common.
7. C. pùmilus, Torr. (Pasture Thistle.) Stem low and stout (1–2° high), hairy, bearing 1–3 very large heads (1½´ broad), which are often leafy-bracted at the base; leaves green, lanceolate-oblong, partly clasping, somewhat hairy, pinnatifid, with short and cut very prickly-margined lobes; outer scales prickly-pointed, the inner very slender; flowers purple or rarely white (fragrant, 2´ long). (Cirsium, Spreng.)—Dry fields, Maine to Penn., near the coast.
[*][*][*][*] Outer scales of the appressed involucre barely prickly-pointed; heads imperfectly diœcious, small and numerous.