§ 1. SCARÌOLA. Achenes very flat, orbicular to oblong, 1-nerved on each face, with a filiform beak; biennial or annual; cauline leaves sagittate-clasping.
L. Scarìola, L. (Prickly Lettuce.) Stem below sparsely prickly-bristly, as also the midrib on the lower face of the oblong or lanceolate spinulose-denticulate vertical leaves; panicle narrow; heads small, 6–12-flowered; achenes striate.—Waste grounds and roadsides, Atlantic States to Mo. and Minn. (Adv. from Eu.)
1. L. Canadénsis, L. (Wild Lettuce.) Mostly tall (4–9° high), very leafy, smooth or nearly so, glaucous; leaves 6–12´ long, pale beneath, mostly sinuate-pinnatifid, the upper lanceolate and entire (rarely all but the lower narrow and entire); heads about 20-flowered, 3–6´´ long, numerous, in long and narrow or diffuse panicles; flowers pale yellow; achene oval, rather longer than the beak.—Rich damp soil, borders of fields or thickets; common.
2. L. integrifòlia, Bigel. Less leafy, 3–4° high, loosely branched above or heads loosely panicled; leaves undivided, oblong-lanceolate, pointed, denticulate or entire; flowers yellow or purplish. (L. Canadensis, var. integrifolia, Torr. & Gray.)—N. Eng. to Ill., and southward.
3. L. hirsùta, Muhl. Rather few-leaved, 2–3° high, commonly hirsute at base; leaves hirsute both sides or only on the midrib, mostly runcinate-pinnatifid; heads in a loose open panicle; achenes oblong-oval, about as long as the beak; flowers yellow-purple, rarely whitish. (L. Canadensis, var. sanguinea, Torr. & Gray.)—E. Mass. to Minn., and southward.
4. L. Ludoviciàna, DC. Glabrous, leafy, 2–5° high; leaves oblong, sinuate-pinnatifid and spinulosely dentate, ciliate; heads in an open panicle; involucre more imbricate; flowers yellow.—Minn., Iowa, and southwestward.
§ 2. LACTUCÁSTRUM. Achenes flat, lanceolate-oblong, tapering to a short slender beak; perennial; flowers blue.
5. L. pulchélla, DC. Pale or glaucous; stem simple, 1–2° high; leaves sessile, oblong- or linear-lanceolate, entire, or the lower runcinate-pinnatifid; heads few and large, racemose, erect on scaly-bracted peduncles; involucral scales imbricated in 3 or 4 ranks. (Mulgedium, Nutt.)—Upper Mich. to Minn.; common on the plains westward.
§ 3. MULGÈDIUM. Achenes thickish, oblong, contracted into a short thick beak or neck; annual or biennial; flowers chiefly blue.
6. L. acuminàta, Gray. Tall biennial (3–7° high), with many small heads in a loose panicle, on diverging peduncles; leaves ovate to oblong-lanceolate, pointed, sharply and sometimes doubly serrate, sometimes hairy on the midrib beneath, contracted into a winged petiole, the lowest occasionally sinuate or cleft at base, and the cauline sagittate or hastate; achenes beakless; pappus white. (Mulgedium, DC.)—Borders of woods, N. Y. to Ill. and Fla.