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..
Tall leafy herbs, with small panicled heads of yellow, white, pink, or blue flowers, and alternate leaves. Involucre cylindric, its bracts imbricated in several series, the outer shorter, or of 1 or 2 series of principal nearly equal inner bracts, and several rows of short outer ones. Receptacle flat, naked. Rays truncate and 5-toothed at the summit. Anthers sagittate at the base. Style-branches mostly slender. Achenes oval, oblong or linear, flat, 3-5-ribbed on each face, narrowed above or tipped by a filiform beak, which is somewhat expanded at the summit into a small disk bearing the copious soft capillary white or brown pappus-bristles. [The Ancient Latin name, from lac, milk, referring to the milky juice.]
Panicle widely branching; achene about as long as its beak.
1. L. virosa.
Panicle-branches nearly erect; achene shorter than its beak.
2. L. saligna.
Native species; heads several- to many-flowered.
Leaves, or their lobes, spinulose-denticulate: stem leafy below.
3. L. ludoviciana.
Leaves entire to pinnatifid, the teeth not spinulose; stem leafy to the inflorescence. Leaves hirsute or bristly on the veins beneath.
4. L. hirsuta.
Plant hirsute below; leaves pinnatifid or the upper entire.
Plant hirsute up to the inflorescence; leaves merely dentate or denticulate.
5. L. Steelei.
Leaves entire to pinnatifid, not sagittate at base.
6. L. canadensis.
Leaves entire or denticulate, sagittate at base.
7. L. sagittifolia.
B. Achenes truncate, or narrowed into stout beaks; rays blue to white. Perennial: achenes flattened.
Annuals or biennials; achenes swollen. Pappus bright white.
Achenes beakless: leaves dentate, acuminate.
9. L. villosa.
Achenes beaked; leaves pinnatifid.
10. L. floridana.
Pappus brown; achenes short-beaked.
11. L. spicata.
Lactuca virosa L. Sp. Pl. 795. 1753. Lactuca Serriola L. Cent. Pl. 2: 29. 1756. Lactuca Scariola L. Sp. Pl. Ed. 2, 1119. 1763.
Biennial, green and glaucous; stem stiff, leafy, usually paniculately branched, glabrous throughout, or bristly-hirsute at the base, 2°-7° high. Leaves oblong or oblong-lanceolate, spinulose-margined, denticulate or pinnatifid, sessile or auriculate-clasping, the lowest sometimes 10' long and 3' wide, the upper much smaller; midrib spinulose or hispid; heads 2"-4" broad, 6-12-flowered; very numerous in an open panicle; involucre cylindric, 1"-1 1/2" thick, its outer bracts about one-third the length of the inner; rays yellow; achenes obovate-oblong, about as long as the filiform beak; pappus white.
In fields and waste places, Vermont to Georgia, west to the Pacific Coast. A troublesome weed. Naturalized from Europe. Consists of several races, differing in the form, lobing and position of the leaves. Aug.-Sept. Milk- or horse-thistle.