Lactuca serriola L.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Species Description - Leafy-stemmed annual or biennial, 3–15 dm, the stem often prickly below, otherwise glabrous; lvs prickly on the midrib beneath, and more finely prickly-toothed on the margins, otherwise generally glabrous, pinnately lobed or lobeless, commonly twisted at base to stand vertically, sagittate-clasping, oblong or oblong-lanceolate in outline, the upper much reduced; heads numerous in a long, often diffuse infl, with (13–)18–24(–27) light yellow fls, drying blue; invol 10–15 mm in fr; achenes gray or yellowish-gray, the body compressed, 3–4 mm, a third as wide, prominently several-nerved on each face, spinulose or hispidulous above at least marginally, the slender beak about equaling the body, rarely twice as long; 2n=18. A weed in fields and waste places; native of Europe, now naturalized throughout most of the U.S. July–Sept. The common form with lobeless lvs, called var. integrata Gren. & Gordon, may reflect introgression from L. sativa L., cultivated lettuce, with which L. serriola hybridizes freely. (L. scariola)