Lactuca canadensis 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 usually biennial 3–25 dm, glabrous or occasionally coarsely hirsute, often ± glaucous; lvs highly variable, entire or toothed to pinnately lobed or cleft, sagittate or sometimes narrowed to the base, 10–35 × 1.5–12 cm; heads numerous, small, with 13–22 yellow fls; invol 10–15 mm in fr; achenes blackish, very flat, with a median nerve on each face, transversely rugulose, 4.5–6.5 mm overall, the slender beak from a little more than half to about as long as the body; mature pappus 5–7 mm; 2n=34. Fields, waste places, and woods; Que. and P.E.I. to Sask., s. to Fla. and Tex. July–Sept. (L. sagittifolia; L. steelei)