10. Lactuca sativa Linnaeus, Sp. Pl. 2: 795. 1753.
Annuals or biennials, (15–)30–70(–100+) cm. Leaves on proximal 1/2–3/4 of each stem; blades of undivided cauline leaves ovate to orbiculate, margins entire or denticulate, seldom prickly, midribs rarely prickly-setose. Heads usually in corymbiform, sometimes in paniculiform, arrays. Involucres 8–13+ mm. Phyllaries usually erect in fruit. Florets 7–15(–30+); corollas yellow (sometimes streaked with violet), usually deliquescent. Cypselae: bodies pale grayish to whitish or tan, ± flattened, obovate, 3–4 mm, beaks ± filiform, 3–5 mm, faces 5–9-nerved; pappi white, 3.5–4+ mm. 2n = 18.
Flowering mostly Jul–Sep. Disturbed sites, abandoned plantings; 10–1000 m; introduced; Ont.; Ala., Calif., Conn., Del., D.C., Idaho, Ill., Ind., Maine, Mass., Mich., Mo., N.H., N.J., N.Mex., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., Vt., Wash., W.Va.; Eurasia; introduced also in Mexico; introduced or ephemeral nearly worldwide.