Description from Flora of China
Lactuca scariola Linnaeus var. sativa (Linnaeus) Moris.
Herbs 25-100 cm tall, annual or biennial. Stem solitary, whitish, yellowish green, or glaucous green, erect, branched apically, glabrous. Leaves green, glabrous, rather soft. Basal and lower stem leaves sessile, oblanceolate to elliptic-oblanceolate, 6-15 × 1.5-6.5 cm, undivided, base cordate to sagittate and semiamplexicaul, margin sinuate-dentate, apex acute, acuminate, or rounded. Upper stem leaves similar to lower stem leaves or lanceolate, smaller; uppermost leaves ovate, very small, base cordate to sagittate and clasping, margin entire. Synflorescence corymbosely paniculiform, with very numerous capitula. Capitula with usually 10-30 florets. Involucre narrowly cylindric and 0.9-1.3 cm at anthesis, ovoid and ca. 6 mm in diam. in fruit. Phyllaries green to pale yellowish green, glabrous, usually erect in fruit, apex obtuse; outer phyllaries broadly triangular to lanceolate, approaching inner phyllaries in length; inner phyllaries ca. 8. Florets yellow. Achene body pale brown, narrowly obovoid, ca. 4 mm, compressed, with 5-7 ribs on either side; beak white, filiform, 2-4 mm. Pappus 3-4 mm. Fl. and fr. Feb-Sep. 2n = 18*.
Lactuca sativa was first known in cultivation in ancient Egypt. It is widely cultivated as a vegetable in China. Leaves of the species contain Vitamins A, B9, C, and K and can be eaten as a salad. Lactuca sativa has many cultivars, which are usually treated as varieties. Lactuca sativa var. angustata Irish ex Bremer is the most commonly cultivated variety in China, where the thick stems are cooked as a vegetable.
Cultivated throughout China [probably originating from E Mediterranean to SW Asia].