lettuce, annual garden plant (Lactuca sativa and varieties) of the family Asteraceae (aster family), probably native to the East Indies or Asia Minor, possibly as a derivative of the widespread weed called wild lettuce (L. scariola). L. sativa has been grown as a salad plant since antiquity and is unknown in the wild. Three types of lettuce are planted: head, or cabbage, lettuce; the leaf, or loose, type; and Cos lettuce, or romaine. The first forms a tight, crisp, white head; the second has many more leaves and a less compact head, which is white toward its center only. Cos lettuce, or romaine, forms long, upright leaves, which, according to variety, may or may not have to be tied up to blanch and form a head. It is not as commonly planted, but is useful where summers are too hot for the other two varieties. As lettuce has increased in popularity in the United States, forcing it for winter use is becoming an extensive industry, especially near large cities. Much of the winter crop comes from Florida and California. The plant is generally eaten as a salad but may be cooked, as it often is in France. A narcotic from the thickened juice of some Lactuca species has been used as an opium substitute. Among the many north temperate species is L. canadensis, the American wild lettuce. Lettuce is classified in the division Magnoliophyta, class Magnoliopsida, order Asterales, family Asteraceae.
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