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(Lactuca sativa), cultivated annual salad plant, probably derived from the prickly lettuce (L. scariola) of the family Asteraceae. Four botanical varieties of lettuce are cultivated: (1) asparagus lettuce (variety asparagina), with narrow leaves and a thick, succulent, edible stem; (2) head, or cabbage, lettuce (variety capitata), with the leaves folded into a compact head; (3) leaf, or curled, lettuce (variety crispa), with a rosette of leaves that are curled, finely cut, smooth-edged or oak-leaved in shape; and (4) cos, or romaine, lettuce (variety longifolia), with smooth leaves that form a tall, oblong, loose head. There are two classes of head lettuce: the butter-head types with soft heads of thick, oily leaves, and crisp-head types with brittle-textured leaves that form very hard heads under proper temperature conditions.

For successful cultivation lettuce requires ample water, especially in warmer weather. During unseasonable weather, protection is furnished and gr