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Lettuce grows with a basal rosette of leaves up to 15 cm (10 in) long, loosely or tightly clustered, and is generally harvested before the flowering stem has bolted. If allowed to bolt, the plant may reach nearly 1 m (3 ft) tall, with yellow composite flowers that develop tan or black achenes (small, one-seeded fruits with hard coverings that do not split open (dehisce) when ripe) with white beaks.
Lettuce cultivars are generally divided into several groups by growth form:
1. Head or cabbage lettuce, which forms dense, tightly packed heads, like cabbage, and includes the common cultivar ‘Iceberg.’
2. Romaine or cos lettuce, with long (often up to 15 cm, or 10 in), upright, broad-stemmed leaves that form loose heads.
3. Curled or leaf lettuce, with a non-heading rosette of round ruffled, fringed, or crisp leaves.
4. Butterhead lettuce, with soft leaves in loose heads.
5. Stem lettuce or celtuce, in which the stems are enlarged, and are the part harvested and eaten.
Lettuce is generally eaten fresh in salads, although there are numerous dishes in which the leaves are stuffed, braised, cooked or softened with cream or butter, or pureed. In stem lettuce, the stems are cooked as a vegetable.
The FAO estimates that total global commercial production of lettuce was 23.6 million metric tons (mmt) in 2010, harvested from 1.1 million hectares. China led production with 12.6 mmt, just over half the world total, while the second-ranked U.S. produced 4.0 mmt, accounting for another 17%. Italy, India, and Spain were the other countries with harvests of more than 800,000 metric tons. (FAO tracks statistics for lettuce production together with chicory, which includes both Cichorium endivia, endive, and Cichorium intybus, chicory—both of which are used as salad greens, although the root of C. intybus is also harvested to be roasted for use as coffee additive or substitute.)
(Bailey et al. 1976, FAOSTAT 2012, van Wyk 2005.)