Lettuce

Lettuce (Lactuca sativa) is a temperate annual or biennial plant of the aster or sunflower family Asteraceae. It is most often grown as a leaf vegetable, but also sometimes for its stem and seeds. Lettuce was first cultivated by the Egyptians, who turned it from a weed whose seeds were used to make oil into a plant grown for its leaves. It then spread to the Greeks and Romans, who gave it the name "lactuca", from which the modern "lettuce" ultimately derives. By 50 AD, multiple types were described, and it appeared often in medieval writings, including several herbals. The 16th through 18th century saw the development of many varieties in Europe, and by the mid-18th century varieties were being described that can still be found in gardens in the 21st century. Europe and North America originally dominated the market for lettuce, but by the late 20th century the consumption of lettuce had spread throughout the world.

Generally grown as a hardy annual, it is easily cultivated, although it requires relatively low temperatures to prevent it from quickly flowering. It can be plagued with numerous nutrient deficiencies, as well as insect and mammal pests and fungal and bacterial diseases. L. sativa crosses easily within the species and with some other species within the Lactuca genus, and although this trait can be a problem to home gardeners attempting to save seeds, biologists have used it to broaden the gene pool of cultivated lettuce varieties. World production of lettuce and chicory for calendar year 2010 stood at 23,620,000 metric tons (23,250,000 long tons; 26,040,000 short tons), with over half coming from China.