Lactuca sativa is a member of the Lactuca (lettuce) genus and the Asteraceae (sunflower or aster) family. Tracing the history of many varieties is complicated by the practice of many companies, particularly in the US, of changing a variety's name from year to year. However, when newly harvested lettuce seed is stored cryogenically, this life increases to a half-life of 500 years for vaporized nitrogen and 3,400 years for liquid nitrogen; this advantage is lost if seeds are not frozen promptly after harvesting..
 The species was first described in 1753 by Carl Linnaeus in the second volume of his Species Plantarum. Plants are massive producers of lobed leaves ranging from bright to deep brilliant green. By 50 AD, many types were described, and lettuce appeared often in medieval writings, including several herbals. Lettuce flowers more quickly in hot temperatures, while freezing temperatures cause slower growth and sometimes damage to outer leaves.  Weeds can also be an issue, as cultivated lettuce is generally not competitive with them, especially when directly seeded into the ground.
, Lettuce also suffers from several viral diseases, including big vein, which causes yellow, distorted leaves, and mosaic virus, which is spread by aphids and causes stunted plant growth and deformed leaves.  The ovaries form compressed, obovate (teardrop-shaped) dry fruits that do not open at maturity, measuring 3 to 4 mm long.  A 2007 study determined that the vacuum cooling method, especially prevalent in the California lettuce industry, increased the uptake and survival rates of E. coli O157:H7. Arugula – A deeply lobed green with a spicy, peppery flavor.  This propensity for crossing, however, has led to breeding programs using closely related species in Lactuca, such as L. serriola, L. saligna, and L. virosa, to broaden the available gene pool.
Romaine lettuce is often used for Caesar salads.
Heat resistant AAS (All American Selections) winner. Weeds also act as homes for insects and disease and can make harvesting more difficult.
Lettuce varieties will cross with each other, making spacing of 1.5 to 6 m (60 to 240 in) between varieties necessary to prevent contamination when saving seeds. Lettuce grows best in full sun in loose, nitrogen-rich soils with a pH of between 6.0 and 6.8.
, Depending on the variety and time of year, lettuce generally lives 65–130 days from planting to harvesting. Circa 50 AD, Roman agriculturalist Columella described several lettuce varieties – some of which may have been ancestors of today's lettuces.
Longer taproots and more extensive secondary systems are found in varieties from Asia.  Plants in hot areas that are provided partial shade during the hottest part of the day will bolt more slowly. Lettuce leaves can also be found in soups, sandwiches and wraps, while the stems are eaten both raw and cooked.  Lettuce was first brought to the Americas from Europe by Christopher Columbus in the late 15th century.  L. scariola is itself a synonym for L. serriola, the common wild or prickly lettuce. There are so many varieties to choose from for leaf lettuces and other leafy greens. In addition to these four main lettuce types, a fifth variety of lettuce, called asparagus lettuce or celtuce, is grown for its thick …  Lettuce is also used as a primary ingredient in the preparation of lettuce soup. Lettuce is the only member of the genus Lactuca to be grown commercially. Post-Roman Europe continued the tradition of poaching lettuce, mainly with large romaine types, as well as the method of pouring a hot oil and vinegar mixture over the leaves. What’s more, it’s full of nutrients and, consequently, one of the lettuce’s healthiest varieties. 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. Red leaf lettuce produces brilliant leaves ranging from deep royal purple to crimson and rust. If you love eating salads, then you need to grow different lettuce varieties. Celtuce, a lettuce variety grown primarily in Asia for its stems, crosses easily with lettuces grown for their leaves.
These upright lettuces were developed by the Egyptians and passed to the Greeks, who in turn shared them with the Romans.
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