In addition to its main use as a leafy green, it has also gathered religious and medicinal significance over centuries of human consumption. 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. Lettuce was first cultivated by the ancient Egyptians who turned it from a weed whose seeds were used to produce oil, into a food plant grown for its succulent leaves and oil-rich seeds.
Lettuce spread to the Greeks and Romans, the latter of whom gave it the name lactuca , from which the English lettuce is ultimately derived. By 50 AD, many types were described, and lettuce appeared often in medieval writings, including several herbals. The 16th through 18th centuries saw the development of many varieties in Europe, and by the midth century cultivars were described that can still be found in gardens.
Generally grown as a hardy annual, lettuce is easily cultivated, although it requires relatively low temperatures to prevent it from flowering quickly. It can be plagued by numerous nutrient deficiencies , as well as insect and mammal pests, and fungal and bacterial diseases. Although this trait can be a problem to home gardeners who attempt to save seeds, biologists have used it to broaden the gene pool of cultivated lettuce varieties.
Lettuce is a rich source of vitamin K and vitamin A , and a moderate source of folate and iron. Contaminated lettuce is often a source of bacterial, viral, and parasitic outbreaks in humans, including E. Lactuca sativa is a member of the Lactuca lettuce genus and the Asteraceae sunflower or aster family. The Romans referred to lettuce as lactuca lac meaning milk in Latin , an allusion to the white substance, now called latex, exuded by cut stems.
Lettuce's native range spreads from the Mediterranean to Siberia , although it has been transported to almost all areas of the world. Some varieties, especially those found in the United States and Western Europe, have long, narrow taproots and a small set of secondary roots.
Longer taproots and more extensive secondary systems are found in varieties from Asia. Depending on the variety and time of year, lettuce generally lives 65— days from planting to harvesting. Because lettuce that flowers through the process known as " bolting " becomes bitter and unsaleable, plants grown for consumption are rarely allowed to grow to maturity. Lettuce flowers more quickly in hot temperatures, while freezing temperatures cause slower growth and sometimes damage to outer leaves.
These include fused anthers that form a tube which surrounds a style and bipartite stigma. As the anthers shed pollen , the style elongates to allow the stigmas, now coated with pollen, to emerge from the tube. The fruits have 5—7 ribs on each side and are tipped by two rows of small white hairs.
The pappus remains at the top of each fruit as a dispersal structure. Each fruit contains one seed, which can be white, yellow, gray or brown depending on the variety of lettuce. The domestication of lettuce over the centuries has resulted in several changes through selective breeding : delayed bolting, larger seeds, larger leaves and heads, better taste and texture, a lower latex content, and different leaf shapes and colors. Work in these areas continues through the present day.
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However, genetically modified lettuce is not currently used in commercial agriculture. Lettuce was first cultivated in ancient Egypt for the production of oil from its seeds. This plant was probably selectively bred by the Egyptians into a plant grown for its edible leaves,  with evidence of its cultivation appearing as early as BC. The plant was thought to help the god "perform the sexual act untiringly. These upright lettuces were developed by the Egyptians and passed to the Greeks, who in turn shared them with the Romans. Lettuce appears in many medieval writings, especially as a medicinal herb.
Hildegard of Bingen mentioned it in her writings on medicinal herbs between and , and many early herbals also describe its uses. Books published in the midth and early 19th centuries describe several varieties found in gardens today. Due to its short lifespan after harvest, lettuce was originally sold relatively close to where it was grown.
The early 20th century saw the development of new packing, storage and shipping technologies that improved the lifespan and transportability of lettuce and resulted in a significant increase in availability. Lettuce is very easy to grow, and as such has been a significant source of sales for many seed companies.
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.
This was done for several reasons, the most prominent being to boost sales by promoting a "new" variety or to prevent customers from knowing that the variety had been developed by a competing seed company. Names also often changed significantly from country to country. A hardy annual, some varieties of lettuce can be overwintered even in relatively cold climates under a layer of straw, and older, heirloom varieties are often grown in cold frames. Lettuce spaced further apart receives more sunlight, which improves color and nutrient quantities in the leaves.
Pale to white lettuce, such as the centers in some iceberg lettuce, contain few nutrients. Lettuce grows best in full sun in loose, nitrogen -rich soils with a pH of between 6.
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Lettuce quickly degrades when stored with fruit such as apples, pears and bananas that release the ripening agent ethylene gas. The high water content of lettuce Lettuce varieties will cross with each other, making spacing of 1. Lettuce will also cross with Lactuca serriola wild lettuce , with the resulting seeds often producing a plant with tough, bitter leaves.
Celtuce , a lettuce variety grown primarily in Asia for its stems, crosses easily with lettuces grown for their leaves. Starting in the s, such programs began to include more distantly related species such as L. However, when newly harvested lettuce seed is stored cryogenically, this life increases to a half-life of years for vaporized nitrogen and 3, years for liquid nitrogen; this advantage is lost if seeds are not frozen promptly after harvesting.
There are several types of lettuce, but three leaf, head and cos or romaine are the most common. The butterhead and crisphead types are sometimes known together as "cabbage" lettuce, because their heads are shorter, flatter, and more cabbage-like than romaine lettuces. Soil nutrient deficiencies can cause a variety of plant problems that range from malformed plants to a lack of head growth.
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For example, the larvae of the ghost moth is a common pest of lettuce plants. Certain varieties contain more than others, and some selective breeding and genetic modification studies have focused on using this trait to identify and produce commercial varieties with increased pest resistance. 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.
Aster yellows are a disease-causing bacteria carried by leafhoppers, which causes deformed leaves.
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Fungal diseases include powdery mildew and downy mildew , which cause leaves to mold and die and bottom rot , lettuce drop and gray mold , which cause entire plants to rot and collapse. Transplanted lettuce started in flats and later moved to growing beds is generally more competitive initially, but can still be crowded later in the season, causing misshapen lettuce and lower yields. Weeds also act as homes for insects and disease and can make harvesting more difficult.
However, this has led to the development of herbicide-resistant weeds and prompted environmental and health concerns. In , world production of lettuce report combined with chicory was 27 million tonnes, with China alone producing Lettuce is the only member of the genus Lactuca to be grown commercially. Spain is the world's largest exporter of lettuce, with the US ranking second. Western Europe and North America were the original major markets for large-scale lettuce production.
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