Lettuce
Overview
Lettuce is a temperate
Temperate
In geography, temperate or tepid latitudes of the globe lie between the tropics and the polar circles. The changes in these regions between summer and winter are generally relatively moderate, rather than extreme hot or cold...

 annual
Annual plant
An annual plant is a plant that usually germinates, flowers, and dies in a year or season. True annuals will only live longer than a year if they are prevented from setting seed...

 or biennial plant
Biennial plant
A biennial plant is a flowering plant that takes two years to complete its biological lifecycle. In the first year the plant grows leaves, stems, and roots , then it enters a period of dormancy over the colder months. Usually the stem remains very short and the leaves are low to the ground, forming...

 of the daisy family Asteraceae
Asteraceae
The Asteraceae or Compositae , is an exceedingly large and widespread family of vascular plants. The group has more than 22,750 currently accepted species, spread across 1620 genera and 12 subfamilies...

. It is most often grown as a leaf vegetable
Leaf vegetable
Leaf vegetables, also called potherbs, green vegetables, greens, leafy greens or salad greens, are plant leaves eaten as a vegetable, sometimes accompanied by tender petioles and shoots...

. It is eaten either raw, notably in salad
Salad
Salad is any of a wide variety of dishes, including vegetable salads; salads of pasta, legumes, eggs, or grains; mixed salads incorporating meat, poultry, or seafood; and fruit salads. They may include a mixture of cold and hot, often including raw vegetables or fruits.Green salads include leaf...

s, sandwich
Sandwich
A sandwich is a food item, typically consisting of two or more slices of :bread with one or more fillings between them, or one slice of bread with a topping or toppings, commonly called an open sandwich. Sandwiches are a widely popular type of lunch food, typically taken to work or school, or...

es, hamburger
Hamburger
A hamburger is a sandwich consisting of a cooked patty of ground meat usually placed inside a sliced bread roll...

s, taco
Taco
A taco is a traditional Mexican dish composed of a corn or wheat tortilla folded or rolled around a filling. A taco can be made with a variety of fillings, including beef, chicken, seafood, vegetables and cheese, allowing for great versatility and variety...

s, and many other dishes, or cooked, as in Chinese cuisine
Chinese cuisine
Chinese cuisine is any of several styles originating in the regions of China, some of which have become highly popular in other parts of the world – from Asia to the Americas, Australia, Western Europe and Southern Africa...

 in which the stem becomes just as important as the leaf. Both the English name and the Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 name of the genus are ultimately derived from lac, the Latin word for “milk
Milk
Milk is a white liquid produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals before they are able to digest other types of food. Early-lactation milk contains colostrum, which carries the mother's antibodies to the baby and can reduce the risk of many...

”, referring to the plant’s milky juice. Mild in flavour, it has been described over the centuries as a cooling counterbalance to other ingredients in a salad.
The lettuce plant has a very short stem initially (a rosette
Rosette (botany)
In botany, a rosette is a circular arrangement of leaves, with all the leaves at a single height.Though rosettes usually sit near the soil, their structure is an example of a modified stem.-Function:...

 growth habit), but when it gradually blooms, the stem and branches lengthen and produce many flower heads that look like those of dandelions, but smaller.
Encyclopedia
Lettuce is a temperate
Temperate
In geography, temperate or tepid latitudes of the globe lie between the tropics and the polar circles. The changes in these regions between summer and winter are generally relatively moderate, rather than extreme hot or cold...

 annual
Annual plant
An annual plant is a plant that usually germinates, flowers, and dies in a year or season. True annuals will only live longer than a year if they are prevented from setting seed...

 or biennial plant
Biennial plant
A biennial plant is a flowering plant that takes two years to complete its biological lifecycle. In the first year the plant grows leaves, stems, and roots , then it enters a period of dormancy over the colder months. Usually the stem remains very short and the leaves are low to the ground, forming...

 of the daisy family Asteraceae
Asteraceae
The Asteraceae or Compositae , is an exceedingly large and widespread family of vascular plants. The group has more than 22,750 currently accepted species, spread across 1620 genera and 12 subfamilies...

. It is most often grown as a leaf vegetable
Leaf vegetable
Leaf vegetables, also called potherbs, green vegetables, greens, leafy greens or salad greens, are plant leaves eaten as a vegetable, sometimes accompanied by tender petioles and shoots...

. It is eaten either raw, notably in salad
Salad
Salad is any of a wide variety of dishes, including vegetable salads; salads of pasta, legumes, eggs, or grains; mixed salads incorporating meat, poultry, or seafood; and fruit salads. They may include a mixture of cold and hot, often including raw vegetables or fruits.Green salads include leaf...

s, sandwich
Sandwich
A sandwich is a food item, typically consisting of two or more slices of :bread with one or more fillings between them, or one slice of bread with a topping or toppings, commonly called an open sandwich. Sandwiches are a widely popular type of lunch food, typically taken to work or school, or...

es, hamburger
Hamburger
A hamburger is a sandwich consisting of a cooked patty of ground meat usually placed inside a sliced bread roll...

s, taco
Taco
A taco is a traditional Mexican dish composed of a corn or wheat tortilla folded or rolled around a filling. A taco can be made with a variety of fillings, including beef, chicken, seafood, vegetables and cheese, allowing for great versatility and variety...

s, and many other dishes, or cooked, as in Chinese cuisine
Chinese cuisine
Chinese cuisine is any of several styles originating in the regions of China, some of which have become highly popular in other parts of the world – from Asia to the Americas, Australia, Western Europe and Southern Africa...

 in which the stem becomes just as important as the leaf. Both the English name and the Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 name of the genus are ultimately derived from lac, the Latin word for “milk
Milk
Milk is a white liquid produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals before they are able to digest other types of food. Early-lactation milk contains colostrum, which carries the mother's antibodies to the baby and can reduce the risk of many...

”, referring to the plant’s milky juice. Mild in flavour, it has been described over the centuries as a cooling counterbalance to other ingredients in a salad.

Description

The lettuce plant has a very short stem initially (a rosette
Rosette (botany)
In botany, a rosette is a circular arrangement of leaves, with all the leaves at a single height.Though rosettes usually sit near the soil, their structure is an example of a modified stem.-Function:...

 growth habit), but when it gradually blooms, the stem and branches lengthen and produce many flower heads that look like those of dandelions, but smaller. This is referred to as bolting. When grown to eat, lettuce is harvested before it bolts. Lettuce is used as a food plant by the larva
Cucullia lactucae
The Lettuce Shark is a species of moth of the Noctuidae family. It is found in most of Europe, Turkey, the Altai mountains and the Caucasus....

e of some Lepidoptera
Lepidoptera
Lepidoptera is a large order of insects that includes moths and butterflies . It is one of the most widespread and widely recognizable insect orders in the world, encompassing moths and the three superfamilies of butterflies, skipper butterflies, and moth-butterflies...

.

Cultivation

Lettuce is grown commercially worldwide.

Lettuce plants should be grown in a light, sandy, fertile, humus-rich soil that will hold moisture in summer. A soil pH of 6.5 is preferred; lime may be added for this purpose. For best eating quality, water is essential; the plants prefer the soil to be moist at all times.

Lettuce plants prefer cool weather, ideally with day temperatures below 23.9°C (75°F) and night temperatures above 4.5°C (40°F). Hot, sunny, or dry conditions may cause the plants to turn bitter and produce a flower shoot, a process known as bolting. Therefore, lettuce is often grown in the coolness of spring and autumn; lettuce sown in summer is often grown in light shade. In addition, bolt-resistant summer cultivars of lettuce may be recommended as temperatures increase.

Lettuce can be directly sown in the garden but lettuce plants are often started in cold frames or greenhouses and the resulting seedlings transplanted to the garden or field. This allows an earlier start, or allows more efficient use of garden space, as the lettuce can be transplanted when growing rapidly, avoiding the use of garden space for germination of seeds.

As another way to allow an earlier crop in cold weather, lettuce is sometimes given glass protection, known as a cloche, or protected with spun material known as a floating row cover. In sufficiently mild-weather climates, these same protective devices (greenhouses, cold frames, cloches, row cover) may be used to protect lettuce throughout the winter, allowing harvest even in near-freezing or freezing weather.

Lettuce is often grown between rows of slower growing plants like brussel sprouts or broccoli. This is called a catch crop
Catch crop
In agriculture, a catch crop is a fast-growing crop that is grown simultaneously with, or between successive plantings of a main crop.For example, radishes that mature from seed in 25–30 days can be grown between rows of most vegetables, and harvested long before the main crop matures...

. It allows more efficient use of garden space, and also provides the lettuce with needed shade in warm weather.

History

The earliest depiction of lettuce is in the carvings at the temple of Senusret I
Senusret I
Senusret I was the second pharaoh of the Twelfth Dynasty of Egypt. He ruled from 1971 BC to 1926 BC, and was one of the most powerful kings of this Dynasty. He was the son of Amenemhat I and his wife Nefertitanen. His wife and sister was Neferu. She was also the mother of the successor Amenemhat II...

 at Karnak
Karnak
The Karnak Temple Complex—usually called Karnak—comprises a vast mix of decayed temples, chapels, pylons, and other buildings, notably the Great Temple of Amun and a massive structure begun by Pharaoh Ramses II . Sacred Lake is part of the site as well. It is located near Luxor, some...

, where he offers milk to the god Min
Min (god)
Min is an Ancient Egyptian god whose cult originated in predynastic times . He was represented in many different forms, but was often represented in male human form, shown with an erect penis which he holds in his left hand and an upheld right arm holding a flail...

, to whom the lettuce was sacred. Lettuce was considered an aphrodisiac
Aphrodisiac
An aphrodisiac is a substance that increases sexual desire. The name comes from Aphrodite, the Greek goddess of sexuality and love. Throughout history, many foods, drinks, and behaviors have had a reputation for making sex more attainable and/or pleasurable...

 food in Ancient Egypt
Ancient Egypt
Ancient Egypt was an ancient civilization of Northeastern Africa, concentrated along the lower reaches of the Nile River in what is now the modern country of Egypt. Egyptian civilization coalesced around 3150 BC with the political unification of Upper and Lower Egypt under the first pharaoh...

, and appears as such in The Contendings of Horus and Seth. Later, Ancient Greek physicians believed lettuce could act as a sleep-inducing agent. The Romans cultivated it, and it eventually made its way to the Papal Court at Avignon, France. Christopher Columbus introduced lettuce to the New World
New World
The New World is one of the names used for the Western Hemisphere, specifically America and sometimes Oceania . The term originated in the late 15th century, when America had been recently discovered by European explorers, expanding the geographical horizon of the people of the European middle...

.

Cultivars

There are six commonly recognised Cultivar Group
Cultivar group
In naming cultivated plants, a Group is a formal classification category, under the International Code of Nomenclature for Cultivated Plants :The term "Group" was introduced in the 2004 ICNCP, replacing the "Cultivar-group" of the 1995 ICNCP.A Group is united by some common trait; for example...

s of lettuce which are ordered here by head formation and leaf structure; there are hundreds of cultivar
Cultivar
A cultivar'Cultivar has two meanings as explained under Formal definition. When used in reference to a taxon, the word does not apply to an individual plant but to all those plants sharing the unique characteristics that define the cultivar. is a plant or group of plants selected for desirable...

s of lettuce selected for leaf shape and colour, as well as extended field and shelf life, within each of these Cultivar Groups:
  • Butterhead (L. sativa var. capitata) forms loose heads. Its leaves have a buttery texture. Butterhead cultivars are most popular in Europe. Popular varieties include Boston, Bibb, Buttercrunch, and Tom Thumb.
  • Chinese lettuce (L. sativa var. asparagina) types generally have long, sword-shaped, non-head-forming leaves, with a bitter and robust flavor unlike Western types, for use in stir-fried
    Stir frying
    Stir frying is an umbrella term used to describe two Chinese cooking techniques for preparing food in a wok: chǎo and bào . The term stir-fry was introduced into the English language by Buwei Yang Chao, in her book How to Cook and Eat in Chinese, to describe the chǎo technique...

     dishes and stew
    Stew
    A stew is a combination of solid food ingredients that have been cooked in liquid and served in the resultant gravy. Ingredients in a stew can include any combination of vegetables , meat, especially tougher meats suitable for slow-cooking, such as beef. Poultry, sausages, and seafood are also used...

    s. Chinese lettuce cultivars are divided into “stem-use” types (called celtuce
    Celtuce
    Celtuse , also called stem lettuce, celery lettuce, asparagus lettuce, or Chinese lettuce, IPA , is a cultivar of lettuce grown primarily for its thick stem, used as a vegetable...

     in English), and “leaf-use” types such as youmaicai or shengcai (生菜/唐生菜), respectively.
  • Crisphead, also called Iceberg, forms tight, dense heads that resemble cabbage
    Cabbage
    Cabbage is a popular cultivar of the species Brassica oleracea Linne of the Family Brassicaceae and is a leafy green vegetable...

    . They are generally the mildest of the lettuces, valued more for their crunchy texture than for flavor. Cultivars of iceberg lettuce are the most familiar lettuces in the USA.
  • Looseleaf (L. sativa var. crispa) has tender, delicate, and mildly flavored leaves. This group includes oak leaf and lollo rosso (Red Leaf) lettuces.
  • Romaine
    Romaine lettuce
    Romaine or cos lettuce is a variety of lettuce which grows in a tall head of sturdy leaves with a firm rib down the center. Unlike most lettuces, it is tolerant of heat...

    (L. sativa var. romana), also called Cos, grows in a long head of sturdy leaves with a firm rib down the center. Unlike most lettuces, it is tolerant of heat.
  • Summer Crisp, also called Batavia or Batavian, forms moderately dense heads with a crunchy texture. This type is intermediate between iceberg and looseleaf types.


Some lettuces (especially iceberg) have been specifically bred to remove the bitterness from their leaves. These lettuces have high water content and so are less "nutritionally dense" than are the more bitter lettuces and those with darker leaves. While all lettuces contain antioxidant
Antioxidant
An antioxidant is a molecule capable of inhibiting the oxidation of other molecules. Oxidation is a chemical reaction that transfers electrons or hydrogen from a substance to an oxidizing agent. Oxidation reactions can produce free radicals. In turn, these radicals can start chain reactions. When...

s and vitamin K
Vitamin K
Vitamin K is a group of structurally similar, fat soluble vitamins that are needed for the posttranslational modification of certain proteins required for blood coagulation and in metabolic pathways in bone and other tissue. They are 2-methyl-1,4-naphthoquinone derivatives...

, romaine and looseleaf lettuce contain five to six times the vitamin C
Vitamin C
Vitamin C or L-ascorbic acid or L-ascorbate is an essential nutrient for humans and certain other animal species. In living organisms ascorbate acts as an antioxidant by protecting the body against oxidative stress...

 and five to ten times the vitamin A
Vitamin A
Vitamin A is a vitamin that is needed by the retina of the eye in the form of a specific metabolite, the light-absorbing molecule retinal, that is necessary for both low-light and color vision...

 of iceberg. Romaine and butterhead lettuce are good sources of folate. Lettuce naturally absorbs and concentrates lithium
Lithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...

.

Breeding

L. sativa can easily be bred with closely related species in Lactuca
Lactuca
Lactuca, commonly known as lettuce, is a genus of flowering plants in the daisy family Asteraceae. The genus includes about 100 species, distributed worldwide, but mainly in temperate Eurasia....

such as L. serriola
Lactuca serriola
Prickly Lettuce is an annual or biennial plant, slightly foetid, that is commonly considered a weed of orchards, roadsides and field crops. The closest wild relative of cultivated lettuce , it grows throughout the temperate regions of all major continents. The leaves grow along a spiny stem and...

, L. saligna
Lactuca saligna
Lactuca saligna is a species of wild lettuce known by the common name willowleaf lettuce, and Least lettuce. It is native to Eurasia but it grows in many other places as an introduced species, including much of North America....

, and L. virosa
Lactuca virosa
Lactuca virosa is a plant in the Lactuca genus, ingested often for its mild psychotropic effects which are often described as being similar to that of opium. It is related to common lettuce Lactuca virosa is a plant in the Lactuca (lettuce) genus, ingested often for its mild psychotropic...

, and breeding programs for cultivated lettuce have included those species to broaden the available gene pool
Gene pool
In population genetics, a gene pool is the complete set of unique alleles in a species or population.- Description :A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can survive bouts of intense selection...

. Starting in the 1990s, such programs began to include more distantly related species such as L. tatarica.

Production

The Food and Agriculture Organization of the United Nations
Food and Agriculture Organization
The Food and Agriculture Organization of the United Nations is a specialised agency of the United Nations that leads international efforts to defeat hunger. Serving both developed and developing countries, FAO acts as a neutral forum where all nations meet as equals to negotiate agreements and...

 reports that world production of lettuce and chicory
Chicory
Common chicory, Cichorium intybus, is a somewhat woody, perennial herbaceous plant usually with bright blue flowers, rarely white or pink. Various varieties are cultivated for salad leaves, chicons , or for roots , which are baked, ground, and used as a coffee substitute and additive. It is also...

 for calendar year 2007 was 23.55 million tons, primarily coming from China (51%), United States (22%) and Spain (5%).
Top ten lettuce and chicory producers — 2007
Country Production (tonnes) Source
12 000 000 FAO estimate
5 105 980 official figure
1 070 000 FAO estimate
850 078 official figure
790 000 FAO estimate
560 000 FAO estimate
471 000 FAO estimate
382 034 official figure
275 000 FAO estimate
185 000 FAO estimate
World 23 550 943 aggregate

Source: Food And Agricultural Organization of United Nations: Economic And Social Department: The Statistical Division


Nutrition

Lettuce is a low calorie
Calorie
The calorie is a pre-SI metric unit of energy. It was first defined by Nicolas Clément in 1824 as a unit of heat, entering French and English dictionaries between 1841 and 1867. In most fields its use is archaic, having been replaced by the SI unit of energy, the joule...

 food and is a source of vitamin A
Vitamin A
Vitamin A is a vitamin that is needed by the retina of the eye in the form of a specific metabolite, the light-absorbing molecule retinal, that is necessary for both low-light and color vision...

 and folic acid
Folic acid
Folic acid and folate , as well as pteroyl-L-glutamic acid, pteroyl-L-glutamate, and pteroylmonoglutamic acid are forms of the water-soluble vitamin B9...

. Lactucarium
Lactucarium
Lactucarium is the milky fluid secreted by several species of lettuce, especially Lactuca virosa, usually from the base of the stems. Lactucarium is known as lettuce opium because of its sedative and analgesic properties...

 (or “Lettuce Opium”) is a mild opiate
Opiate
In medicine, the term opiate describes any of the narcotic opioid alkaloids found as natural products in the opium poppy plant.-Overview:Opiates are so named because they are constituents or derivatives of constituents found in opium, which is processed from the latex sap of the opium poppy,...

-like substance that is contained in all types of lettuce. Both the Romans and Egyptians took advantage of this property by eating lettuce at the end of a meal to induce sleep.

Religious restrictions

The Yazidi
Yazidi
The Yazidi are members of a Kurdish religion with ancient Indo-Iranian roots. They are primarily a Kurdish-speaking people living in the Mosul region of northern Iraq, with additional communities in Transcaucasia, Armenia, Turkey, and Syria in decline since the 1990s – their members emigrating to...

 of northern Iraq consider eating lettuce taboo
Taboo food and drink
Taboo food and drink are food and beverages which people abstain from consuming for religious, cultural or hygienic reasons. Many food taboos forbid the meat of a particular animal, including mammals, rodents, reptiles, amphibians, bony fish, and crustaceans...

.
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