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Lettuce

Lettuce (''Lactuca sativa'') is a temperate annual plant|annual or biennial plant of the daisy family Asteraceae. It is most often grown as a leaf vegetable. In many countries, it is typically eaten cold and raw, in salads, hamburgers, tacos, and many other dishes. In some places, including China, lettuce is typically eaten cooked and use of the stem is as important as use of the leaf. Both the English name and the Latin name of the genus are ultimately derived from ''lac'', the Latin word for “milk”, 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.Grigson, p. 313 In his humorous essay ''100 Things I Hate'', filmmaker John Waters (filmmaker)|John Waters refers to iceberg lettuce as "the polyester of greens".

Description

The lettuce plant has a short stem initially (a Rosette (botany)|rosette growth habit), but when it blooms the stem lengthens and branches, and it produces many flower heads that look like those of dandelions, but smaller. This is called wiktionary:bolt#Verb|bolting. When grown to eat, lettuce is harvested before it bolts. Lettuce is used as a food plant by the larvae of some Lepidoptera. The largest lettuce head, of the Salad Bowl cultivar, weighed 11 Kilogram|kg (25 Pound (mass)|lb) grown by Colin Bowcock of Willaston, England, in 1974.

Cultivation

Lettuce is grown commercially worldwide, hardy to hardiness zone|Zone 6, requiring light, sandy, humus rich, moist soil. It dislikes dry conditions and can cause plants to bolt (go to seed). Early and late sowing in sunny positions or summer crops in shade. It is considered fairly easy to grow and a suitable crop for beginners. Planting Depth: Some resources suggest planting seeds by covering lightly with soil while others suggest a depth of 1/4 to 1/2 inch Planting Spacing: Thin seedlings to 10 cm (4 in) apart for leaf lettuce sq. m (9/sq ft) and 6 to 8 inches apart for Cos or Butterhead (4/sq ft - 1/sq ft), transplant Crisphead seedlings 10 to 12 inches apart in the row (1/sq ft). Row Spacing: 12 - 18 inches apart

History

The earliest depiction of lettuce is in the carvings at the temple of Senusret I at Karnak, where he offers milk to the god Min (god)|Min, to whom the lettuces was sacred. Later, Ancient Greek physicians believed lettuce could act as a sleep-inducing agent. The Romans cultivated it, and it eventually made its way to France cultivated of the Papal Court at Avignon.Grigson, p. 312 Christopher Columbus introduced lettuce to the new world.

Cultivars

There are six commonly recognised Cultivar Groups of lettuce which are ordered here by head formation and leaf structure; there are hundreds of cultivars of lettuce selected for leaf shape and colour, as well as extended field and shelf life, within each of these Cultivar Groups:
- '''Butterhead''' forms loose heads; it has a buttery Texture (food)|texture. Butterhead cultivars are most popular in Europe. Popular varieties include '''Boston''', '''Bibb''', '''Buttercrunch''', and '''Tom Thumb'''.
- '''Chinese lettuce''' types generally have long, sword-shaped, non-head-forming leaves, with a bitter and robust flavour unlike Western types, appropriate for use in stir frying|stir-fried dishes and stews. Chinese lettuce cultivars are divided into “stem-use” types (called celtuce in English), and “leaf-use” types such as ''youmaicai'' () or ''shengcai'' (生菜).
- '''Crisphead''', also called '''Iceberg''', which form tight, dense heads that resemble cabbage. They are generally the mildest of the lettuces, valued more for their crunchy Texture (food)|texture than for flavour. Cultivars of iceberg lettuce are the most familiar lettuces in the USA. The name Iceberg comes from the way the lettuce was transported in the US starting in the 1920s on train-wagons covered in crushed ice, making them look like icebergs.
- '''Looseleaf''', with tender, delicate, and mildly flavoured leaves. This group comprises oak leaf and lollo rosso lettuces.
- '''Romaine lettuce|Romaine''', 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 '''Batavian''', which form 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 a high water content with very little nutrient value. They cost a lot more than regular lettuce also, this is despite the fact they contain more water (which is free). The more bitter lettuces and the ones with pigmented leaves contain antioxidants. Image:Lettuce Cultivars by David Shankbone.JPG|Some lettuce cultivars Image:Romaine.jpg|A Romaine lettuce Image:Lactuca compounds.svg|Chemical compunds which occur in lettuce:
1: α-Lactucerol (=Taraxasterol); 2: β-Lactucerol (=Lactucon, Lactucerin); 3: Lactucin; 4: Lactucopicrin. Image:Lettuces.JPG|More lettuce cultivars

Breeding

L. sativa can easily be bred with closely related species in ''Lactuca'' such as ''Lactuca serriola|L. serriola'', ''Lactuca saligna|L. saligna'', and ''Lactuca virosa|L. virosa'', and breeding programs for cultivated lettuce have included those species to broaden the available gene pool. Starting in the 1990s, breeding programs began to include more distantly related species such as ''Lactuca tatarica|L. tatarica''.

Seed Saving

Inbreeding plant, flowers form in heads of 10-25 individual florets of perfect flowers. Considered suitable for seed-saving beginners

Production

The Food and Agriculture Organization|Food and Agriculture Organization of the United Nations reports that world production of lettuce and chicory for calendar year 2007 was 23.55 million tonnes, primarily coming from China(51%), United States(22%) and Spain(5%).

Nutrition

Lettuce is a fat free, low calorie food. It is a valuable source of vitamin A and folic acid. Lactucarium (or “Lettuce Opium”) is a mild opiate-like substance that is contained in all types of lettuce. Both the Romans and Egyptians took advantage of this property eating lettuce at the end of a meal to induce sleep. "Lettuce - ''Lactuca sativa'' - Daisy family". Hamilton, Dave (2005).