Lettuce

Lettuce is an annual or biennial plant that forms a rosette of leaves and, once it bolts, a tall flowering stem with many small yellow dandelion-like flowers. The seeds are slender, flattened achenes about 3 to 4 millimetres long, pale grey, cream, brown or black according to variety, with fine ridges and a feathery pappus that is lost in processing. They taste faintly bitter and nutty. Seeds are sold raw in packets for gardeners, as pelleted seeds for commercial growers, and occasionally as sprouting or microgreen seeds. Many types exist, including crisphead, romaine, butterhead, loose-leaf and stem lettuce.

Lettuce was domesticated from prickly lettuce (Lactuca serriola), a wild plant widespread in the Mediterranean, western Asia and North Africa. Tomb paintings in ancient Egypt show a tall, upright lettuce type that was grown for seed oil and as a plant associated with fertility. The name Lactuca comes from the Latin lac, milk, referring to the milky latex in the stems, and the word lettuce reached English through Old French laituës. The species name sativa means cultivated. Wild relatives still grow along roadsides and waste ground across Europe and Asia, and crosses with them have supplied disease-resistance genes to modern breeders.

In ancient Egypt, lettuce was linked to the god Min, associated with fertility, and early cultivated forms were tall and were likely grown for the oil in their seeds and for leaves. Greek and Roman writers such as Hippocrates and Pliny described lettuce, and Romans ate it at the end of meals as a soporific. The Emperor Augustus is said to have built a statue to lettuce after it helped him recover from an illness, according to Suetonius.

Lettuce spread through medieval Europe, and by the sixteenth century many varieties were recorded, including head forms. Columbus brought lettuce to the Americas, where selection created the crisphead types, and in the twentieth century iceberg lettuce grew popular in the United States because it ships well. In China, stem lettuce (celtuce or wosun) was grown for its thick stem.

Seed production now concentrates in places with dry summers and reliable seed harvests, including California, the Netherlands and Italy, and the seed industry supplies the world's growers. Lettuce is also a model species in plant biology, and seed germination research on lettuce led to the discovery of the role of light and phytochrome in plant development.

Lettuce seeds are mostly used to grow lettuce. Sprouted seeds and microgreens are eaten in salads and sandwiches, though they are less common than other sprouts. Historically, seeds were pressed for a mild oil used for lamps and food in Egypt, and in parts of the Middle East and Asia seed oil has been recorded in folk use. Leaves are eaten raw in salads, wraps and sandwiches or cooked in soups and braises, and stem lettuce is cooked or pickled. Lettuce seeds require light to germinate, so they are sown very shallowly.

The leaves provide water, folate, vitamins A and K and small amounts of fibre, especially in darker varieties. Lettuce seeds are not an established dietary source of nutrients, and little research exists on their use as food. Traditional medicine has used lettuce seed in teas for sleep or digestion, but the evidence is lacking. Wild lettuce latex and extracts are sold as supplements with claims of sedation and pain relief that are not supported by good clinical evidence.