Until the mid-twentieth century, lettuce was the most important food plant in the Asteraceae. Today, despite its low nutritional value, lettuce is the world's most widely consumed salad plant. In 2013, nearly 25 million tonnes of the vegetable were harvested globally, mostly from a limited range of commercial varieties, such as Cos and Iceberg.
The garden lettuce evolved from a group of closely-related, bitter-tasting wild species distributed around the Mediterranean Basin and south west Asia. Early farmers selected lettuces with low bitterness, although there is evidence they also selected lettuce as an oil seed. One wild species, Lactuca virosa, is rich in sesquiterpene lactones, such as lactucin and lactucopicrin, and was used as the mild sedative, lactucarium.
Classical literature is replete with references to lettuce diversity, and their culinary, laxative, aphrodisiac and soporific uses. By the seventeenth century, the English apothecary and gardener John Parkinson could state: 'so great diuersitie of Lettice, that I doubt I shall scarce be beleeued ... some [are] of little use ... some that are of excellent use and service'. He mentioned varieties such as red and white Romane, Virginia, common Lumbard, Venice, Cabbage, Curled and winter lettuce. At the end of the century, many of these were being grown in the Oxford Physic Garden, as John Evelyn published his paean to salads, Acetaria. A discourse of sallets, praising lettuce as a panacea 'so harmless that it may safely be eaten raw'.
The herbalist John Gerard recognised that some of these differences were caused by the conditions under which lettuces were grown: 'by manuring, transplanting ... the leaues of the artificiall Lettuce are oftentimes transformed into another shape'. By the end of the nineteenth century, French seed merchants were offering more than one hundred different types of lettuce for sale, and lettuces were available throughout the year as different cultivars came into season during the winter, spring and summer months.
One of the more curious associations of lettuce is with the Ancient Egyptian fertility cult of Min. Images of the god are often accompanied by a leafy plant, interpreted as a Romaine-like lettuce. Unfortunately, other than a few fruits, no physical remains of Ancient Egyptian lettuces have been found. It is tempting to speculate the association between lettuce and fertility emerges from the observation that lettuces bleed white latex when they are damaged. Indeed, 'milk' is the derivation of plant's scientific name, Lactuca ('milky plant').
de Vries IM (1997) Origin and domestication of Lactuca sativa L. Genetic Resources and Crop Evolution 44: 165-174.
Koopman, WJM et al. (1998) Phylogenetic relationships among Lactuca (Asteraceae) species and related genera based on ITS-1 DNA sequences. American Journal of Botany 85: 1517-1530.
Zohary, D (1991) The wild genetic resources of lettuce (Lactuca sativa L.). Euphytica 53: 31-35.