Lettuce is the great paradox of the kitchen garden: the most ubiquitous salad plant on earth, eaten raw and cold by the tonne, descends from a tall, bitter, prickly roadside weed that no one in their right mind would eat. The cultivated lettuce, Lactuca sativa, is a member of the daisy and sunflower family, the Asteraceae, and it was bred from the wild prickly lettuce, Lactuca serriola, a coarse plant of disturbed Mediterranean and Near Eastern ground whose leaves bristle with spines along the midrib and whose every cut surface bleeds a bitter white latex. The two will still cross freely, and the wild plant grows as a weed in the very fields where its cultivated child is raised.

The original attraction was almost certainly not the leaf at all but the seed and the stem. The earliest clear evidence of cultivation comes from Old Kingdom Egypt, where wall paintings of around 2680 BCE show a tall, upright lettuce with a thick stem and narrow leaves, grown both for the oil pressed from its seeds and as a sacred plant of the fertility god Min. From that erect, stem-forming ancestor the whole family of lettuces would be bred, the long centuries of selection working to suppress the bitterness, the spines, and the milky latex, to delay the bolting to seed that turns a lettuce woody and acrid, and to swell first the leaves and then, in one branch, the stem.

Unlike the plum or the bean, lettuce was domesticated only once, and it remains a single species; its bewildering variety is the work of the cultivator's hand rather than of separate wild origins. Four great forms grew out of the one plant, and each became the staple of a different cuisine. The oldest is the romaine or cos (var. longifolia), the tall, upright, sweet-hearted lettuce that descends most directly from the Egyptian original and carries the name of the Greek island of Kos. From it the cultivators of Europe bred the soft, loosely folded butterhead and, later still, the tight, pale, intensely crisp crisphead or iceberg (both gathered under var. capitata), and the open, non-hearting looseleaf lettuces (var. crispa) in their reds and greens and frills. At the eastern end of the plant's long journey, in China, selection ran in a wholly different direction, swelling not the leaves but the stem into the thick, crunchy, jade-green celtuce or stem lettuce (var. angustana), and so returning, by a curious circle, to something close to the stem vegetable of ancient Egypt.

Lettuce travelled out of its Near Eastern and Egyptian homeland in two directions, and along the way each branch of the family settled into the cuisine that would make it a staple.

The western and Mediterranean stream is the older and the better documented. From the sacred gardens of Egypt the upright stem lettuce passed to the Greeks, who grew it on Kos and across the Aegean, gave the cos lettuce its lasting name, and wove it into both medicine and myth. Rome received it with enthusiasm: the Romans named it lactuca for its milky latex, served it to open the banquet as an appetite-whetter, and prized its reputation as a bringer of sleep. From Rome the lettuce spread through the orchards and potagers of medieval and early modern Europe, and it was above all in France that the soft butterhead and the art of the dressed green salad reached their height, in the kitchen gardens of Paris and the great potager du roi laid out for Louis XIV at Versailles. Carried across the Atlantic by European colonists, the lettuce found in California a climate and an industry that would transform it: in the Salinas Valley, the 'Salad Bowl of the World', the crisp, heat-tolerant iceberg was perfected and, packed under crushed ice for the long rail journey east, gave the crisphead its modern name. The romaine line came to its most famous moment just over the border, in Tijuana, where in 1924 the Italian-born restaurateur Cesare Cardini tossed the first Caesar salad.

The eastern stream carried lettuce out of the same Near Eastern homeland into Persia, where the sweet romaine, the kahu, became the centrepiece of the summer refreshment kahoo sekanjabin, dipped leaf by leaf into a syrup of mint and vinegar. From Persia the plant travelled the Silk Road eastward to reach Tang China, where the cultivators did something done nowhere else: they bred the lettuce for its stem, creating the celtuce, and at the same time made the leaf lettuce a beloved stir-fried green whose name, shengcai, puns so neatly on the words for 'growing wealth' that no Chinese New Year table is complete without it. From China lettuce passed to Korea, where it became the indispensable wrapping leaf of the ssam, and southward into Vietnam, where it joined the great raw-herb plate, the rau sống, set beside every grilled and rolled dish.

Lettuce is now one of the most widely grown vegetables on earth, and the geography of its production has been quietly reordered. China is by a wide margin the world's largest grower, raising more than half of the global crop, much of it the stem lettuce or celtuce that is little known in the West but a true staple of the Chinese table, eaten stir-fried, pickled, and cold-dressed. After China come the United States and Spain, whose Californian and Murcian fields supply the salad lettuces of the whole Western world; California's Salinas Valley alone earns its old title as the Salad Bowl of the World.

The plant keeps its four culinary identities largely distinct. The romaine or cos, sweet and upright, is the lettuce of the Greek maroulosalata, the Persian kahoo sekanjabin, and, above all, the Caesar salad, the single most imitated salad in the world. The butterhead, soft and yielding, is the salad and braising lettuce of France, melted into petits pois à la française and folded into the gentle potage de laitue. The crisphead or iceberg, crunchy and watery and durable, is the American lettuce of the wedge salad, the Cobb, and the BLT, where its crispness is the entire point. The looseleaf lettuces fill the salad bowls and sandwich shops of the world. And the Chinese stem lettuce stands somewhat apart, grown for its peeled, crisp, faintly sweet stem rather than its leaves. Across East and South-East Asia the leaf is also a vessel and a wrapper: the cool, raw lettuce leaf folded around grilled meat and rice in the Korean ssam, cupped around minced pork in the Cantonese san choy bau, and laid amongst the herbs of the Vietnamese table. Few plants travel so easily between the salad bowl, the wok, and the wrapping hand.