or cos lettuce
L. var. longifolia
) is a variety of lettuce
which grows in a long head of sturdy leaves with a firm rib down the center. Unlike most lettuces, it is tolerant of heat.
Origin and etymology
Most dictionaries trace the word cos to the name of the Greek island of Cos, from which the lettuce was presumably introduced. Other authorities (Davidson) trace it to the Arabic word for lettuce, خس khus (xus)
It apparently reached the West via Rome, as in Italian it is called lattuga romana and in French laitue romaine, hence the name 'romaine', the common term in American English. (Davidson)
The thick ribs, especially on the older outer leaves, should have a milky fluid which gives the romaine the typically fine-bitter herb taste. Romaine is the standard lettuce used in Caesar salad.
Romaine lettuce is often used in the Passover Seder as a type of bitter herb, to symbolise the bitterness inflicted by the Egyptians whilst the Israelites were slaves in Egypt.
The American Institute for Cancer Research includes Romaine Lettuce in its list of foods that fight Cancer.