Wikipedia Article About Romaine lettuce on Wikipedia Romaine or Cos lettuce (often called simply Romaine or Cos) (Lactuca sativa L. var. longifolia) is a variety of lettuce which grows in a long head of sturdy leaves with a firm rib down the center. It is said to have originated on the Aegean island of Cos, which is why it is also known as Cos lettuce. Romaine's elongated head has dark green outer leaves that lighten to pale celadon in the center. The leaves are crisp and slightly bitter and the crunchy midrib is particularly succulent. Romaine adds crunch and flavor to mixed green salads and is the lettuce of choice for Caesar salads.