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. Unlike most lettuces, it is tolerant of heat.

Cuisine

The thick ribs, especially on the older outer leaves, have a milky fluid which is unpleasant, so they should not be used. Neither should the leaf-tips because those can be bitter.Romaine is the standard lettuce used in Caesar salad.