Because it's said to have originated on the Aegean island of Cos, romaine is also called 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.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.