Romaine lettuce

Romaine lettuce grows in a long head of sturdy leaves which have a firm rib down their centers. These thick ribs, especially on the older outer leaves, have a milky fluid which is unpleasant, so they should not be used, nor should the leaf-tips, which can be bitter. (Although these dark green outer leaves are the most nutritious). The interior leaves are paler in color, and more delicate in flavor. There is also a milder tasting variety with red tipped leaves and a sweet romaine, which is even sweeter than regular romaine. Romaine is the classic lettuce of choice for Caesar salads. Because it's said to have originated on the Aegean island of Cos, Romaine is also called Cos lettuce.