Romaine or cos lettuce has elongated upright leaves forming a loose, loaf-shaped head. The outer leaves are usually dark green. Leaf lettuce comes in many varieties, none of which form a head. The next 3 types are seldom seen in the United States. Latin lettuce looks like a cross between romaine and butterhead.
Stem lettuce has long, narrow leaves and thick, edible stems.
Oilseed lettuce is a primitive type grown for its large seeds that are pressed to obtain oil. Lettuce plants are illustrated along with the hieroglyphs of the ancient Egyptians. They were probably cultivating and enjoying ancient forms of lettuce some 4,500 years ago along the green ribbon of the Nile Valley. The ancient Egyptian illustrations are cos-like and may also have represented stem lettuce. Stem lettuce and oilseed lettuces are still cultivated in Egypt.