Romaine also known as Cos, this variety of head forming lettuce has deep green, long leaves with a crisp texture and deep taste. Romaine lettuce is an excellent source of vitamin A (in the form of carotenoids), vitamin K, folate and molybdenum. In addition, romaine lettuce is a very good source of dietary fiber, manganese, potassium, biotin, vitamin B1, copper, iron and vitamin C. It is also a good source of vitamin B2, omega-3 fatty acids, vitamin B6, phosphorus, chromium, magnesium, calcium and pantothenic acid.