One variety, the celtuce (asparagus lettuce) is grown for its stems, which are eaten either raw or cooked. Europe and North America originally dominated the market for lettuce, but by the late 20th century the consumption of lettuce had spread throughout the world. Lettuce is a rich source of vitamin K and vitamin A, and a moderate source of folate and iron. Christopher Columbus was the person who actually introduced lettuce to America during his second voyage to the New World in 1494. Dark green lettuce leaves are more nutritious than light green leaves.