Lettuce, Lactuca sativa, is a leafy herbaceous annual or biennial plant in the family Asteraceae grown for its leaves which are used as a salad green. The lettuce plant can vary greatly in size, shape and leaf type but generally, the leaves of the plant form a dense head or loose rosette. The stem of the plant is short, with larger leaves arranged at the bottom and becoming progressively smaller further up the stem. Leaves can be smooth or curly and are usually green or red in color.