It is the stem. Stems that change in form and structure due to changes in function are called metamorphic stems, and the metamorphic organ of lettuce is the stem. Lettuce is an annual or biennial herb of the Asteraceae family. It is 25-100 cm tall. The stem is upright and solitary. The basal leaves and lower stems and leaves are large, undivided, oblanceolate, oval or elliptical. shape.