Lactuca sativa (Lettuce)

This annual or biennial herb is cultivated worldwide for its leaves and, in some forms, its thick stems. Soft leaves gather near the base before the plant produces an upright, branching flowering stem. Numerous small flower heads carry yellow strap-shaped florets. The flattened, ribbed fruits have slender beaks and white tufts of hairs that aid dispersal. Cultivated forms differ greatly in leaf shape, color, and head structure. Heat and dry soil encourage bolting, when the flowering stem rises and the leaves become bitter. Outer leaves of loose-leaf types can be harvested while the centre continues growing. Photographed in Colombia.