Expansins are extracellular proteins that increase plant cell wall extensibility in vitro. Beads loaded with purified expansin induced bulging on the leaf-generating organ, the apical meristem, of tomato plants. Some of these bulges underwent morphogenesis to produce leaflike structures, resulting in a reversal of the direction of phyllotaxis. Thus, expansin can induce tissue expansion in vivo, and localized control of tissue expansion may be sufficient to induce leaf formation. These results suggest a role for biophysical forces in the regulation of plant development.