In their development leaves exhibit plastic responses in both shape and size. Variations in shape are often associated with changes in size also but the reverse is not always true. Plastic responses in leaf form resulting from ontogenetic or external influences are initiated very early in primordial development and are brought about by effects on the rate and direction of cell division and expansion in different regions of the primordium. Effects on leaf size are often induced over much longer periods including the phase of lamina expansion. In Phaseolus vulgaris the primary leaves exhibit increases in size when one of the pair is removed or when the stem is decapitated above the primary leaf node. These compensatory growth effects are not the result of a change in cell number but are caused by an increase in mean cell size. Cell wall extensibility is not increased by treatment and the evidence suggests that a small increase in the (turgor-wall yield stress) term may be the cause of the very rapid response to defoliation. The usefulness of leaf systems for the analysis of plastic responses of shape and size is indicated and the importance of a better understanding of the factors determining the siting and development of the cell wall is stressed.