Phentypic plasticity for leaf size at high versus low light in three Polygonum species. Leaf size (photosynthetic surface area) strongly affects plant light capture and transpirational water loss, and thus is a major determinant of function in different light environments. P. cespitosum (solid squares) invariably produces large leaves and is restricted to shade habitats; P. hydropiper (solid triangles) invariably produces narrow leaves and is restricted to high-light habitats; and P. persicaria (open symbols; three populations shown) alters leaf size plasticity depending on growth conditions. This species is an ecological "generalist" and occurs in many diverse habitats.