Long leaves in terrestrial plants and their submarine counterparts,
algal blades, have a typical, saddle-like midsurface and rippled
edges. To understand the origin of these morphologies, we dissect
leaves and differentially stretch foam ribbons to show that these
shapes arise from a simple cause, the elastic relaxation via bending that follows either differential growth (in leaves) or differential
stretching past the yield point (in ribbons). We quantify these different modalities in terms of a mathematical model for the shape
of an initially flat elastic sheet with lateral gradients in longitudinal growth. By using a combination of scaling concepts, stability
analysis, and numerical simulations, we map out the shape space
for these growing ribbons and find that as the relative growth
strain is increased, a long flat lamina deforms to a saddle shape
and/or develops undulations that may lead to strongly localized
ripples as the growth strain is localized to the edge of the leaf. Our
theory delineates the geometric and growth control parameters
that determine the shape space of finite laminae and thus allows
for a comparative study of elongated leaf morphology.