Abstract
Leaves of most monocotyledonous plants gain structural stiffness from curling or folding. This is shown to be true of Phormium spp., where the optimisation seems to be for the leaf to gain as much stiffness as possible by curling longitudinally whilst keeping the maximum amount of projected area available for intercepting light. Smaller plants with less leaf fibre have to fold about the midrib to gain this stiffness, which will reduce their projected area more than curling would but reduces the investment in fibre content.