Microscale geometrical features in the plant leaf epidermis confer enhanced resistance to mechanical failure

In many plant species, epidermal tissues of leaves and petals feature irregular wavy cell geometries forming jigsaw puzzle patterns. At the origin of plant tissues are simple polyhedral progenitor cells that divide and grow into a kaleidoscopic array of morphologies that underpin plant organ functionality. The wide prevalence and great diversity of the wavy cell shape in the plant kingdom point to the significance of this trait and its tunability by environmental pressures. Despite multiple attempts to explain the advent of this complex cell geometry by evolutionary relevant functionality, our understanding of this peculiar tissue patterning preserved through evolution remains lacking. Here, by combining microscopic and macroscopic fracture experiments with computational fracture mechanics, we show that wavy epidermal cells toughen the plants protective skin. Based on a multi-scale approach, we demonstrate that, biological and synthetic materials alike can be toughened through an energy-efficient patterning process. Our data reveal a ubiquitous and tunable structural-mechanical mechanism employed in the macro-scale design of plants to protect them from the detrimental effects of surface fissures and to enable and guide the direction of beneficial fractures. We expect these data to inform selective plant breeding for traits enhancing plant survival under changing environmental conditions. From a materials engineering perspective, this work exemplifies that plants hold sophisticated design principles to inspire human-made materials.