The mechanics of leaf growth on large scales

Leaf growth is a process during which the leaf expands by many orders of magnitude, while maintaining a proper shape. Complex mechanisms which are not fully understood regulate the local, individual expansion of multiple cells into an organ with a well-defined shape. Various factors of mechanical and biochemical nature play a role in the growth process. The mechanical factors include the cell wall properties, osmotic pressure, and interactions between neighboring cells. This happens under the strict restrictions stemming from geometric considerations, which are dictated by the three-dimensional shape of the leaf. In this chapter, we review this topic focusing on its geometric and mechanical aspects, such as stress-strain response, viscous and elastic moduli, and typical statistical properties of the growth field. We view the leaf as a sheet of active matter, capable of adjusting and responding to signals originating both from its natural internal growth processes and from its surrounding. We aim at a better understanding of the growth patterns and mechanical properties of the tissue as a whole.