Plant Biomechanics

Abstract

The mechanical design of plants is based on the tensile properties of cellulose molecules which combine to form stiff nanofibrils from which the wall surrounding all plant cells is mainly constructed. Simple plants such as liverworts are made from few cell types and rely for their stiffness on the osmotically maintained pressure (turgor) of water inside the cell reacting against the stiff cellulose. As plants evolved and grew towards the light they developed larger fibres which transmit the forces more effectively and convert the plant into a complex prestressed structure, but still largely dependent on water pressure for its rigidity. Increased structural complexity leads to increased fracture resistance and durability. Lignin glues the nanofibrils together so that the fibres, and then the cell walls in general, can take compressive forces, and tall herbs and trees evolve. Some plants (lianas, creeping herbs, Spanish moss, etc.) can grow large relying on simple tension under gravitation.