Ingo Burgert & Peter Fratzl

Max Planck Institute of Colloids and Interfaces, Potsdam, Germany

Plant cell walls consist of a few nanometer thick semi-crystalline cellulose fibrils embedded in amorphous matrix polymers such as hemicelluloses, pectins or lignin. The orientation of the parallel cellulose microfibrils in secondary cell walls, known as microfibril angle (MFA) is one of the main parameter to adjust mechanical properties and to specifically actuate organ movement. Several plant organ movements are due to moisture changes in cell walls which means that they do not require any metabolism and even dead tissues can be actuated. The specific movement is controlled by the architecture of the cell walls by means of the orientation of stiff cellulose fibrils embedded in swellable matrix polymers. Recent findings on the movement and stress generation in tension wood which enables hardwoods to orient leaning stems and branches and the bending movement of wheat awns which provide motility to the seed in a daily cycle are presented. The general principle by which active gels can be directed in their swelling behaviour by stiff fibres distributed in a suitable way makes humidity-based actuation systems in plants particularly interesting for biomimetic materials research.