Matthew J. Harrington*, Peter Fratzl, Christoph Neinhuis and Ingo Burgert

Abstract: Plants are able to adapt the geometry of their organs and tissue properties to cope with external and internal stresses and to actuate organ movement. Actuated movements due to moisture changes in cell walls are controlled by stiff cellulose fibrils embedded in a swellable matrix. Notably, many of these actuated movements function in the absence of an active metabolism, making them not only a fascinating natural phenomenon, but also an attractive model system for biomimetic technology transfer. A prime example of this actuation behavior is observed in the mature (non-living) seed capsules of Mesembryanthemaceae, which undergo a complex hydration-dependent movement in which protective valves reversibly fold open when wetted and close when dried. This movement permits a special mode of seed dispersal known as ombrohydrochory in which the plant utilizes the kinetic energy from raindrops to eject seeds long distances and which has been suggested to be a major factor in the rapid evolution and success of this plant group in arid and semi-arid climates. Here, data will be presented on the macroscale to nanoscale hierarchical organization of the seed capsule, as well as on the molecular level mechanisms that control their sophisticated hydration-dependent unfolding. These data were attained through the use of several in situ characterization techniques including environmental SEM, Raman and IR spectroscopy, and X-ray diffraction.