Helical morphing of <i>Pelargonium</i> seed awns

Seeds of some plant species, when left on a dry land, propel themselves into the soil to secure warmth and humidity needed for germination. In particular, seeds of Pelargonium species possess a long awn that are responsible for screwing motion of the seeds. The awn consists of two layers, one of which swells when wet but the other is insensitive to humidity. The coiling (dry) - uncoiling (wet) deformation of the individual cells in the hygroresponsive layer has been previously analyzed, which is owing to a tilted arrangement of cellulose microfibrils in the cell wall. Here we construct a novel mechanical model to explain the global helical deformation of the awn consisting of hundreds of hygroresponsive cells aligned in one direction. We also fabricate a phytomimetic actuator capable of helical coiling in response to environmental humidity change, with a controlled pitch and radius. The mathematical model constructed for the seed is shown to work equally well for the artificial actuators.