Seed ejection mechanism in an Oxalis species

With millions of years’ evolution, plants and fungi have developed a variety of ballistic dispersal structures for seeds or spores. One typical example is the catapult of an Oxalis sp., which can realize a consecutive seed ejection by triggering only one seed. If the protrusion on an aril, a specialized outgrowth covering a seed, is disturbed, cracks would occur and cause the opening of the aril. Subsequently, the whole aril snaps and transforms its stored strain energy to eject the inside seed with an optimal launching angle. Once the first seed is triggered, its curly aril will contact the next seed’s protrusion and induce its firing. This chain effect will further trigger the remaining seeds in turns, within 0.1 s. Inspired by this phenomenon, we invented a bionic ejection device to launch projectiles with high efficiency. This exploration is promising for a number of applications, such as drug delivery and oil displacement.