The Effect of Seed Quantity and Material Density on Dispersal from Wet Splash Cup Plants

Splash cup plants harness energy from falling rain to disperse their seeds distances up to 10x their plant height; the resulting splash accelerates the raindrop's initial velocity up to 5.5x. Understanding the biomechanics involved in this unique process could shed light on the evolution of these plants, how foliar diseases spread, and the mechanics of soil erosion. Splash cup plants are <10cm tall with 5mm diameter conical flowers–the cup–which house its seeds and are ubiquitous where potential splashing can occur. In nature, the cup can be wet before any dispersal occurs. Seed material density may also change dispersal distance. Previous work examined the effect of seed density in wet cups using 5 seed mimics and found that lighter seeds went longer distances. These plants may start with varied numbers of seeds. So, we examined the effect of seed density in wet cups for varying numbers of initial seeds. We used 3D-printed cup models half full of water to compare the dispersal distance of glass seed mimics to that of much lighter plastic mimics. We also varied the cup's internal cone angle and initial raindrop impact location to examine their interplay with seed density. Our results demonstrate that plastic seed mimics traveled about twice as far as the denser glass mimics.