Splash Cup Seed Dispersal Depends on Cup Size

Splash-cup plants are plants that have adapted to form a conical-shaped seed cup that takes advantage of the kinetic energy of incoming rain droplets to disperse seeds. This type of dispersal method has evolved separately several different times in families of flowers and fungi. It exists in a variety of habitats, including grasslands, wetlands, forests, deserts, and more. Splash cups are small, with diameters ranging from 3 to 5 mm and plant heights mostly under 10 cm. Despite this, they disperse their seeds distances up to and beyond 1 m away. This splashing mechanism is not well understood, and previous theoretical models did not match experimental results well. Understanding this system can lead to insight into splash-cup plant evolution and a better understanding of other similar splashing systems, such as soil erosion and the spread of foliar disease.

Previous studies have examined the relationship between seed dispersal distance and cup opening angle, drop impact location, cup contents, and cup shape. Here, we study the relationship between cup size and seed dispersal, motivated by the small variance in diameter observed in nature.

We released water droplets onto 3D-printed cup models, which contained 5 plastic seed mimics. Four different 3D cup models were used with diameters of 3, 5, 7, and 9 mm. We recorded the incoming drop impact with a high-speed camera. We then measured the resulting dispersal distance of each ejected seed. We found that larger cups dispersed seed mimics farther than smaller cups on average. We also compared our results to a simple theoretical model based on the volume of the incoming raindrop that contributes to the splashing.