A biological hydraulic accumulator: How the squirting cucumber, Ecballium elaterium, squirts its seeds

TL;DR

This study uncovers the hydraulic and mechanical mechanisms behind seed dispersal in the squirting cucumber, revealing how elastic energy stored in fruit tissues is explosively released to propel seeds at high velocities for effective dispersal.

Contribution

It provides detailed insights into the fluid mechanics and physiological processes enabling explosive seed dispersal, integrating high-speed imaging, microtomography, and pressure measurements.

Findings

Fruits store elastic energy as turgor pressure up to 99 kPa.

Seeds are expelled at velocities up to 30 m/s.

Dispersal covers a broad spatial cone.

Abstract

Seed dispersal is a fundamental process that allows offspring to reach suitable habitats and colonize new environments. While most plants rely on external vectors, some have evolved mechanisms that employ the buildup of liquid pressure in a closed compartment and its explosive release to disperse their seeds. This form of energy storage, reinvented by humans for engineering applications, is termed a hydraulic accumulator. Here we investigated the fluid mechanics involved in dispersal in the squirting cucumber, Ecballium elaterium integrating high-speed videography (up to 10 000 fps), microtomography, and internal pressure sensors. We recorded long-term pressure time series showing that E. elaterium exhibits circadian (24 hour) and ultradian (short-period) rhythms. Remarkably, the measurements revealed a lack of correlation between fruit and stem turgor; while the stem showed strong circadian cycles, the fruit often did not, suggesting isolated physiological processes in different tissues. The fruit's spongy wall tissue stores elastic potential energy as turgor pressure builds to nearly one atmosphere (92-99 kPa). Upon detachment, this energy is rapidly released to expel a turbulent, particle-laden liquid jet. Microtomography revealed that the seeds are packed around a central funiculus, a configuration that optimizes their exit through the basal orifice at velocities of up to 30 m/s. Seeds eventually move faster than the liquid droplets during the later stages of ejection as they shed their liquid coating. This sophisticated mechanism ensures a broad dispersal cone, effectively spreading offspring across space and environmental conditions.