A new study has finally shed light on one of nature’s most remarkable botanical mysteries: how the squirting cucumber (Ecballium elaterium) propels its seeds with such explosive force.
The research, led by a team at the University of Oxford, combines cutting-edge experiments, high-speed videography, image analysis and mathematical modelling to uncover the intricate mechanics behind this extraordinary phenomenon.
Published in The Proceedings of the National Academy of Sciences, the study reveals how this Mediterranean native uses a unique, pressurised system to ensure its seeds are dispersed far and wide, enhancing its chances of survival and reproduction.
A natural catapult
The squirting cucumber gets its name from its dramatic seed dispersal strategy. The plant’s fruit, a small ovoid structure, bursts from the stem when ripe, ejecting seeds in a high-pressure jet of mucilage. The whole event takes place in just 30 milliseconds, launching the seeds at speeds of up to 20 metres per second. These projectiles can travel as far as 10 metres – an impressive 250 times the fruit's length.
This ballistic launch system has long fascinated naturalists and scientists. Even Pliny the Elder, the Roman naturalist, noted the plant’s remarkable behaviour in the first century AD, writing, “Unless, to prepare it, the cucumber be cut open before it is ripe, the seed spurts out, even endangering the eyes.” But until now, the precise mechanics behind this explosive ejection have remained a mystery.
Experiments at the Botanical Garden
To investigate, researchers from the University of Oxford and the University of Manchester conducted a series of experiments using squirting cucumber specimens grown at the University of Oxford Botanic Garden.
The team captured the seed ejection process with a high-speed camera, recording up to 8,600 frames per second to scrutinise every fraction of a millisecond. They also used CT scans, indentation tests, and time-lapse photography to study the fruit and stem before and during dispersal.
To complement their observations, the researchers developed advanced mathematical models to simulate the internal pressures and trajectories involved.
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A sophisticated dispersal system
The study identified four key components that contribute to the squirting cucumber’s explosive seed dispersal:
- Pressurised build-up: As the fruit matures, mucilaginous fluid accumulates, creating high internal pressure.
- Fluid redistribution: In the days leading up to dispersal, some of this fluid is transferred to the stem. This process lengthens, thickens, and stiffens the stem while tilting the fruit to an angle of around 45°—the optimal position for seed ejection.
- Rapid recoil: At the moment of ejection, the stem recoils, causing the fruit to rotate sharply in the opposite direction.
- Variable launch angles: As seeds are ejected sequentially, the internal pressure diminishes. This means the first seeds travel the furthest, while later seeds are launched at higher angles and shorter distances. This variation ensures seeds are distributed over a wide, ring-shaped area, typically between 2 and 10 metres from the parent plant.
This unique dispersal strategy allows the squirting cucumber to maximise the spread of its seeds, ensuring they don’t compete with each other or the parent plant for resources.
Fine-tuned by nature
The researchers also explored what might happen if the plant’s dispersal system were altered. For example, they found that a stiffer stem resulted in seeds being launched almost horizontally, limiting their spread. Conversely, reducing fluid transfer to the stem caused seeds to be ejected at steeper angles, shortening the dispersal range.
These simulations suggest that the squirting cucumber’s system has been finely tuned by evolution to ensure the optimal spread of seeds for survival.
“For centuries people have asked how and why this extraordinary plant sends its seeds into the world in such a violent way," explains Dr Chris Thorogood, Deputy Director and Head of Science at Oxford Botanic Garden. "Now, as a team of biologists and mathematicians, we’ve finally begun to unravel this great botanical enigma.”
Dr Derek Moulton, Professor of Applied Mathematics at Oxford, adds: “The first time we inspected this plant in the Botanic Garden, the seed launch was so fast that we weren’t sure that it had actually happened. It was very exciting to dig in and uncover the mechanism of this unique plant.”
Beyond botany
Interestingly, the findings could inspire innovations in engineering and material sciences. Co-author Dr Finn Box of the University of Manchester highlights potential applications in bio-inspired technology, such as on-demand drug delivery systems. These might use mechanisms similar to the squirting cucumber’s to precisely release nanoparticles.
A botanical wonder
The squirting cucumber is part of the gourd family (Cucurbitaceae), which includes familiar crops like melon, pumpkin and courgette. While native to the Mediterranean, where it is often regarded as a weed, this plant’s remarkable dispersal mechanism has captured human imagination for centuries.
Thanks to this study, we now have a clearer understanding of how this botanical marvel works. As Dr Thorogood summarises, “it’s fascinating to see how something so small can exhibit such sophisticated engineering. It’s a wonderful reminder of the ingenuity of the natural world.”
The research offers a glimpse into the interplay between biology and physics, highlighting how plants like the squirting cucumber have evolved to thrive in their environments through ingenious strategies.
Main image: A still showing the jet ejected from a squirting cucumber, which carries its seeds distances of up to 10m away from the mother plant/Dominic Vella
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