Scientists solve centuries-old mystery of how a cucumber explosively squirts its seeds

(CNN) — An odd little gourd called the squirting cucumber has an explosive reproductive strategy that has intrigued naturalists since the days of the Roman Empire. Squirting cucumbers blast their seeds over distances hundreds of times their length, and now scientists know how the plants do it.

A squirting cucumber in action is a remarkable sight, but if you blink, you might miss it — the eruption lasts about 0.03 second (30 milliseconds). When ripe, the hairy green fruit measures about 1.6 inches (4 centimeters) long. As it drops from a stem, it launches a fountain of seeds and sticky liquid at a velocity of around 45 miles per hour (20 meters per second), with seeds reaching distances of about 33 feet (10 meters).

Squirting cucumbers, also known by the scientific name Ecballium elaterium, are found across the Mediterranean and Europe, and in parts of northern Africa, Australia, Asia and North America. They are members of the gourd family (Cucurbitaceae) and relatives of zucchini, squash and pumpkins, but their seed jets are unique in the group and rare among plants.

Ancient Roman naturalist Pliny the Elder (AD 23 to 79) was the first to describe squirting cucumbers, warning that “the seed spurts out, even endangering the eyes.” Studies in the 19th century provided clues about the cucumbers’ high-pressure internal launch system, but until now, the details of their seed spurts were poorly understood.

Recently, researchers blew this ballistic mystery wide open. They used high-speed video, time-lapse photography, CT scans and digital 3D reconstructions to analyze squirting cucumbers before, during and after a squirt and created mathematical models to describe the fruits’ geyser-like eruptions.

Their findings, published Monday in the journal Proceedings of the National Academy of Sciences, explain that the squirting mechanism is more complex than a simple buildup of liquid and release of internal pressure. Along with cucumber engorgement, other physical changes in the plant before and during the explosion shaped the angle, height and distance of a successful seed spew, according to the study.

“It is a new idea that optimal seed dispersal is not only about building the greatest pressure in the fruit,” said Dr. Angela Hay, a research group leader at the Max Planck Institute for Plant Breeding Research in Cologne, Germany. Hay has previously written about explosive seed dispersal but was not involved in the new research.

Rather, Hay told CNN in an email, the study suggests that the most effective seed launch relies on a trade-off between pressure from fluid accumulation and other distinct changes that prepare the fruit for that eruptive moment.

‘A tiny little squirt’

Study coauthor Dr. Derek Moulton, a professor of applied mathematics at the UK’s University of Oxford, was introduced to the squirting cucumber in the summer of 2022 by senior study author Dr. Chris Thorogood, a deputy director and head of science at the Oxford Botanic Garden and Arboretum.

“We’ve worked on some other projects together, and have a really good collaborative relationship,” Moulton told CNN in an email. “He shows me a plant that does something interesting, and then I try to use mathematics to understand some aspect.”

Thorogood brought Moulton and other researchers to see squirting cucumbers in the Oxford garden, assuring them that the unassuming gourds were more bizarre than they looked. When they were ripe, all it took was a touch to trigger their high-powered launch.

“He bent down and gave one a squeeze, and you could hear a tiny little squirt, but we couldn’t see anything happen, it was too fast,” Moulton said. “So we took some fruits back to the lab, where we filmed the ejection with high speed cameras. It was amazing. We were hooked, and we wanted to understand it.”

High-speed videos of the cucumbers were just the beginning. The researchers used CT scans to study the internal arrangement of the seeds, tested the stiffness of plant parts and captured time-lapse videos in the days leading up to a seed launch to track physical changes in the stems and fruits. With mathematical models based on their data, the scientists created simulations of seeds taking flight and predicted where they would land.

“Our mathematical framework came from thinking about the physics of the process, both before and during the seed ejection, and included mathematical descriptions of both the fruit and the stem and how they interact,” Moulton said.

Lift and twist

Days before a seed launch, a lot was going on inside the cucumbers, the researchers observed. As the fruits swelled with fluid, some of that fluid also flowed into the stem, thickening and stiffening it so that the dangling fruit lifted from a near-vertical hanging position to an angle of about 45 degrees. Squirting from this angle, rather than straight up, is a better strategy for the plant, enabling it to spit the seeds farther, according to the study.

The scientists also saw that the stem gave the fruit a spin as it detached, by shrinking away from the fruit. Their models showed that this spin would disperse the seeds in a wider arc.

“It’s not enough to just fire your seeds far — you also want them well spread out if you want to increase the likelihood that some of them survive to make new plants,” Moulton said. “There is this whole orchestration of activity that contributes to where the seeds go. And since all of that detail lived in our mathematical model, we were able to show how fine tuned Ecballium is for successful dispersal.”

Scientists often look to model organisms — certain species that are well-studied — to answer biological questions. But there’s also much to be learned by studying outliers that receive less attention, such as the squirting cucumber, Hay said.

“Many interesting traits are not found in model organisms,” she said. “This paper shows how we can learn from diversity.”

The new study also illustrates that even though plants typically don’t command center stage for odd behavior, they can still demonstrate “some pretty amazing feats,” Moulton said.

“How many people have ever associated cucumbers with rapid motion?” he asked. “I was quite happy just to be able to uncover something about the mechanism of this fascinating little cucumber.”

Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American and How It Works magazine.

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