Spray cucumbers (Ecballium elaterium) release their seeds in a powerful jet of liquid. (Photo credit: Left: MSchauer/Getty Images; Right: Dominic Vella)
Spray cucumbers get their name from the unique way they eject their seeds in a powerful jet of liquid. Now, scientists have finally uncovered the mysterious mechanism behind these explosive emissions.
New research shows that these inedible cucumbers (Ecballium elaterium), whose name translates as “to throw out” in Greek, accumulate liquid in their fruit, creating so much pressure inside the cucumbers that they eventually burst. Interestingly, the scientists found that this pressure is relieved shortly before the seeds are released, when the liquid moves from the fruit into the stem, changing its shape and pushing the cucumbers out in such a way that they disgorge their contents.
The results obtained answer questions that have long worried researchers.
“For centuries, people have wondered how and why this unusual plant so vigorously releases its seeds into the world,” study co-author Chris Thorogood, deputy director and scientific director of the Oxford Botanic Garden and Arboretum in the U.K., said in a statement. “Now, as a team of biologists and mathematicians, we have finally begun to unravel this great botanical mystery.”
Spray cucumbers belong to the Cucurbitaceae family, which also includes melons, pumpkins, and zucchini. To understand how these small, furry cucumbers release their seeds, Thorogood and his colleagues used time-lapse photography, CT scanning, and a high-speed camera capable of recording 8,600 frames per second. The seed-ejection process lasts just 30 milliseconds — too fast to be seen with the naked eye, according to the statement.
The team also measured the volume of cucumbers and stems before and after the plants released their seeds. The researchers then fed this data into mathematical models to map the mechanics and trajectories of seed release.
The results, published Monday (Nov. 25) in the journal PNAS, show for the first time how spray cucumbers eject their seeds “with astonishing speed and precision,” study co-author Finn Box, a research fellow in the Fluid and Soft Matter Physics Group at the University of Manchester in the U.K., said in a separate statement.
Fluid builds up inside cucumbers for weeks, creating the pressure needed to release the seeds. But in the days leading up to release, some of the fluid moves into the stem, causing it to grow longer, wider, and stiffer. This process is “almost unheard of in the plant world,” and it changes the angle of the cucumbers in a way that maximizes seed dispersal once the fruit bursts, Box said.
Microseconds before the seeds are released, the stem tip bounces off the cucumbers, causing them to spin as they are catapulted. This twisting motion causes spray cucumbers to launch their seeds up to 33 feet (10 meters) from the parent plant at speeds of 66 feet per second (20 meters per second).
“The cucumber's explosive seed-release mechanism has evolved over many generations to help the plant survive,” Box said. “These mechanisms allow the plant to spread seeds over a wide area and reduce crowding and competition between offspring and other nearby plants, giving the next generation a better chance of survival.”
Even small changes in these mechanisms can reduce a seed's chances of survival, according to the study. In their models, the researchers examined the effects of creating pressure in the fruit without redistributing fluid into the stem, which resulted in the seeds being more densely packed around the mother plant and reduced seedling survival.
The results solve a long-standing mystery, but could also inspire the development of new technologies, such as medical devices that
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