Three summers ago, Kiel University biomechanist Stainslav Gorb, returned to Germany after a vacation in Turkey with a present for Ph.D. student Helen Gorges.
“He came back and was like, ‘Helen, I brought you something. They might explode, but maybe you can work on them,'” she recalls.
He presented her with a handful of squirting cucumbers, a smaller, hairier cousin of what one would find in the produce aisle — except that it’s toxic.
But what the squirting cucumber lacks in palatability, it makes up for in a feat of ballistic seed dispersal. When the fruit is ripe, it’s puffed up with fluid, pressurized, and ready to burst.
The merest disturbance or nudge causes the cucumber to pop open and expel its seeds in a fraction of a second, sending seeds on a germination journey that can reach some distance away, reducing competition with their parent.
Once Gorges learned about this botanical feat, she was hooked. “We just wanted to know how the whole explosion works,” says Gorges.
She and her colleagues have done just that in a talk she gave at last week’s Society for Experimental Biology conference in Belgium and an upcoming publication in the journal npj Science of Plants. “The plant is crazy,” summarizes Gorges. “The fruit really prepares for perfect dispersal — as far as possible so that the plant will grow everywhere.”
A cucumber’s perfect ballistic angle
Gorges and her colleagues started figuring out how the cucumber worked by taking a few hundred photos of the plants — some from Turkey and others from the local botanical garden in Kiel in northern Germany — to measure the angle between the stem and the ripened fruit.
It was consistently a touch more than 50 degrees. “With air resistance,” she says, “it just is the perfect angle so that they can shoot as far as they can — the perfect ballistic angle for the perfect shooting parabola.”
Next, Gorges examined the fifty-some seeds inside the fruit. But cut one open, and the whole thing explodes. So she took micro-CT scans of the fruit’s interior, which revealed the seeds lined up in rows, awaiting their launch, round end first. “The seeds, they really all come out in the same direction,” says Gorges.
To examine the expulsion of the seeds in greater detail, she used high speed videography.
When Gorges later slowed down the video, she observed the stem lifting off and a jet of fluid spurting out, accompanied by one seed shooting out at a time. She calculated that the seeds can reach velocities of up to nearly 30 miles an hour and they can launch themselves almost 40 feet.
The results are consistent with a study published in 2024 in the journal PNAS by a different team in England that calculated an average launch angle of about 43 degrees, velocities of 37 miles per hour, and launch distances of almost 40 feet.
“It’s actually very difficult to throw a small object a large distance,” says Dwight Whitaker, a physicist at Pomona College who didn’t contribute to either research project. “To my knowledge, [this is] one of the few plants, if not the only plant, that uses a fluid to eject its seeds — this pressurized fluid that sort of pushes them and accelerates them as they come out.”
A supremely sticky seed
Gorges then examined the seeds’ slimy coat.
When she placed a seed on a glass slide and let it dry, she found its adhesive quality was so strong that a single seed could support as much as a six-pound weight.
“If adhesion is what it’s all about,” says Sheila Patek, a biologist at Duke University who wasn’t involved in the research, “then in fact, the remarkable launch velocities may have more to do with making sure that when that seed hits something, it sticks” — so that it might then be carried farther by whatever it’s stuck to.
To test this idea, future work could consider how well the seeds adhere to various surfaces when launched at different speeds.
Patek says that the discoveries about the squirting cucumber may well have many applications. “These types of materials are really translatable in today’s world of material science,” she says, including soft robotics that might disperse seeds on farms or deliver drugs in the human body.
And these seeds, whose stickiness changes when wet, could help inspire materials designed to work differently in damp or dry conditions.
Not bad for a little squirt.
Transcript:
JUANA SUMMERS, HOST:
All right, Ailsa, pop quiz – how many cucumbers can you name?
AILSA CHANG, HOST:
(Laughter) Well, there are Persian cucumbers – the itty-bitty ones. And then there’s the super long English cucumber, right?
SUMMERS: Go on. Go on.
CHANG: That’s all I got, Juana.
SUMMERS: Well, Ailsa, our next story is about a kind of cucumber that science reporter Ari Daniel says uses ballistics to spread its seeds.
ARI DANIEL, BYLINE: Three summers ago, when Helen Gorges’ Ph.D. supervisor returned from his annual vacation to coastal Turkey, he had a present for her.
HELEN GORGES: He came back and was like, Helen, I brought you something. They might explode, but maybe you can work on them.
DANIEL: He brought her a handful of squirting cucumbers.
GORGES: It looks like a small, hairy cucumber, actually. So this cucumber is toxic, so you shouldn’t eat the fruit, you shouldn’t eat the seeds and also not the leaves.
DANIEL: The squirting cucumber doesn’t get its name for nothing, and it’s why her supervisor, who studies biomechanics, was so excited about them. Gorges searched for the plant on YouTube and turned up this David Attenborough video.
(SOUNDBITE OF MUSIC)
DANIEL: Attenborough reclines next to a patch of the cucumbers. Each one is puffed up with fluid and seeds, pressurized and ready to burst.
(SOUNDBITE OF TV SHOW, “THE GREEN PLANET”)
DAVID ATTENBOROUGH: All they need is just a slight nudge.
DANIEL: He barely touches a stick to one, and it pops open…
(SOUNDBITE OF TV SHOW, “THE GREEN PLANET”)
ATTENBOROUGH: (Laughter).
DANIEL: …Expelling its seeds in a fraction of a second, which he says allows them to germinate some distance away, reducing competition with their parent. Gorges, who’s based at Kiel University in Germany, was hooked.
GORGES: We just wanted to know how the whole explosion works, actually.
DANIEL: The first thing that she and her colleagues did was take a few hundred photos of different cucumber specimens to measure the angle between the stem and the ripened fruit, which was consistently a touch more than 50 degrees.
GORGES: With air resistance, it just is the perfect angle so that they can shoot as far as possible.
DANIEL: Gorges then wanted to examine the ejection of the seeds in greater detail. But you can’t expect to see anything by just watching a squirting cucumber in action. It’s way too fast.
GORGES: I haven’t seen it before, how the explosion really looks like.
DANIEL: So Gorges used high-speed videography. When she later slowed the video down, she observed the stem lifting off and a jet of fluid spurting out, accompanied by one seed shooting out at a time.
GORGES: The seeds, they really all come out in the same direction.
DANIEL: Gorges calculated the seeds can reach velocities of up to nearly 30 miles an hour, and they can launch themselves almost 40 feet. CT scans of the fruit’s interior revealed the seeds lined up in rows, awaiting their ejection, round end first. The results are consistent with a study published late last year by a different team in England.
GORGES: So the plant is crazy. So the fruit really prepares for perfect dispersal – as far as possible so that the plant will grow everywhere.
DANIEL: Gorges and her colleagues then went on to examine the seed’s mucilaginous coat and how strong it was when it’s stuck to a glass slide and then dried. She found the adhesive was so strong that a single seed could support as much as a 6-pound weight. Gorges presented her results at last week’s Society for Experimental Biology conference in Belgium. Sheila Patek is a biologist at Duke University who wasn’t involved in the research.
SHEILA PATEK: If adhesion is what it’s all about, then, in fact, the remarkable launch velocities may have more to do with making sure that when that seed hits something, it sticks.
DANIEL: So that it might be carried farther by whatever it’s stuck to – Patek says the discoveries may well have applications.
PATEK: These types of materials that these folks identified in this study are really translatable in today’s world of material science.
DANIEL: Including soft robotics that might disperse seeds on farms or deliver drugs in the human body. And these seeds, whose stickiness changes when wet, could help inspire materials designed to work differently in damp or dry conditions – not bad for a little squirt. For NPR News, I’m Ari Daniel.
(SOUNDBITE OF MUSIC)
