A new study reveals the explosive secret of the Devil’s cucumber, which ejects its seeds with a pressurized fluid jet reaching up to 10 meters. It is also referred to as Cucumis metuliferus, commonly known as the kiwano or horned melon, due to its spiky appearance and vibrant orange color. The name “devil’s cucumber” is a colloquial term that highlights its unusual, otherworldly look. This fruit is native to Africa and notable for its bright green jelly-like interior. Despite its somewhat intimidating appearance, the kiwano is edible and has a mildly sweet and tart flavor.
A team headed by Oxford University has resolved a mystery that had intrigued scientists for centuries. How on earth does the devil’s cucumber eject its fluid? Results obtained through a combination of experiments, along with high-speed videography, image analysis, and advanced mathematical modeling will be published today, November 25th, in Proceedings of the National Academy of Sciences (PNAS), which is known for its rigorous peer-review process and its role in disseminating groundbreaking research.
The devil’s cucumber, bitter worm or squirting cucumber (Ecballium elaterium), in Greek, received its name from the ballistic method used by the species to disperse its seeds. When ripe, the ovoid fruit detaches from the stem and expels the seeds explosively in a stream of mucilage, a viscous substance at high pressure. This projectile launch, which lasts just 30 milliseconds, causes the seeds to reach speeds of about 20 meters per second and land at distances of up to 250 times the length of the fruit, which is more or less around 10 m. Up till recently, the exact mechanism of seed dispersal of the devil’s cucumber and how it affected its reproductive success was not well understood.
Seed dispersal mechanism
In the new study, researchers from the University of Oxford, along with the University of Manchester, conducted several experiments with specimens of Ecballium elaterium cultivated at the University of Oxford Botanic Garden. Among other things, they filmed seed dispersal with a high-speed camera (capturing up to 8600 frames per second), measured fruit and stem volume before and after dispersal, performed indentation tests and computed tomography of an intact cucumber, and monitored the fruit with time-lapse photography in the days prior to release. With this combined approach, the team elucidated the key components of the plant’s dispersion strategy.
Redistribution of the Fluids
In the weeks prior to seed dispersal, the fruits become pressurized due to the accumulation of the mucilaginous liquid. In the days prior to the dispersal, some of this fluid is redistributed from the fruit to the stem, making it longer, thicker, and stiffer. This takes the fruit from being nearly vertical to a near 45-degree angle, a key element necessary for a successful launch. In the first hundred microseconds of ejection, the tip of the stalk recoils away from the fruit, causing the fruit to rotate in a counterclockwise direction.
Due to the components mentioned above, the seeds are ejected with an exit velocity and an angle of release that depends on their sequence. With the following seeds, the exit velocity decreases (because the pressure of the fruit capsule, now empty, decreases), while the launch angle increases (due to the fruit rotation). This causes the initial seeds to reach the farthest distance, and the later seeds to land closer. As multiple fruits are distributed around the center of the plant, the overall result is a wide and almost uniform distribution of seeds covering a ring-shaped area at a distance of 2 to 19 meters from the mother plant
Together, these components constitute a sophisticated seed dispersal system. In particular, the redistribution of fluid from the fruit to the stem is believed to be unique in the plant kingdom. It is fascinating and unique at the same time, as it utilizes explosive ejection to spread its seeds. It is a mechanism that is both effective and dramatic. The devil’s cucumber’s method of seed dispersal is a remarkable adaptation, helping it to effectively spread its progeny across its environment.
The secret to the plant’s success
Using the mathematical model, the researchers explored the consequences of artificially altering different parameters. This revealed that the seed projection method of the devil’s cucumber has been fine-tuned to ensure near-optimal dispersal and plant success over generations. For instance, by making the stem thicker and stiffer, the seeds were thrown almost horizontally, as the fruit rotated less during discharge. This would cause the seeds to be distributed over a narrower area, with less chance of survival.
Meanwhile, the reduction in the amount of fluid distributed from the fruit to the stem resulted in an over-pressurized fruit, which caused the seeds to be expelled at a higher velocity but with a near-vertical angle of release. Consequently, the seeds would not disperse far enough away from the mother plant, and again, few would survive. Its author, Dr. Chris Thorogood (deputy director and head of Oxford Botanic Garden Science), has stated, “For centuries, people have wondered how and why this extraordinary plant sends its seeds out into the world in such a violent way. Now, as a team of biologists and mathematicians, we have finally begun to unravel this great botanical enigma.”
In the words of Dr. Derek Moulton (professor of applied mathematics at Oxford Mathematical Institute, and co-author of the study), “When we first inspected this plant in the botanical garden, the seed release was so rapid that we were not sure it had actually occurred. It was exciting to investigate and discover the mechanisms of this unique plant.”
According to Finn Box, co-author of the study (Royal Society University Research Fellow, University of Manchester), “This research offers potential applications in bio-inspired engineering and material science, particularly in on-demand drug delivery systems, e.g., micro-capsules that deliver nanoparticles where precise control of rapid and directional release is crucial.”
Ecballium elaterium belongs to the gourd family (Cucurbitaceae), to which melon, pumpkin, and squash belong as well. The species is native to the Mediterranean, where, thanks to its effective seed dispersal strategy, it is often considered a weed. The plant was described by the ancient Greeks and Romans: The naturalist Pliny the Elder (23–24 to 79 AD) wrote, “Unless, in order to prepare it, the cucumber is cut before it is ripe, the seed sprouts, endangering even the eyes.
