Blink and you might miss how fast this is squirting cucumber (Ecballium elaterium) sheds its seeds. The mechanisms behind this natural wonder were biologically difficult to crack. Now, using a combination of experiments, advanced mathematical models and high-speed cameras, a team from the University of Oxford and the University of Manchester in Britain has looked behind the surface to see what makes squirting cucumbers squirt. Ecballium elaterium uses four key components to successfully disperse the seeds that have been refined over time to ensure the best chances of seed survival. The findings are detailed in a study published Nov. 25 in the journal Proceedings of the National Academy of Sciences (PNAS).
“For centuries, people have wondered how and why this extraordinary plant sends its seeds into the world in such a violent way,” said Chris Thorogood, co-author of the study and deputy director and chief science officer at Oxford Botanic Garden, said in a statement. “Now, as a team of biologists and mathematicians, we have finally begun to unravel this great botanical mystery.”
Results of a computed tomography (CT) scan showing the internal organization of the seeds in the fruit of the spurting cucumber. CREDIT: Elizabeth Evans
VIDEO: Results of a computed tomography (CT) scan showing the internal organization of the seeds in the fruit of the spurting cucumber. CREDIT: Elizabeth Evans
What is the spray cucumber?
Ecballium elaterium is a member of the gourd family Cucurbitaceae which also includes melon, pumpkin and pumpkin. It is native to the Mediterranean region and is often considered a weed due to the good distribution and dispersal of its seeds. It was even described by the ancient Romans and Greeks. Around 77 AD, The Roman author and naturalist Pliny the Elder wrote“Unless, in order to prepare it, the cucumber is cut open before it is ripe, and the seed squirts out, and even endangers the eyes,” in his volumes on natural history.
When ripe, the spurting cucumber will detach from the stem and shoot out its seeds in a high-pressure jet of slime. In just 30 millisecondsit can send its seeds about 20 meters per second. How it achieves this feat has baffled scientists for centuries.
“The first time we inspected this plant in the Botanic Garden, the seed launch was so rapid that we were not sure it had actually happened,” study co-author and University of Oxford applied mathematician Derek Moulton. said in a statement.
Under pressure
In this new studya team from Oxford and the University of Manchester conducted several experiments Ecballium specimens grown in the Botanic Garden of the University of Oxford.
They filmed the seed dispersal with a high-speed camera, capturing up to 8,600 frames per second. They then measured fruit and stem volume both before and after dispersal, performed notch tests, acquired CT scans of an intact cucumber, and monitored the fruit with time-lapse photography in the days leading up to seed launch.
[Related: Watch ‘tiny tornadoes’ spread plant pathogens.]
Based on this data, they developed a series of mathematical models to understand the functioning of the pressurized fruit, the stem and the ballistic trajectories of the seeds. The combined approach helped them identify the problems key components of the spreading strategy of the spray cucumber–a pressurized system, fluid redistribution, rapid recoil and variable launch.
In the weeks leading up to seed dispersal, a mucus-like fluid builds up to form a pressurized system, such as an air compressor or scuba tank. This pressure ensures that the seeds are released so quickly.
Then, during the days before propagation, some of this fluid will be redistributed from the fruit to the stem. This makes the stem longer, thicker and stiffer. When the liquid is redistributed, the fruit rotates from nearly vertical to an angle of about 45 degrees. This acute angle is an important element necessary for successful seed launch, as it balances the vertical and horizontal elements of the plant.
Only during the first few hundred microseconds of ejection does the tip of the stem recoil from the fruit. This lightning-fast recoil causes the fruit to spin in the opposite direction.
The seeds are all ejected at a speed and launch angle that depends on the order in which they are released. The exit velocity of a seed decreases because the pressure of the deflating fruit capsule decreases. At the same time, the launch angle increases due to the rotation of the fruit. So the earliest seeds reach the greatest distance and the next seeds land closer to the plant. Having such a large distribution area ensures that more seeds have a chance to survive.
While several fruits are distributed around the center of the plant, a wide and nearly uniform distribution of seeds covers one annular area approximately 2.5 and 32 meters away of the mother plant.
When used together, these elements form a sophisticated seed dispersal system. The redistribution of moisture from the fruit back to the stem takes place is believed to be unique within the plant kingdom.
Everything for the seeds
Once the team developed the mathematical model of this dissemination process, they investigated what would happen if they changed different parts of the process. They found that the spray cucumber’s seed projection method has been refined to ensure near-optimal seed dispersal and the plant’s success over subsequent generations.
For example, by making the stem thicker and stiffer, the seeds were launched almost horizontally, because the fruit would rotate less during release. These seeds would then be distributed over a narrower areawith less chance of survival.
Meanwhile, reducing the amount of fluid redistributed from the fruit to the stem resulted in a fruit that was overpressured. This ejected the seed at a higher velocity, but at a nearly vertical launch angle. With this combination the seeds would are not spread far enough of the parent plant, so few would survive.
In addition to unraveling a biological mystery, outlining this process could have future applications in human medicine.
“This research offers potential applications in bioinspired engineering and materials science, especially on-demand drug delivery systems, for example microcapsules that eject nanoparticles where precise control of rapid, targeted release is crucial,” study co-author and physicist at the University of Manchester. FinnBox said in a statement.
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