Blink and you might miss how quickly this squirting cucumber (Ecballium elaterium) ejects its seeds. The mechanics behind this natural marvel have been a tough biological case to crack. Now, with a combination of experiments, sophisticated mathematical modeling, and high-speed cameras, a team from the the University of Oxford and the University of Manchester in the UK have peered behind the surface to see what makes squirting cucumber squirt. Ecballium elaterium uses four key components to successfully disperse its seeds that have been refined over time to ensure the best chance of the seeds’ survival. The findings are detailed in a study published November 25 in the journal Proceedings of the National Academy of Sciences (PNAS).
“For centuries people have asked how and why this extraordinary plant sends its seeds into the world in such a violent way,” Chris Thorogood, study co-author and Oxford Botanic Garden Deputy Director and Head of Science, said in a statement. “Now, as a team of biologists and mathematicians, we’ve finally begun to unravel this great botanical enigma.”
VIDEO: Results of a computed tomography (CT) scan showing the interior organisation of the seeds within the fruit of the squirting cucumber. CREDIT: Elizabeth Evans
What is the squirting cucumber?
Ecballium elaterium is a member of the gourd family Cucurbitaceae which also includes melon, pumpkin, and squash. It is native to the Mediterranean and it is often regarded as a weed due to how well it disperses its seeds and spread. It was even described by the ancient Romans and Greeks. Around 77 CE, Roman author and naturalist Pliny the Elder wrote, “Unless, to prepare it, the cucumber be cut open before it is ripe, the seed spurts out, even endangering the eyes,” in his volumes on natural history.
When it is ripe, the squirting cucumber will detach from the stem and shoot out its seeds in a high-pressure jet of mucus. In only 30 milliseconds, it can send its seeds about 65-feet-per second. How it achieves this feat has stumped scientists for centuries.
“The first time we inspected this plant in the Botanic Garden, the seed launch was so fast that we weren’t sure that it had actually happened,” study co-author and University of Oxford applied mathematician Derek Moulton said in a statement.
Under pressure
In this new study, a team from Oxford and the University of Manchester conducted several experiments on Ecballium specimens grown at the University of Oxford Botanic Garden.
They filmed the seed dispersal using a high-speed camera, which captured up to 8,600 frames per second. They then measured the fruit and stem volume both before and after dispersal, performed indentation tests, took CT scans of an intact cucumber, and monitored the fruit with time-lapse photography in the days leading up to the seed launch.
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From this data, they developed a suite of mathematical models to describe the mechanics of the pressurized fruit, the stem, and the ballistic trajectories of the seeds. The combined approach helped them pinpoint the key components of the squirting cucumber’s dispersal strategy–a pressurized system, fluid redistribution, rapid recoil, and variable launch.
In the weeks leading up to the seed dispersal, a mucilaginous fluid builds up and forms a pressurized system like an air compressor or SCUBA tank. This pressure is what helps the seeds release at such a fast pace.
Then, during the days before the dispersal, some of this fluid will be redistributed from the fruit to stem. This makes the stem longer, thicker, and stiffer. When the fluid is redistributed, the fruit rotates from being almost vertical to a roughly 45 degree angle. This acute angle is a key element needed for successful seed launch because it balances out the vertical and horizontal elements of the plant.
During only the first hundreds of microseconds of the ejection, the tip of the stem then recoils away from the fruit. This lightning-fast recoil rotates the fruit in the opposite direction.
The seeds are all ejected with a speed and launch angle that depends on the order they are released. A seed’s exit speed decreases because the pressure of the emptying fruit capsule decreases. At the same time, the launch angle increases because of the fruit’s rotation. So the earliest seeds reach the furthest distance and the following seeds land closer to the plant. Having such a wide area of dispersal ensures that more seeds have a chance to survive.
While multiple fruits are distributed around the center of the plant, a wide and nearly uniform distribution of seeds covers a ring-shaped area roughly 6.5 and 32 feet away from the mother plant.
When used together, these elements make up a sophisticated seed dispersal system. The redistribution of fluid from the fruit back into the stem is believed to be unique within the plant kingdom.
All for the seeds
Once the team developed the mathematical model of this dispersal process, they explored what would happen if they altered different parts of the process. They found that the seed projection method of the squirting cucumber has been refined to ensure a near-optimal seed dispersal and the success of the plant over subsequent generations.
For example, making the stem thicker and stiffer caused the seeds to be launched almost horizontally, because the fruit would rotate less during the discharge. These seeds would then be distributed over a more narrow area, with fewer likely to survive.
Meanwhile, reducing the amount of fluid that is redistributed from the fruit to the stem resulted in an over-pressurized fruit. This ejected the seed at a higher speed, but at a nearly vertical launch angle. With this combination, the seeds would not be dispersed far enough away from the parent plant, so few would survive.
In addition to cracking a biological mystery, outlining this process could have future applications in human medicine.
“This research offers potential applications in bio-inspired engineering and material science, particularly on-demand drug delivery systems, for instance microcapsules that eject nanoparticles where precise control of rapid, directional release is crucial,” study co-author and University of Manchester physicist Finn Box said in a statement.
