A group of researchers, spearheaded by the University of Oxford, has finally unveiled the fascinating mechanism behind the squirting cucumber’s explosive seed dispersal. The study, which employed a mix of experimental testing, high-speed videography, image analysis, and intricate mathematical modeling, was released on November 25th in The Proceedings of the National Academy of Sciences (PNAS).
The squirting cucumber, more formally known as Ecballium elaterium (borrowed from the Greek ‘ekballein’, meaning ‘to throw out’), is known for its unique seed dispersal technique. When ripe, the elliptical fruits detach from the stem, expelling the seeds in a high-pressure, sticky jet. This rapid ejection, lasting a mere 30 milliseconds, propels the seeds at speeds up to 20 meters per second, landing them as far as 10 meters away, or 250 times the length of the fruit.
Despite its fascinating method of seed distribution, the inner workings of the squirting cucumber’s system were not well-understood until now. The research team, consisting of experts from the University of Oxford and the University of Manchester, conducted a series of experiments on Ecballium specimens cultivated in the University of Oxford Botanic Garden.
The team employed several techniques to study the plant, including high-speed videography (capturing up to 8600 frames per second), volume measurements before and after seed dispersal, indentation tests, CT scans, and time-lapse photography. These observations were then used to develop mathematical models that describe the mechanics of the fruit’s pressure system, the role of the stem, and the trajectory of the seeds.
The findings revealed the key elements of the plant’s dispersal strategy, providing insight into a sophisticated seed dispersal system, and suggesting a unique fluid redistribution system within the plant kingdom.
Using their mathematical models, the researchers simulated the effects of modifying various parameters. They found that the squirting cucumber’s seed dispersal mechanism has been finely calibrated over generations to ensure optimal seed dispersal and survival.
For instance, altering the stem to be thicker and less flexible resulted in seeds being launched almost horizontally, covering a smaller area and reducing the likelihood of survival. Similarly, reducing the fluid redistribution from fruit to stem created an over-pressurized fruit, which ejected seeds at higher speeds but at a nearly vertical angle, limiting their dispersal range and survival.
Dr. Chris Thorogood, co-author of the study and Deputy Director and Head of Science at Oxford Botanic Garden, commented on the significance of these findings. “For centuries, this plant’s violent seed dispersal method has puzzled observers. Now, our interdisciplinary team of biologists and mathematicians has begun to solve this botanical mystery.”
Co-author Dr. Derek Moulton, a Professor of Applied Mathematics at the Oxford Mathematical Institute, shared the excitement of their discovery. “Initially, the seed launch was so swift that we doubted our observation. It was thrilling to delve deeper and uncover the unique mechanism of this plant.”
Dr. Finn Box, co-author and Royal Society University Research Fellow at the University of Manchester, noted that the research could have implications beyond botany, potentially influencing bio-inspired engineering and the development of drug delivery systems.
The squirting cucumber is a member of the gourd family (Cucurbitaceae), which includes melons, pumpkins, squashes, and courgettes. Native to the Mediterranean, it has been regarded as a weed, thanks to its effective seed dispersal technique. The plant, known to the ancient Greeks and Romans, was described by naturalist Pliny the Elder, who warned about the potential danger from the explosively ejected seeds.
