One of the great mysteries of the botanical world involves a familiar component found in many of our salads: cucumbers. Except the specific cucumber species in question has a unique way of dispersing its seeds—in a ballistic jet of mucus.
An interdisciplinary team of scientists led by the University of Oxford has shed light on the appropriately named squirting cucumber’s aggressive and poorly understood seed dispersal method. A study published on November 25 in the journal PNAS suggests the findings could inspire bio-engineering innovations, like mechanisms for the precise release of medication. It also explains how it’s even possible for a cucumber to launch its seeds a distance 250 times the length of its body.
“For centuries people have asked how and why this extraordinary plant sends its seeds into the world in such a violent way. Now, as a team of biologists and mathematicians, we’ve finally begun to unravel this great botanical enigma,” Chris Thorogood of the Oxford Botanic Garden, a scientist involved with the study, said in a university statement.
The squirting cucumber, a Mediterranean gourd, has been known since antiquity. Pliny the Elder (23/24 CE to 79 CE) once reflected on his experience with the plant, saying: “Unless, to prepare it, the cucumber be cut open before it is ripe, the seed spurts out, even endangering the eyes.”
The plant is hairy, grows yellow flowers, and its fruit—the cucumbers—grow up to a couple of inches in length. When they’re ripe, they shoot off the tops of their thick stems and eject an exceptionally fast jet of seed-filled mucus. The seeds can reach speeds of up to 65.6 feet (20 meters) per second and land up to 32.8 feet (10 meters) away. No wonder Pliny feared for his eyes.
To study this mechanism more closely, scientists from the University of Oxford and the University of Manchester used a series of experiments, mathematical modeling, high-speed videography, CT scanning, and photography.
Here’s what they learned: In the weeks before the seed ejection, the cucumber builds up fluid and becomes highly pressurized. Days before the event, some of that mucus goes from the fruit to the stem, tilting the cucumber about 45 degrees from its previous, almost vertical position, making the stem longer, thicker, and stiffer. When the cucumber and stem finally burst apart, the stem tip jerks away, causing the cucumber to rotate in the opposite direction.
“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. It was very exciting to dig in and uncover the mechanism of this unique plant,” said Derek Moulton of the University of Oxford, who also worked on the study.
The seeds’ direction and speed depend on their timing. For example, seeds that exit the fruit first are the fastest, have the lowest launch angle, and land the farthest. The seeds that follow are slower (because of the cucumber’s now-decreased pressure), have a higher launch angle (because of the cucumber’s rotation), and therefore land closer.
In this way, a single squirting cucumber plant with multiple fruits can distribute its seeds in an almost even ring between 6.6 feet (2 meters) and 32.8 feet (10 meters) away. While the entire mechanism is surprising, the step that involves passing fluid from the fruit back to the stem might be “unique within the plant kingdom,” the researchers wrote in the statement.
The team also developed mathematical models to simulate this mechanism and test how changing the parameters might impact the outcome. It turns out, however, that the seed dispersal system is already near-optimal.
If the stem was thicker and stiffer, the cucumber would rotate less and the seeds would launch horizontally. If the stem received less fluid before the ejection, the fruit would have higher pressure and shoot the seeds out faster but more vertically. Both scenarios limit the dispersal range, meaning that fewer seeds would be likely to survive because of factors including increased competition for resources.
While the explanation behind strange plant behaviors (like why this flower smells like rotting flesh) might not have obvious applications for daily life, perhaps we’ll soon discover more writings by Pliny the Elder revealing that the Roman army incorporated the squirting cucumber’s ballistic seeds as a special legionary weapon.
