Humans have known about the squirting cucumber plant for over 2,000 years. Ancient Roman naturalist Pliny the Elder warned that the seeds of this plant can often suddenly burst out, even posing an injury risk for the eyes. Pliny was right about one thing: the seeds really do burst out, but it wasn’t clear how. In the 19th century, naturalists tried to further explain this mechanism but didn’t really succeed. Now, researchers have finally elucidated the unique seed dispersal mechanism of the squirting cucumber (Ecballium elaterium).

The key element is pressure.

As the fruit grows, internal pressure continues to build inside it. This pressure is highest when the fruit is fully ripe. At this stage, the fruit detaches from the stem and shoots multiple seeds in a ballistic fashion at incredible speeds — around 20 meters/second, which is equivalent to a car traveling at 72 km/h (or 45 mph).

Within 30 milliseconds, the fruit has shot seeds tens of meters away from the original plant. This mechanism ensures higher chances of survival as closely located cucumber plants compete for resources like sunlight, water, and nutrients. 

But how do you study this in more detail? There’s no way to open the ripe fruit and study its pressure dynamics without causing it to burst. Studying it is further complicated because the squirting action occurs at lightning speed. That’s why understanding the science behind this mechanism has been so challenging. Now, a series of clever experiments conducted by researchers at the University of Oxford and the University of Manchester have solved that problem. 

Studying the speedy plant

“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,” Derek Moulton, one of the researchers and a professor at the Oxford Mathematics Institute, said.

The approach Moulton and his team employed to understand the squirting action of E. elaterium plants grown at Oxford University involved a combination of advanced tools and technologies. For instance, they recorded the seed dispersion action with a high-speed camera capable of taking 8,600 frames per second. 

Next, they performed CT scans and indentation tests to study what was going on inside the fruit and the changes it went through that eventually led to dispersion. Finally, they developed some mathematical models to have a clear understanding of the pressure dynamics of the fruit and the motion of the ejected seeds. When the results from all these tools and methods were combined, they uncovered a full picture of what actually makes the squirting cucumber squirt.

The step-by-step squirting action

The researchers noticed the occurrence and increase in the concentration of a mucilaginous fluid inside the fruit weeks before the dispersal event. They suggest that this fluid is responsible for creating high-pressure conditions within the fruit. The mucilaginous fluid gradually fills the fruit and stem, causing the stem to become stiff and the fruit to rotate at 45 degrees from its original position.  

“In the first hundreds of microseconds of ejection, the tip of the stem recoils away from the fruit, causing the fruit to counter-rotate in the opposite direction,” the study authors note.

Finally, as the seeds are ejected explosively, their speed decreases due to the gradual drop in internal pressure, while their launch angles increase. This causes the initial seeds to travel the farthest, with later seeds landing closer to the original plant.

“As multiple fruits are distributed around the center of the plant, the overall result is a wide and nearly uniform distribution of seeds covering a ring-shaped area at a distance of between 2 and 10 m from the mother plant,” they added. 

The significance of this rare behavior

Seed squirting is this plant’s specialty, but why have scientists been so keen to understand this mechanism in detail? Well, this is because insights from this unique seed dispersal method could be useful in various fields, especially where controlled, rapid release, and precision targeting are essential. 

For instance, drones, underwater vehicles, and fluid-based propulsion systems could use similar principles (that govern the squirting action) for controlled bursts of energy to move or change direction swiftly in different environments.

Similarly, a clear understanding of the fruit’s internal pressure dynamics can lead to the creation of materials that expand, store energy, and release it in controlled bursts. These could be used in soft robotics, where materials replicate biological movements for delicate tasks.

The cucumber’s controlled ejection process, on the other hand, can lead to the development of agricultural tools that could disperse materials such as fertilizers evenly over a specified area, reducing waste and improving crop yields. Moreover, inspired by the pressure-driven seed ejection of the cucumber, scientists could also design tiny capsules and robots capable of releasing medicine directly to a targeted area in the body. 

‘This research offers potential applications in bio-inspired engineering and material science, particularly on-demand drug delivery systems,” Finn Box, lead study author, and a research fellow at the University of Manchester, said.

The study is published in the journal PNAS.