Imma Perfetto
Cosmos science journalist
Squirting cucumber explosive seed dispersal (over 300x slowed down). Credit: Helen Gorges (CC BY-NC-ND)
The squirting cucumber (Ecballium elaterium) ejects high-speed explosions from its tip, spreading seeds far and wide.
I swear I’m not being appropriate. These are real plants that exist on Earth.
“Many factors have to interact perfectly to disperse the seeds in the most efficient way, while not destroying the whole plant too early,” says Helen Gorges, a PhD student at the Department of Functional Morphology and Biomechanics in the Zoological Institute of Kiel University, Germany.
Plants have evolved a multitude of mechanisms to disperse their seeds as far as possible. This reduces direct competition for space and resources between parent and offspring.
Some species’ seeds float on the wind, others disperse via water. Animals may also be recruited to courier fruits and seeds in their fur or stomachs to deposit them elsewhere.
Some plants take a self-propelled approach: ballistic dispersal.
Gorges and her colleagues captured every moment of the squirting cucumber’s explosive event with high-speed cameras.
“We recorded the explosion of the fruit with high-speed videos at 1,000 frames per second (fps) and 10,000 fps to calculate the speed of the seeds and the possible shooting distances,” says Gorges.
They used microcomputed tomography (micro-CT) to create 3D models of the fruit at rest and during ejections.
“We also analysed pictures during the ripening of the fruits to measure the curvature of the fruit stem and the angle between fruit and stem,” says Gorges.
The team did this to explore the mechanisms which control the ripening process and maximise the fruit’s chances of successfully dispersing seeds.
Squirting cucumber explosive seed dispersal in real time and slowed down. Credit: Helen Gorges (CC BY-NC-ND)
As the oval-shaped fruit ripens “turgor pressure” builds. Eventually, it explodes – detaching from the stem and shooting out seeds one by one.
“It’s super interesting to watch the explosions through high-speed recordings, as the explosions happen way too fast to see anything in real-time!” says Gorges, who found that the seeds shot from the end of the cucumber reach speeds of up to about 46km/h (29 miles/h) and distances of up to 12m.
This smooth exit is made possible by the fruit’s mucus – a thick, gluey liquid produced by the plant. Gorges and her team found that this mucilage becomes an adhesive when it dries and improves conditions for germination.
Experiments also revealed the cucumber’s stem straightens up to an average angle of 53° during ripening, close to the 50° angle which would theoretically maximise the distance travelled by the seeds.
The researchers say their findings could have applications in bio-inspired launching systems for medical tools and microrobots.
“There are also many applications in soft robotics, drug delivery systems and similar devices where energy-efficient launching systems are desired,” says Gorges.
The findings were presented at the Society for Experimental Biology Annual Conference in Antwerp, Belgium on July 8.