The hairy, ground-hugging vines of the squirting cucumber Ecballium elaterium might seem like an ordinary weedy plant. But take a closer look at the plant, which grows in dry, barren areas in the Mediterranean, and you’ll find “an extraordinary thing,” said Chris Thorogood, a botanist at the University of Oxford Botanic Garden and Arboretum.
The roughly 1.5-inch fruits of the squirting cucumber contain toxic chemicals and are distinguished in another way from the green rounds you might put on a salad. “When they’re ripe, they eject their seeds very violently in a stream of mucilage,” Dr. Thorogood said. They can shoot up to almost 40 feet.
The Roman naturalist Pliny the Elder wrote about these plants around 2,000 years ago. It’s long been a mystery why and how the plants can carry out this act of vegetative bombardment, Dr. Thorogood said.
In a paper published Monday in the Proceedings of the National Academy of Sciences, Dr. Thorogood and his colleagues share how the squirting cucumber goes ballistic.
Researchers knew that pressure builds up inside the fruits before they blow. “Initially we all thought it was rather straightforward,” said Finn Box, a physicist at the University of Manchester and another author of the study. But they decided to take a closer look, recording high-speed video of the cucumber cannons in action.
The ripe fruit detaches from the stalk, opening a hole in the cuke’s base. From there, the researchers found, seeds emerge at speeds of up to 66 feet per second. As the fruit detaches, the stem recoils. The fruit rotates backward, changing the angle at which seeds shoot. As slimy fluid spews out, depressurizing the fruit, the seeds’ speed drops. The whole process takes about 30 milliseconds.
The group also made time-lapse photos of stalks bearing fruit for several days. In the run-up to a seed-squirting event, the ovoid fruits moved from dangling nearly straight down to a roughly 45-degree angle with the stem. “That was just something that blew our minds,” Dr. Box said.
“There’s a perfect angle and an organism finds it,” said Marcus Roper, a mathematician at the University of California, Los Angeles, who wasn’t involved with the work. If the seeds shot out nearly vertically, they wouldn’t have enough forward velocity to go far. If spit nearly horizontally, they quickly fall to the ground. But the squirting cucumber lines up along the sweet spot, an intermediate angle.
The team’s time-lapse imagery provided clues about how this occurs. Before the launch, the plant’s stem becomes wider, straighter and longer. The cucumber appears to contract. That suggests that liquid moves from fruit to stem, stiffening the stem so it can hoist the fruit. It’s still not clear what biological mechanism the plant might use to bulk up its stem.
The researchers also calculated the pressure within the cucumber and found that it’s similar to that of an inflated mountain bike tire, Dr. Box said. Using that and other data they collected on the plant, the researchers worked out the math behind the dispersal of seeds. That allowed them to simulate how hypothetical mutant plants would spread differently.
For instance, a stiffer stem would disperse most seeds very near the parent plant. Alternatively, a more pressurized fruit would shoot seeds forward and backward. Over several generations, these mutants wouldn’t spread as far as the squirting cucumber that occurs naturally, the team found.
“It gave us a sense of how fine-tuned evolution has made this particular species for dispersal,” said Derek Moulton, an applied mathematician at the University of Oxford and another author of the research.
The squirting cucumber shows how plants evolve ways to spread their seeds while being rooted. “For plants to generate rapid motion, they have to do something a bit clever,” Dr. Moulton said.
Finding the physical basis of “weird biological things” can be useful, said Angela Hay, a plant biologist at the Max Planck Institute for Plant Breeding Research in Cologne, Germany, who wasn’t part of the new study. The squirting cucumber has already inspired at least one strategy for delivering drugs within the body. Because of the process and time scale of evolution, she said, it can often “come up with design principles that, as engineers, we haven’t thought of.”
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