High-flying physics for a wild petunia
When it’s time for the hairyflower wild petunia to pass its genes to the next generation, it does it with a bang.
To reproduce, the plants flings tiny seeds from a small torpedo-shaped fruit more than 20 feet through the air. That’s not an easy task.
The seeds are discs about a 10th of an inch in diameter – smaller than the circles that fall out of a hole punch – and 1/50th of an inch thick, the equivalent of three sheets of paper.
“It’s like throwing confetti,” says Dwight Whitaker, a professor of physics at Pomona College in Claremont, California.
But somehow these seeds slice smoothly through the air.
In an article published recently in the Journal of the Royal Society Interface, Whitaker and a trio of undergraduate physics majors worked out what happens in that moment of explosion that launches the seeds so far.
The seeds sit within a small fruit that is a bit over 1 inch long. A spine along each half of the fruit is made of three layers, which shrink at different rates as they dry. That creates a strain that bends them outwards. The two halves remain held together by glue.
Drip some water onto it, the glue dissolves and the fruit violently splits in half.
With ultrahigh speed video – up to 20,000 frames a second – Whitaker and his students slowed down the action, watching as hooks in the fruit accelerated the seeds to speeds of more than 30mph, similar to how the curved scoops used in the sport of jai alai can accelerate a ball to more than 100 mph.
“It just looks like this gentle, beautiful motion,” Whitaker says.
When they did the calculations, they were stunned to find that some of the seeds were spinning at a rate of more than 1,600 revolutions a second. Those at the top of the fruit tended to fly the farthest, while those at the bottom of the fruit tended to wobble and land closer to where they started.