Wild petunia passes genes on with a bang
Flower can scatter disc-like seeds in excess of 20 feet.
When it’s time for the hairy flower wild petunia to pass its genes to the next generation, it does it with a bang.
To reproduce, the plant flings tiny seeds from a small torpedo-shape fruit more than 20 feet through the air. That’s not an easy task.
The seeds are discs about a tenth of an inch in diameter — smaller than the circles that fall out of a hole punch — and 1/50th an inch thick, the equivalent of three sheets of paper.
“It’s like throwing confetti,” said 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 this week in the Journal of the Royal Society Inter- face, Whitaker and a trio of undergraduate physics majors worked out what happens in that moment of explosion that launches the seeds so far.
“It’s really cool,” said Sheila Patek, a professor of biology at Duke University who was not involved with the research.
While scientists know that plants use a variety of strate- gies to disperse seeds, Whita- ker and his students “really did an exceptional job of showing how this comes together in this particular plant species,” Patek said.
Whitaker became intrigued with the hairy-flower wild petunia more than five years ago when botanists at Rancho Santa Ana Botanic Garden, less than 1 mile from campus, told him about the plant.
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 outward. 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 more than 30 mph, similar to how the curved scoops used in the sport of jai alai can accel- erate a ball to more than 100 mph.
“It just looks like this gentle, beautiful motion,” Whita- ker said.
When they did the calcula- tions, they were stunned to find that some of the seeds were spinning at a rate of more than 1,600 revolutions a second. The discs were oriented vertically, spinning like a bicycle wheel headed backward. Those at the top of the fruit tended to fly the farthest, while those at the bottom of the fruit tended to wobble, and nicknamed floppers, and land closer to where they started.
Next, they wondered why. Why the vertical orientation? Why backspin? Why do the seeds spin so fast?
For that, they turned to an obsolete piece of 20th century technology: the compact disc. Carolyn Cross, who graduated from Pomona in 2013, cannibalized a boom box and built a disc-launching apparatus.
She also performed more prosaic experiments: flinging CDs down a hallway or off the roof of a campus building.
No matter how she launched the CDs, the spinning discs would invariably tip over to a vertical orientation, always with backspin, not topspin.
The aerodynamics of a flat disc are different from a Frisbee, Whitaker said. The Frisbee’s lip creates a stabilizing turbulence that keeps it flying horizontally (at least when properly thrown).
Eric Cooper, a senior at the college, worked out the mathematics of the seed’s flat disc. The backspin keeps the seed oriented so that it slices through the air with minimal drag.
Lucinda McDade, executive director of the Rancho Santa Ana Botanic Garden, said even the floppers might be part of the plant’s reproductive strategy, taking advantage of a spot that is already known to be fertile in addition to spreading outward.f them have really remarkable ways to move around.”