New Study: Incredible Backspin Is Secret to Wild Petunia Species' Launch of High-Speed, Disc-Shaped Seeds

Hairyflower Wild Petunia

Amazing backspin allows the Hairyflower Wild Petunia to launch its seeds at high speeds, reaching distances of up to 23 feet through the air, new research from Pomona College shows.

Using high-speed video analysis, Pomona College researchers have discovered that Ruellia ciliatiflora explosively launches its seeds with a rotation rate unmatched in the plant world, according to a paper published today (March 7) in the Journal of the Royal Society Interface. 

“This rotation rate is the fastest of any projectile launched in nature and matches the fastest rotations in the natural world,” writes Pomona College Physics Professor Dwight Whitaker in the paper co-authored with undergraduate researchers Eric Cooper ’18, Carolyn Cross ’13 and Molly Mosher ’18. “By staying streamlined during flight, the seeds are able to travel twice as far as a seed launched without spin, thus increasing the seed dispersal range for this plant.”

Seed dispersal is a key evolutionary adaptation for plants. While some plants rely on animals, water or the wind to spread speeds far and wide, the Hairyflower Wild Petunia’s seeds explosively launch with streamlined efficiency. The seeds travel with a backspin of up to 100,000 r.p.m., which gyroscopically stabilizes them in their most streamlined orientation. 

Professor Whitaker studies rapid biological movements, working to understand how the morphology and physical properties of a plant enable quick movement. Learning about the explosive qualities of the Acanthaceae family while visiting the near-to-campus Rancho Santa Ana Botanic Garden in Claremont, Calif., Whitaker and his students soon went to work studying and videoing the explosive fruits collected from the greenhouse, where Ruellia ciliatiflora or Hairyflower Wild Petunia, was cultivated. (As noted in the paper, Ruellia ciliatiflora is a perennial herb native to South America but today the plant is also found in the Southeastern U.S.)

Slowing down the video to less than 1/600th of the actual speed, the team looked closely, frame by frame, at how the seeds launch through the air. They began to theorize about the disc-shaped seeds, aerodynamics and drag. “It took us a long time to understand the significance of the rotation rates,” says Whitaker.

Professor Whitaker and his undergraduate researchers eventually came to realize that, instead of moving with the topspin or horizontal spin of a Frisbee, successfully-fired seeds relied on backspin to stay stable and avoid wobble as the seeds shot through the air at speeds up to 22 m.p.h. 

In fact, Whitaker and Cross began the project in 2012, and the gyroscopic theorizing in the early days at times involved tossing old CDs down empty hallways; at one point Cross even built a machine to shoot out CDs and study their paths. Cooper and Mosher later joined the team, and high-school students in the summer Pomona Academy for Youth Success (PAYS) participated as well.

Questions for further study remain. While sending seeds far and wide would seem to yield an obvious evolutionary advantage, Whitaker notes that other plants have found ways to take their seeds even farther, whether on the wind or dispersed by animals. Why does this particular species depend so much on perfect backspin? Next, he will be studying a number of closely-related species to learn more. “Stay tuned,” Whitaker says.