Pomona Students Discover Record-Breaking Physics Of Petunia’s Explosive Seed Dispersal

Leaves of ruellia ciliatiflora, commonly known as the hairyflower wild petunia.
The explosive launch mechanism of the seeds of Ruellia ciliatiflora, commonly known as the hairyflower wild petunia, is the focus of Pomona College Physics Professor Dwight Whitaker and various Pomona students’ study published last month. (Photo courtesy of Pomona College)

A flower species’ seeds are breaking records in physics, blasting out as rapidly spinning projectiles that travel up to 20 feet, and Pomona College discovered it first.

A study published last month by Pomona students and physics professor Dwight Whitaker attracted national attention, including from The New York Times, when it revealed that the seeds of the Ruellia ciliatiflora — commonly known as the hairyflower wild petunia — rotate at the record-breaking rate of 1,660 times per second.

Though the seeds are only a tenth of an inch across, the plant’s backspin and vertical orientation generated by the launch mechanism contribute to the striking phenomenon.

“This rotation rate is the highest-reported rotation rate for any biological projectile,” said Eric Cooper PO ’18, a student who worked on the project.

Whitaker began researching the physics behind ruellia seeds at Rancho Santa Ana Botanic Garden in 2012.

“We started looking at them in the lab, and we realized that they went really fast, and they blew up, and they were kind of beautiful-looking,” Whitaker said, “but it took us a long time to realize what was actually scientifically interesting about them.”

Carolyn Cross PO ’13, Molly Mosher PO ’18, and Cooper, some of Whitaker’s physics students, joined the project soon after.

To understand the fluid dynamics of the disc-shaped seeds, Cross and Whitaker threw CDs down hallways, off the tops of buildings, and from a CD launcher hand-built by Cross. They observed that the CDs flew more stably when thrown with the same characteristics as ruellia seeds: oriented vertically and thrown with a backspin.

Cooper and Whitaker worked out the equation of motion for rotating discs, proving that they will naturally assume a vertical orientation. This minimizes air resistance and allows the discs, or seeds, to travel farther.

The team then began to record the seeds being launched to analyze them in slow-motion.

To the team’s excitement, The New York Times reported on the study March 9.

“I didn’t even realize, when we were doing the research, that that was the magnitude of what our results would be,” Mosher said. “I didn’t realize that it would be that impactful.”

Cross said the attention the study received reassured her that the public is interested in science.

“It sort of reminds me that … sciences and physics and biology are really interesting to everyone out there,” she said. “That’s where I get really proud — bringing science to everyday people and inspiring them to look at the world around them.”

Whitaker said there are still important questions left unanswered, particularly related to the biological and ecological aspects of this phenomenon.

“It’s great that we can measure all these aerodynamic properties of seeds, but the scientific questions we’re trying to answer are ‘why does the seed do this [and] what’s the purpose of exploding and sending your seeds far?’” he said. “I won’t pretend that we’ve answered that question at all yet.”

To address this question, Whitaker and Cooper are currently working on a comparative study of other plants in the acanthus family in collaboration with Erin Tripp, a professor of evolutionary biology at the University of Colorado, Boulder.

In the comparative study, they will pursue the environmental applications of the first study. “It will be one of the first times we’ll be able to quantify seed dispersal,” Cooper said.

Whitaker said they will also look into evolutionary applications of acanthus plants’ ballistic seed launching.

“If nature’s done it a bunch of times, there must me some good reason for it,” he said.

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