Somewhere in the wetlands of South Carolina, a buzzing fly alights on a rosy-pink surface. As the fly explores the strange scenery, it unknowingly brushes a small hair sticking up like a slender sword. Strolling along, the fly accidentally grazes another hair. Suddenly, the pink surface closes in from both sides, snapping shut like a pair of ravenous jaws. The blur of movement lasts only a tenth of a second, but the fly is trapped forever.
“We don’t think plants move at all, yet they can move so fast you can’t catch them with the naked eye,” says Joan Edwards, a botanist at Williams College in Williamstown, Mass.
We tend to picture plants as static life-forms rooted in place until they die. To describe something boring, we say it’s “like watching grass grow.” But this is a stale view of plant life.
All plants grow, a rather slow form of motion, but many can also move rapidly. The snapping jaws of the Venus flytrap (Dionaea muscipula) are the most famous example, but far from the only one. The botanical world offers plenty of equally impressive feats. The explosive sandbox tree (Hura crepitans), also known as the dynamite tree, can launch seeds far enough to cross an Olympic-sized swimming pool; sundews (genus Drosera) have sticky tendrils that curl around prey; and the touch-me-not (Mimosa pudica) folds in its compound leaves within seconds of a touch.
“Plants have evolved a number of different approaches and mechanisms for movement,” Edwards says. This variety has resulted in a huge spectrum of plant speed, from the crawl of roots (1 millimeter per hour) to the explosive launch of seeds (tens of meters per second).
TAKEN FOR A SPIN Watch as the fruit pod of the hairyflower wild petunia explosively splits open, sending its seeds spinning. Moisture usually triggers this action, but in this experiment, a pinch worked as well. Some seeds have a rapid backspin that gives them stable flight, while other seeds, known as floppers, are unstable and don’t fly as far.
The most dynamic plant movements have long entranced researchers.
Fascinated with the Venus flytrap’s fast, forceful snap, Charles Darwin called the plant “one of the most wonderful in the world.” He performed all manner of flytrap-focused experiments, described in his 1875 book Insectivorous Plants. Darwin baited the plants with raw meat, prodded them with objects as fine as human hairs and even tested how the plants’ traps reacted to drops of chloroform. Although Darwin didn’t fully unlock the flytrap’s secrets, he understood that its speed had to do with the geometry of its leaves.
Modern research on rapid plant movement has precision that Darwin would envy. A little over a decade ago, scientists began using high-speed digital cameras and computer modeling to get a new view on plant motion. Frame-by-frame analyses, along with improved resolution, at long last offered a detailed look at the mechanisms that give plants their speed.
Most recently, evidence points to the existence of a startling variety of these mechanisms. In the last few years alone, researchers have discovered contraptions that kick like a soccer player, throw like a lacrosse player and even generate heat to launch seeds explosively.
Nearly 150 years after Darwin’s work, the impetus for such research remains the same — a fascination with the movement of plants.
Moving without muscles
Yoël Forterre was a postdoc at Harvard University in the early 2000s when his adviser was given a Venus flytrap as a gift. Never having seen the plant before, Forterre was amazed at its ability to move without muscles. He soon realized that the motion could be understood through the lens of his own specialty: soft matter physics, a field concerned with the mechanics of deformable materials like liquids, foams and some biological tissues.
Forterre published a study in 2005 in Nature that was among the first to leverage both high-speed cameras and computer modeling to study mechanisms of rapid plant movement (SN: 1/29/05, p. 69).
“The big transformation was digital high-speed cameras,” says Dwight Whitaker, an experimental physicist at Pomona College in Claremont, Calif. Around this time, the cameras were making their way into academic labs. “With film, you get one chance,” he says. Everything has to be arranged in advance, “which is why directors need to say ‘lights, camera, action!’ in that order.”
With the new technology, Forterre and colleagues could track the tiniest changes in the curvature of the flytrap’s leaves, which face each other like two halves of a book. This allowed the team to see how the plant’s speed relies on the special geometry of those leaves. When the trap is triggered by a fly or other wayward prey, cells on the green outer surfaces of the leaves expand while the pink inner surfaces don’t. This creates a tension as the outer surface pushes inward. Eventually, the pressure becomes too great and the leaves, originally convex in shape, rapidly flip to concave, slamming the trap shut in a process known as snap-buckling.
One way of understanding this elastic motion is to look at a popular children’s toy, says Zi Chen, an engineer at Dartmouth College who also studies the flytrap. Rubber poppers are little rubber hemispheres that can be inverted. Like a compressed spring, the inverted toys have a lot of potential energy. The poppers convert that energy into kinetic energy as they revert to their original shape, launching several feet into the air. Similarly, potential energy from the tension of the outer surfaces against the inner surfaces of a flytrap’s leaves is converted to kinetic energy, allowing the trap to slam shut in about a tenth of a second.
NUCLEAR DISPERSAL Captured here, the peat moss Sphagnum affine explodes into a mushroom cloud that carries spores 20 times higher than they would otherwise go.
Around the same time Forterre was scrutinizing flytraps, Edwards and her husband were at Lake Superior’s Isle Royale, leading a group of budding researchers doing fieldwork on native plants.
As Edwards tells it, a student stuck her head down to sniff a flower of the bunchberry dogwood (Cornus canadensis) and announced that “something went poof.” Intrigued by this distraction, the team brought specimens back to the lab to capture the behavior on video camera. But whatever triggered the dogwood poof wasn’t visible. So Edwards upgraded to a 1,000-frames-per-second camera.
“It was still blurry, so I thought something was wrong with the camera,” she says.
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