Meet the speedsters of the plant world

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).
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.
Blasting off
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.

She brought the problem to Whitaker, who was then at Williams. It turned out the plant was moving too quickly for the camera to capture. Edwards ordered a special 10,000-frames-per-second camera — then top of the line — and for the first time saw the mechanism clearly (SN: 6/11/05, p. 381).

Four petals fused together barely hold down four bent, armlike stamens that protrude from the petals’ embrace. When disturbed — by a fat bumblebee or the nose of an inquisitive human — the petals split apart, freeing the stamens. The stamens flip outward, accelerating to a g-force of 2,400, each flinging a pollen sack attached to the tip. (For comparison, fighter pilots can handle a g-force of about 9 before passing out.) This flower trebuchet launches the pollen at whatever triggered the burst, or into the wind.
This early work signaled the start of a now-flourishing research area. High-speed cameras and other high-tech equipment were soon used to study more plants, revealing the secrets of their speed.

Edwards and Whitaker, for example, discovered that, like a detonating nuclear bomb, a peat moss named Sphagnum affine explodes into a mushroom cloud. On dry, sunny days, tiny, bloated spore capsules dotting the moss’ surface dehydrate, shrinking down and increasing the air pressure within the capsules to several atmospheres. When the pressure becomes too great, a capsule explodes into a cloud of spores. With the help of computer modeling, the duo reported in 2010 that the ominously shaped explosion granted the spores 20 times the height they would otherwise have, boosting their chances of catching a good breeze.

Some plants manage such impressive motion underwater. Bladderworts (genus Utricularia) come in aquatic forms, with flowers thrusting up from freshwaters and thin leafy stalks below the surface. The stalks are dotted with traps that are a few millimeters in size and shaped like a sack with a hinged lid. To set a trap, a plant pumps out water from inside the sack, which inverts its sides like a pair of sucked-in cheeks. When prey such as mosquito larvae trigger hairs at the trap’s mouth, the lid opens. Water from outside rushes in, pulling in the prey, which is trapped when the lid closes. From open to close, bladderworts can trap prey in about a millisecond.
Water is, in fact, a key player in the most fundamental of plant movements: growth.

“Growth occurs when water moves into a cell and inflates it,” says Wendy Kuhn Silk, a biologist at the University of California, Davis. “The speed of most growth responses is determined by the rate of water movement in a tissue.”

By moving water from cell to cell, plants can push out their branches and send their roots through the soil or angle their leaves toward the sun. But such movements are only so fast; a Venus flytrap relying on water-driven motion might take 10 seconds to close its trap. It’s hard to imagine even the most lethargic fly falling for this kind of slow-motion ambush.

Plants overcome these constraints through mechanical instabilities, created by storing energy through growth. Like the string of a bow pulled until taut, plants can store up potential energy. When the string is pulled too far, or nudged enough, it releases, transforming potential energy into kinetic energy.

Mechanical instabilities give the flytrap its snap, and even allow some plants to jump. Commonly known as the horsetail plant, Equisetum releases microscopic spores shaped like a bendy X. When wet, the legs of Equisetum spores curl up. As the spores dry, their legs uncurl. The curling and uncurling that come with humidity changes let the spores skitter around. Sometimes the legs compress before releasing, a forceful kick that sends a spore hopping into the wind.
Myriad mechanisms
The variety of mechanisms has proven as impressive as the speed. To the snap traps and catapults that were the focus of the initial inquiries, in just the last few years researchers have added mechanisms that rely on explosive heat, kicking teeth and lacrosselike flicks. “What we know today is the sheer diversity of it,” says Whitaker, before rattling off half a dozen different plant species, all with different mechanisms for rapid movement.

The American dwarf mistletoe (Arceuthobium americanum) had long been known to harbor rapid movement. Studies in the 1960s found that the parasitic plant, which grows in bulbous sprigs from the branches of pine trees on the West Coast, could disperse its seeds up to about 20 meters per second. But in 2015, after studying the mistletoe with thermal imaging that could detect minute changes in temperature across areas smaller than a millimeter, researchers reported in Nature Communications that the dispersal was triggered by self-produced heat. About a minute before the mistletoe releases its seeds, the plant warms up by roughly 2 degrees Celsius, thanks to a heat-producing reaction in its mitochondria. Like a lit fuse, this reaction triggers a gooey gel in the plant to expand, launching the seeds explosively.
One of the smallest and strangest mechanisms yet discovered was reported this February in the journal AoB Plants. Using cameras that can record microscopic movements at 1,000 frames per second, researchers found that the moss Brachythecium populeum is a star soccer player that can kick with its “teeth,” pliable structures of tissue that surround the spores. When the plant’s microscopic teeth absorb water, they bend and warp. As they dry, the teeth flick outward, lifting the spores to get caught in the wind.

Soon after, in March, researchers described a mechanism similar to how a lacrosse stick flings a ball, in the hairyflower wild petunia (Ruellia ciliatiflora). The flower (which despite its name, isn’t part of the petunia family) has elongated seedpods. Each pod holds about 20 disk-shaped seeds in hooks. As the seedpod grows, it strains at its seams, which can be weakened by water. When the pod splits in two, the hooks fling the seeds, giving them a dizzying spin of nearly 100,000 revolutions per minute, the researchers reported in the Journal of the Royal Society Interface. This spin, which is the fastest yet observed in any plant or animal, keeps the seeds in stable flight.
In search of speed
Despite all these efforts, the physicists, botanists and engineers who have taken part in these studies are still a disparate group. “I wander around, a bit like an outcast,” Whitaker admits. Whether it’s a biology conference or a physics conference, people are interested but unsure what he’s doing there. “This is a very young field.”

And there’s a lot still to figure out, adds Forterre, now at Aix-Marseille University in Provence, France. The Venus flytrap, extensively studied, still holds mysteries.

Researchers know that an electrical signal is sent to the plant’s leaves when a fly brushes the trap’s hairs. Somehow, the plant cells expand, resulting in now-understood snap-buckling. But researchers aren’t sure what the electrical signal is telling the cells or how exactly the cells expand.

One theory proposes that the electrical signal triggers the release of an acid that weakens the cell walls. Another posits that the electrical signal causes the plant to pump water into the cells of its outer surface, beginning the snap-buckling. Forterre is attempting to use cell pressure probes to settle the dispute, but getting the tool to work in a moving plant is easier said than done.
Researchers are also keen to understand how plants evolved their myriad movement methods. For a lot of plants, the “why” is fairly clear: Plants that can quickly capture insects get a good source of nutrients — nitrogen and phosphorus — that the plants might not be able to get in abundance from the soil. Similarly, plants that move quickly may disperse their seeds farther, gaining an evolutionary advantage over those that don’t.

But understanding how the speed came to be is much trickier. A recent clue might help piece together the evolutionary story of bladderworts. In a 2017 overview published in Scientific Reports, Anna Westermeier reported something intriguing: one species that had the architecture of the trap, but did not open or close.

This species appears to be a more primitive form in which a trap developed but did not become fully functional, according to Westermeier, of the University of Freiburg in Germany. Identifying relatives that have pieces of mechanisms could help reveal how rapid motions evolved. Whitaker is hoping to find similar potential clues by looking more broadly at plants in the family Acanthaceae, which includes the hairyflower wild petunia and thousands of other species of flowering plants, nearly all of which have some form of explosive seed dispersal.

The great diversity so far uncovered is impressive, but is it unexpected?

“It’s not surprising at all,” says Karl Niklas, a plant expert at Cornell University. Niklas has studied plant evolution for over four decades. “It’s human ego,” he says, dismissing the idea that animal movement is anything special. “And I think plants will be around a lot longer than we will.”