The plant world has some true speed demons

Author: Dan Garisto / Source: Science News for Students

Somewhere in the wetlands of South Carolina, a fly alights on a pink surface.

As it explores the scenery, the fly unknowingly brushes a small hair. It’s sticking up from the surface like a slender sword. As the fly continues to stroll along, it grazes a second hair. All at once, the pink surface closes in from both sides. Two leaves have snapped shut like a huge pair of botanical jaws.

This blur of movement lasted only a tenth of a second. But this fly will never leave this death trap.

“We don’t think plants move,” says Joan Edwards. She’s a botanist at Williams College in Williamstown, Mass. Yet some plants, she notes, “can move so fast you can’t catch them with the naked eye.”

We tend to picture plants as largely unmoving — rooted in one place until they die. To describe something boring, we say it’s “like watching grass grow.” But such phrases offer a naïve view of the plant world.

All plants grow, a rather slow form of motion. Many also have the capacity to move rapidly. The snapping jaws of the Venus flytrap (Dionaea muscipula) are perhaps the most famous example. But they are far from the only one. Plants exhibit plenty of impressive actions. Consider the explosive sandbox tree (Hura crepitans). Also known as the dynamite tree, it can fling seeds the length of an Olympic-sized swimming pool.

Sundews (genus Drosera) have sticky tendrils that curl around prey. And within seconds of being touched, the aptly named touch-me-not (Mimosa pudica) folds its compound leaves.

Plants have evolved a broad range of approaches to movement. It spans an equally huge spectrum in terms of speed. Roots crawl through the soil at only about 1 millimeter (0.04 inch) per hour. In contrast, some plants have found a way to shoot their seeds into the environment at speeds of tens of meters (more than 100 feet) per second.

The most dynamic plant movements have long captivated scientists.

Take Charles Darwin. Of all plants, he described the Venus flytrap as “one of the most wonderful in the world.”

In his 1875 book Insectivorous Plants, he described tests he conducted on this curiosity. He baited some with raw meat. He prodded others with objects as fine as human hairs. He even tested how the plants’ traps reacted to drops of chloroform. Darwin never fully unlocked the plant’s secrets. Still, he understood that the shape of its leaves played some role in how speedily they could trap prey.

Modern researchers can study rapid plant movements with a precision that Darwin would envy. A little more than a decade ago, scientists began using high-speed digital cameras and computer modeling to home in on plant motion. Frame-by-frame analyses, along with high-resolution lenses, at long last offered a detailed look at what gives plants their speed.

Emerging evidence now points to a surprising variety of mechanisms. Researchers have turned up contraptions that kick like a soccer player or throw like a lacrosse player. One plant even generates heat to explosively launch its seeds.

Nearly 150 years after Darwin’s work, what drives such research remains the same — a fascination with fast-action plants.

Story continues below video.

Putting the moves on

In the early 2000s, Yoël Forterre was a young scientist at Harvard University in Cambridge, Mass. His adviser, another scientist, made him a gift of a Venus flytrap. The plant, new to him, amazed Forterre at its ability to move without muscles.

The scientist soon realized that its motion could be understood by thinking of it through his own specialty: soft matter physics. This field investigates how certain materials — such as liquids, foams and some biological tissues — can deform, or change shape.

Forterre published a study in 2005 that was among the first to rely on both high-speed cameras and computer modeling to study how speedy plants move.

High-speed digital cameras made such research possible, notes Dwight Whitaker. He’s an experimental physicist at Pomona College in Claremont, Calif. Around this time, cameras were making their way into university labs. “With film, you get one chance” to catch some fleeting action, he notes. Everything has to be arranged in advance. That’s why movie directors call out to their crews “lights, camera, action!” — and in that order.

A flytrap’s leaves face each other like two halves of a book. With super-fast cameras and computers, Forterre’s team could track the tiniest changes in the curve of those leaves. This allowed his group to see how the plant’s speed relied on the special shape of those leaves.

When a fly or other prey triggers the trap, cells on the green outer surfaces of the leaves expand. Cells on the pink inner surfaces don’t. This creates a tension that pushes the outer surface inward. Eventually, the pressure becomes too great. The leaves, originally convex in shape (bowing outward) now rapidly flip to concave (curving a bit inward like a bowl). This slams shut the trap in a process known as snap-buckling.

One way to understand this motion is to look at a popular child’s toy, says Zi Chen. An engineer at Dartmouth College in Hanover, N.H., he has studied the flytrap. Rubber poppers are little rubber half spheres that can be inverted (turned inside out). Like a compressed spring, the inverted toys have a lot of stored energy, which is known as potential energy. As they snap back to their original shape, the poppers convert that stored energy into kinetic energy — the energy of motion. This can launch the toys several feet into the air.

Similarly, potential energy builds up as the outer surfaces a flytrap’s leaves press against the inner ones. But they can be near-instantly converted to kinetic energy. This is what slams shut the leafy trap within a tenth of a second or so.

Blast off!

Around the same time Forterre was studying flytraps, Edwards and her husband were leading a group of budding researchers at Lake Superior’s Isle Royale. They were scouting native plants.

As Edwards tells it, a student stuck her head down to sniff a flower of the bunchberry dogwood (Cornus canadensis). Suddenly, she noted, “something went poof.” Intrigued by this distraction, the team brought specimens back to the lab. They wanted to capture the behavior on video. But whatever triggered the dogwood poof wasn’t visible. So Edwards upgraded to a camera that could take 1,000 images per second.

“It was still blurry,” she recalls. “I thought something…

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