In a Snap: Leaf geometry drives Venus flytrap’s bite

In a mere tenth of a second, without any muscles, a Venus flytrap’s jawlike leaves can imprison a hapless insect. Since the time of Charles Darwin, scientists have struggled to understand this feat.

OPEN AND SHUT CASE. Tickling the inner surfaces of an open Venus flytrap (top) causes mechanical tension to build in the leaves until they buckle and snap shut (bottom). Forterre, et al./Nature

In the Jan. 27 Nature, researchers offer a possible explanation: With its peculiar leaf geometry, the flytrap Dionaea muscipula achieves fast, springlike action that’s usually off-limits to plant tissue.

Plant motions are typically slow, notes study coauthor Lakshminarayanan Mahadevan of Harvard University, a mathematician who specializes in mechanics. Many botanical movements take place as plants’ internal plumbing systems gradually redistribute water among cells. However, a flytrap’s snap requires motions at least 10 times as fast.

In 2003, Mahadevan and his coworkers began investigating the plant’s quick motion when they were at the University of Cambridge in England. The researchers had become fascinated with a Venus flytrap that was sitting in a flowerpot in their shared office.

“I was watering it. We started talking about it,” recalls Jan M. Skotheim, now at Rockefeller University in New York City.

Soon, the researchers were painting reference dots of fluorescent paint onto curved flytrap leaves and taking high-speed videos of snapping traps. The researchers also used a microscope to accurately measure leaf stretching.

Now, a fast-action picture of the flytrap’s capturing mechanism has emerged.

First, the cells in the outer surface of each open leaf elongate, Mahadevan and his colleagues report. An insect or another object landing in the trap stimulates hairs that then trigger pumping of fluid into the outer cells. Meanwhile, the leaves’ inner surfaces don’t change.

That uneven elongation puts mechanical pressure on the leaves to bend inward, says team member Yoël Forterre, now at the University of Provence in Marseille, France. But the open leaves are curved in such a way that they resist the bending force. As a result, tension grows in the leaves.

The team’s measurements show that after nearly 1 second of buildup, the leaves can resist no more. Suddenly, their shape becomes unstable and they buckle, flipping into their cupped, closed form.

An elastic mechanism for the flytrap’s snap “is the right explanation,” comments mechanical engineer Julian F.V. Vincent of the University of Bath in England.

“This is a convincing demonstration of a most unusual application of mechanical instability in plants,” adds Charles R. Steele of Stanford University.

It’s likely that other rare, fast plant motions—including the explosive spewing of seeds by species such as the squirting cucumber—depend on similar mechanisms, Mahadevan says. It may even be possible for people to adapt the process to technological uses such as sensors or novel valve mechanisms in microfluidic devices, he adds.

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