Researchers explore mystery, and say gotcha

A team of scientists led by a Harvard mathematician announced yesterday that they had solved one of the plant world's most intriguing mysteries: how the Venus flytrap snaps shut.

At least since Charles Darwin, scientists have been puzzled by the carnivorous plant, which can close its fanged leaves on an insect in a fraction of a second -- without any muscle. Using a high speed video camera and computer modeling, the team found that the flytrap employs an ingenious trick to slowly build up elastic pressure in its leaves, like the stretching of a rubber band, and then snap at the slightest provocation.

The flytrap experiments, which brought together mathematicians, engineers, and biologists, are part of a growing interest in biomechanics as engineers design devices as small as cells and look to nature for inspiration.

But the real driving force behind the work was something that may seem quaint in an era of particle accelerators and interplanetary space probes: curiosity about the everyday world. For Lakshminarayanan Mahadevan, who led the team, the discovery joins a list of projects that have brought surprising insights, and publications in high-powered scientific journals, on topics as seemingly mundane as the crumpling of paper and the pouring of honey.

"People assume that because it is familiar it is understood," said Mahadevan, who is a professor of applied mathematics and mechanics. "But if you really probe, there are mysteries."

Darwin himself considered the Venus flytrap a worthy mystery, declaring in the 19th century that the plant was "one of the most wonderful in the world." In the many years since, plant biologists uncovered some pieces to the puzzle of the flytrap. Although plants do not have a nervous system in the conventional sense, they are able to send signals in the form of electrical pulses. Studies of the Venus flytrap showed that it, too, used such pulses, explaining how it might send the signal to close when an insect crosses one of the tiny filaments on the leaf.

Scientist have long suspected that the plant creates forces on the leaf by moving water into certain cells, or by changing the strength of the walls that surround the cells, said Mahadevan. Denying water to a common house plant, for example, will make its leaves move: They wilt.

The question has been how the flytrap can move before its prey buzzes along, because it seemed impossible that the plant could create such force so quickly.

To find an answer, the team painted a field of dots on the outside of the leaf, according to the paper, which is published in today's issue of the journal Nature. From a video of the leaves clamping shut, they then tracked the path of each of the dots. This, in turn, allowed them to reconstruct the shape of the leaf at each moment, and build a computer model of the entire process.

The plant's secret lies in the elasticity and curvature of its leaf, which is somewhat analogous to a soft contact lens. When a contact lens is pushed, it first holds steady, and then dramatically flips around.

Strain within the Venus flytrap leaf, probably created by water pressure, keeps the leaf poised near the point at which it will flip. Then, when an insect lands on the leaf and triggers an electrical signal, it takes only a tiny change in pressure to push the leaf over the brink, slamming it shut.

"Their contribution is to explain, in beautiful detail, how the mechanical structure actually operates," said Karl J. Niklas, a professor of plant biology at Cornell University.

The exact mechanism the flytrap uses to change the pressures within the leaf remains unknown, Mahadevan and other scientists said.

Mahadevan said that the team, which included Yoel Forterre of the University of Provence in France and Jan M. Skotheim and Jacques Dumais of Harvard, did not envision any immediate application of the work.

But the insight might prove useful in microfluidics, a relatively new field concerned with creating devices that manipulate the flow of very small amounts of liquids and gases. The plant's ability to create a dramatic movement, without muscle, might inspire designs for such devices, Mahadevan said.

Gareth Cook can be reached at cook@globe.com