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Snap! How can the Venus flytrap indulge its taste for insect flesh? The secret is the cunning construction of its leaves

Natural History, June, 2005 by Adam Summers

Although plants are firmly rooted in the ground, they do move: sunflowers track the Sun across the sky; daffodils turn their floral faces away from the wind as it blows. Most plant motion is either quite slow (the sunflower), or driven by external factors (the wind on the daffodil). Herbal hustle caused by internal forces is uncommon. That's no surprise, really: plants have neither nerves nor muscles, nor do they have other obvious mechanisms for generating force rapidly.

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Yet despite the lack of muscle, several plant lineages have independently evolved some capacity for rapid movement. The trigger plants of Australia, for instance, slap a dab of pollen on visiting bees. More morbidly, the Venus flytrap slams two halves of a leaf shut on nutritious insects. Recently, investigators discovered that the flytrap owes its quick grasp to a "bistable configuration" of its leaves, whereby small movements can trigger much larger ones.

The Venus flytrap (Dionaea muscipula) is native to verdant, boggy coastal plains of North and South Carolina. Bogs are more acidic and have fewer nutrients than most plants can tolerate, and so it's no coincidence that several bog plants supplement their root-gathered nutrition with insect snacks. That makes a bog into a minefield for winged and walking arthropods. Bladderworts, pitcher plants, and sundews all indulge their carnivorous tastes. Among those refugees from the Little Shop of Insect Horrors, though, the flytrap has a uniquely dynamic method for catching prey.

The flytrap features a set of inch-long, heart-shaped capture leaves, each fringed with trigger hairs and bisected by a deep fold. Any insect unwary enough to bend a single hair is a goner. The two halves of the leaf snap shut along its fold in just 100 milliseconds, swiftly enveloping the animal. The trigger hairs become the bars of a prison. In the ensuing few hours the trap seals itself airtight, and digestive glands in the leaf secrete enzymes that reduce the insect to a dry husk.

Botanists discovered the Venus flytrap several hundred years ago, and its behavior has fascinated people ever since. It may come as a surprise, then, that until recently no one knew how a flytrap, unthinking and without muscles, could move fast enough to capture flies. The mystery prompted Yoel Forterre, a physicist at the University of Provence in Marseille, France, and his colleagues to take up the case.

To improve visibility, the team began by daubing flytrap leaves with dots of paint that glows under ultraviolet light. Then they shot videos of the leaves closing, at 400 frames per second (a somewhat smaller video file, showing the action at 125 flames per second is available online at www.nature.com/natu re/journal/v433/ n7024/suppinfo/nature03185.html). Watching the videos in slow motion, and tracing the path of each painted dot in three dimensions, the investigators discovered that what appears to be a quick, fluid snap of the two halves of the leaf is actually a three-phase process.

In its initial, open configuration, the capture leaf looks like a paperback book that has been splayed open by breaking its binding, and further insulted by bending its spine into an arc. The two halves of the leaf also curve away from each other; if you were to see them from the hapless insect's point of view, they would appear to be convex, with the center of each leaf toward you and the edges curving away.

The first phase of the leaf closure begins when a trigger hair is disturbed. The capture leaf begins to close slowly for half a second or more, reducing the "gape" of the leaf by a few hundredths of all inch. As it does so, the spine straightens slightly, its curvature resisting the closure like a spring. The leaf halves retain their convex curvature even as they rotate slightly toward each other.

Suddenly the leaf crosses a critical threshold, and the second phase begins. The two halves buckle outward, into a new, concave configuration (again, from the insect's point of view), and the leaves snap shut.

During the third phase the leaf slowly continues to close. That process can last a long time; the trap keeps closing for hours and remains closed for days.

Because both the open, convex configuration and the closed, concave one resist any rotation of the leaf" halves about the spine, both configurations are stable. The leaf is therefore bistable.

To get a better picture of the idea, think about the shape of a toilet plunger, or plumber's helper. Stored next to the toilet, a plunger is in a stable, concave configuration. When you use it on a clogged toilet, as long as you don't push too hard, the plunger is stable enough to spring back to its resting state. Push too hard, though, and the rim of the plunger will snap back along the handle, and the whole thing will buckle into a convex shape [see illustration above]. Then, unfortunately, you must flip it back by hand. Much the same effect drives the sudden closure of the flytrap. The closing pushes the structure to the edge of stability slowly enough that the hapless insect never notices. Then suddenly, in becomes out, out becomes in, and the prey is neatly trapped.

Snap! How can the Venus flytrap indulge its taste for insect flesh? The secret is the cunning construction of its leaves

Natural History, June, 2005 by Adam Summers

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