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.
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 Yoël 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 frames per second is available online at www.nature.com/nature/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.
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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 an 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 below]. 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.
Earlier botanists had proposed that the flytrap, lacking muscles, relied on cellular water pumps called vacuoles to drive the leaf closure. That hypothesis had been met with some skepticism, because the change in leaf shape seemed to require that a large volume of fluid move rapidly from one place to another. Water-powered movement can drive slow motions, such as that of a sunflower tracking the Sun. But no one could see how cells could gain volume fast enough to close the flytrap.
Forterre and his colleagues, however, have demonstrated that fast pumping isn’t needed. Just a small change in the shape of the leaf cells—which needn’t be powered by a flow rate any higher than that of the water in and out of sunflower cells—can cause the trap to snap quickly. Pushed to the point of instability, the leaf halves are forced to buckle into a new shape. The continuing slow rotation of the leaf halves in the third phase is also consistent with a slow flow rate of water into the leaf cells.
The pretty yet creepy Venus flytrap illustrates a principle applicable to a variety of self-assembling structures. Take my self-erecting shelter for the beach. In its storage configuration it looks like several flat discs of nylon fabric. But when I grab one layer and shake it, the fiberglass supports suddenly reconfigure, and a two-person hut stands ready for use. The Venus flytrap makes me wonder how hard it would be to add low-force actuators to my sunshade—for rapid repacking or, with an unsuspecting person sitting inside, simply for entertainment.