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.