Snap, Buckle, Pop: The Physics of Fast-Moving Plants

May 26, 2005

Fleet-footed animals, such as gazelles and cheetahs, aren't the only livings things that rely on speed for their survival. The same is true for some plants and fungi.

Consider the Venus flytrap, the poster child for carnivorous plants: Its jaw-like leaves can ensnare insects in an eye-blurring one-tenth of a second.

Other plants employ similar lightning-quick movements, if not to hunt, than to spread their seeds, squirt pollen, or shake off predators.

Plants don't have muscles. So how can some plants move so quickly?

Using the laws of physics, two scientists have detailed the mechanical design principles that govern these speedy plant moves.

"To understand biology, it is always useful to come up with general principles as we have in this case," said Lakshminarayanan Mahadevan, a professor of applied mathematics and mechanics at Harvard University in Cambridge, Massachusetts.

Mahadevan and his student, Jan Skotheim, report their findings in tomorrow's issue of the research journal Science.

The scientists divided plant speedsters into two groups: those that swell and shrink their cells to generate movements and those that use swelling and shrinking to release stored energy in a quick snap, buckle, or explosion.

The waterwheel plant (Aldrovanda), a cousin of the Venus flytrap, belongs to the first group. The carnivorous plant is so small and thin that its cells can swell with water fast enough to close its leaves quickly and smoothly around the small aquatic invertebrates that the plant feeds on.

"Bigger plants can't move water quickly enough to do that, so water triggers an [elastic] instability and that's what gets them over the barrier," Mahadevan explained.

As for the Venus flytrap, it falls in the second group. With its leaves spring-loaded like a contact lens pushed inside out, the plant lies in wait for insect prey. When a fly or spider lands on a flytrap's leaf, the stimulus triggers the leaf to rapidly swell with extra water. This forces the leaf to snap back to its original position, trapping its insect meal.

Karl Niklas, a plant biologist at Cornell University in Ithaca, New York, said the new classification system "gives me a nice, formal mathematical way to describe what people have seen for a very long time."

Continued on Next Page >>




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