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

John Roach
National Geographic News

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."

Extreme Movements

Previously, Mahadevan led a research team that investigated how the Venus flytrap works. The research, published earlier this year in the journal Nature, led the scientists to ask more general questions about the limits of plant mechanics, Mahadevan said.

His student, Skotheim, spent hours in the library poring over data on the size, design, and tissue structure of a variety of plants and fungi. Together they found "the physical basis on how to classify [their] movements," Mahadevan said.

Plants and fungi that move only by shrinking and swelling are limited by the speed with which water can move from one tissue area to another. As a result, only the smallest plants and fungi can shrink or swell as rapidly as the waterwheel plant.

Larger plants, such as the Venus flytrap, rely on elastic instabilities, or spring-loaded force. In these plants, water simply takes too long move from one tissue to another.

The researchers further classify these elastic instabilities as either snap-buckling or explosive fractures. Both classifications rely on plant designs that permit the gradual storing of elastic energy and its sudden release.

The difference between these two types of elastic instabilities is how the energy is released.

Snap-buckling refers to a rapid change in plant shape that does not tear any plant tissue, such as that of the Venus flytrap. Explosive fractures involve a rapid shape change from tissue tearing.

For example, the Brazilian tree Hura crepians uses an explosive fracture to spread its seeds. While the seedpods are still on the tree, they bake in the sun. As a result, the outside cells of the seedpod lose water and shrink more than the cells lining the seedpod. This creates stress that grows and grows until the pod explodes and sends its seeds flying.

While Niklas, the Cornell University plant biologist, said the classification system derived by Mahadevan and Skotheim is well-grounded in the mechanisms of hydraulics, he thinks the researchers "missed a few things. There are probably three different mechanisms operating here."

He believes the three mechanisms include the comparatively slow movement of water, an electrical signal, and a third that involves the release of stored-up strain energy.

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