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

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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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