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How Is a Maple Seed Like a Moth?

12 June 2009 (All day)
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David Lentink

Easy glider. Paired on the tree, these seeds will split when they fall, spinning and generating lift to slow their descent.

Each spring, winged maple seeds twirl like helicopters as they glide to the ground. A seed can drift up to a kilometer on a windy day, landing on a fresh patch of earth to start its own tree. To travel so far, a seed relies on a tiny spiral of air that forms above its papery wing and provides lift, a new study shows. Such an air vortex helps moths, bats, and birds stay aloft, but its use in seeds marks a first for plants.

To make the discovery, zoologist David Lentink of Wageningen University in the Netherlands and colleagues first created an artificial maple seed in the lab. They needed something big to actually quantify the air flow patterns, so they made a seed about 10 times the size of a maple seed out of acrylic. The team then attached the seed to a robotic arm that spun it in a container of mineral oil filled with thousands of tiny glass spheres. When the fluid was lit by a laser, it illuminated the glass beads and revealed a tornado-like vortex that ran parallel to the seed's wing. The vortex, Lentink says, decreases the pressure above the seed, essentially sucking it upward to slow its decent.

But the model still wasn't the real deal, so Lentink and his colleagues built a wind tunnel, filled it with smoke, and watched as an actual maple seed rotated in midair. The team reports in today's issue of Science that the smoke patterns revealed the same tornado-like effect--also known as a leading-edge vortex--as seen in the glass bead experiment, confirming that their model was correct.

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Exposed. Smoke particles reveal the tiny, side-lying tornado-like vortex that forms over the wing of a freely falling maple seed.
Credit: David Lentink

The fact that the leading-edge vortex also enhances the flight of insects, bats (ScienceNOW, 10 May 2007), and probably birds indicates that it is an important universal mechanism for generating high lift forces, says Anders Hedenström, a theoretical ecologist at Lund University in Sweden. "People don't give plants enough credit for 'smarts'" in spite of having similar demands as animals, adds biomechanist Steven Vogel of Duke University in Durham, North Carolina. Lentink says that employing the leading-edge vortex in flight design could lead to efficient new forms of aircraft, such as parachutes and monocopters used to probe other planets.

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