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Horsetail spores don't need legs to jump

Plant's curly, humidity-controlled ribbons propel epic leaps

horsetail spore

BOING  The tiny spore of a horsetail uncurls its four ribbonlike elaters as humidity drops. If the elaters uncurl  suddenly, they can launch the spore in an extreme jump.

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They look like lifeless green dust, but the spores of spiky horsetail plants can jump about 200 times their body length. That’s true even though they lack legs.

Equisetum spores trail long, double-layered ribbons called elaters that wrap around the main body of each spore. Biologists have previously seen elaters uncurl, nudging the spore along in a small walking step.

Now high-speed cameras have captured occasional jumps too fast for the human eye to see, says physicist Philippe Marmottant of the University of Grenoble in France. The speedy leaps happen after spores get soaked; when the elaters dry out they can unfurl so suddenly that a 50-micrometer spore jumps as high as a centimeter. That’s enough to launch it into a breeze that could carry it to a new home, Marmottant and his colleagues report September 10 in the Proceedings of the Royal Society B.

“The spores don’t get tired,” he says. Elaters can curl back up as humidity rises and get ready to take another step or jump the next time they dry out.

“When we look at a spore, we never know whether it’s going to walk or jump,” Marmottant says. Uncurling starts when humidity drops below about 75 percent, he found. Then the spore takes a tumble in an unpredictable path; he demonstrates in a Skype conversation a series of poses involving an arm, his head or a blur of torso angling across the screen. These poses produce (for the spore) a random series of short steps and occasional extreme leaps that are a fine way for a brainless dot to search for a good position.

Spores sometimes clump, and to his surprise, Marmottant found that clumps move faster than individuals. More elaters boinging outward seem to give the cluster an advantage over singletons.

Elaters’ cuplike ends make them “look like bras that open at the front” Kathleen Pryer of Duke University tells students when they are hunting for spores through a microscope. The elaters are unique to horsetails, which Pryer considers a kind of fern because DNA evidence suggests that horsetails and ferns are the closest relatives of plants with seeds. Other ferns have evolved different ways of launching their spores, such as little catapults.

Spores in motion have inspired Marmottant and his colleagues to start developing a robot that moves like a horsetail spore. “We don’t know what to do with it yet,” he says, “but there will be something.”

TINY DANCER A slow-motion video shows horsetail spores unfurling their ribbonlike elaters for small position changes, epic jumps and group scuttling.
Credit: Courtesy P. Marmottant

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