Plants’ reproductive weaponry unfurled

Botanical tricks include adhesion and bubbles to spread their spores

BOSTON — Plants appear to live the ultimate sedentary lifestyle, but they’ve got mad aeronautical skills when it comes to their progeny. Two new studies — one investigating the catapult action of ferns, and another examining the “jumping spores” of a species of horsetail — reveal surprising botanical mechanical tricks for dispersing future offspring.

Miniscule spores released from this horsetail plant (Equisetum arvense) have tiny arms that use friction to launch themselves, studies with high-speed cameras reveal. leezsnow/istockphoto

Consider Equisetum’s jumping spores. Known as horsetail or scouring rush, Equisetum bears spores that are decked out with four slender, arm-like projections called elaters. Botanists have long presumed that the elaters, which are curled around the spore and unfurl as local humidity drops, give the spores lift, akin to the tail or wings of a kite. But new research suggests that before the spores go airborne, the elaters provide a sticky friction that helps launch the spores aloft.

The spores are a mere 50 micrometers across. At that size, adhesion forces dominate, making small things — like the spore itself — stick to surfaces. But once the elaters have unfurled, the slender arms can provide some alternative grip. When the elastic energy stored by the elaters is released, the spores launch, jumping several times their height, physicist Philippe Marmottant explained March 1 at a meeting of the American Physical Society.

“This is a way to overcome the adhesion,” said Marmottant, of the University of Grenoble in France. “It’s a little kick to put you in the air.”

A separate study exploring how a species of Polypodium fern launches several spores at once reveals catapult action to rival the ancient Greeks. While man-made catapults require some sort of cross bar to hold the arm back before hurling the projectile, ferns manage the same trick with a bit of water, Xavier Noblin of France’s National Center for Scientific Research reported March 2 at the meeting. Once the fern’s capsule of spores has opened and drawn back into ready-to-launch mode, tiny bubbles appear along the ridge of the capsule, which draw in a bit of water. These bubbles break the tension, leading to an explosive flinging forward of spores, experiments with a high-speed camera reveal.

Such studies may lead to new tricks for designing materials, said biomechanics expert Etienne Reyssat, of ESPCI ParisTech in France. But researchers also study such things “because it is fun,” he said.

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