Moss counters shortness with A-bomb-style clouds

Sphagnum overcomes drag by launching spores in vortex rings

SEATTLE — A little bog moss turns out to be a master of throwing up.

UP The plump round capsules of a sphagnum moss launch spores upward when ripe. New work on several moss species finds a vortex-ring mechanism that amplifies the height of the spores’ launch. Kristian Hassel, NTNU Vitenskapsmuseet / Flickr

GETTING LOW Sphagnum moss species grow low to the ground on the forest floor, one reason that launching its spores high in the air helps disperse its offspring. A new study shows how the moss spores reach such heights. treerunner / Flickr

Sphagnum moss shoots its spores extra high by using a trick, called a vortex ring, previously known only from animals, says plant ecologist Joan Edwards of Williams College in Williamstown, Mass. The additional loft gives spores a better chance of rising into the zone of turbulent air and catching currents for a ride to new homes.

Throwing dust, or the tiny dustlike spores produced by the moss, any meaningful distance is hard. A lot of punch just leads to poof.

Yet the round capsules of sphagnum moss blow their lids and launch a burst of reddish spores relatively high in “a beautiful mushroom cloud,” Edwards said January 4 at the annual meeting of the Society for Integrative and Comparative Biology. That cloud, which is a vortex ring, lowers the drag that normally reduces dust tosses to ho-hum puffs.

In the vortex’s roiling mass, sphagnum spores reach heights 10 to 20 times greater than predicted for a similarly fast-moving spore shot like a bullet into the air, according to calculations by Edwards’ collaborator, physicist Dwight Whitaker of Pomona College in Claremont, Calif.

Vortex rings often show up in the wake of jet-propelled jellyfish and squid, comments Patrick Martone, a phycologist and biomechanist at the University of British Columbia in Vancouver, Canada. “This is the beauty of the SICB conference: squid researchers and moss researchers learning from one another,” he said.

The sphagnum species investigated are the first plants shown to create a vortex ring, as far as Edwards knows, she said. A high-speed camera with just the right photographic exposure revealed the doughnut-shaped mass with a trailing stem. Sphagnum spores form the ring as they rise in a mass, rolling upward to the top of the doughnut and then curling around underneath for another loop, and another. The ring only forms if an abrupt burst of air rushes out of a cylinder, like a spore-launching capsule or a smoker’s lips, at just the right speed.

The images captured of the moving cloud of spores have yet to be released by the research team, pending acceptance of the work by a scientific journal.

Sphagnum spores average some 16 meters per second (around 36 miles per hour) when they leave the capsule. Without boiling up in their vortex ring, they would probably not reach even a centimeter in height at that speed, according to Whitaker’s work. Yet Edwards reported that the sphagnum spores, moving at the speed of clogged freeway traffic, launch to a height of some 16 centimeters.

Adding more speed alone wouldn’t help the spores disperse farther, according to another team’s studies of the flowers of white mulberry trees. The flowers shoot out pollen without a vortex ring, but at some 237 meters per second. Breaking out of the flower at more than half the speed of sound, the mulberry pollen moves only some 7 centimeters away from its flower.

For more than a century, researchers have believed that pressure building up in the moss capsules finally popped the top and ejected the spores. A recent paper disputed that, and Edwards said she’s now planning on using her high-speed images of the capsule bursts to sort out the mechanism.

Susan Milius

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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