Explosions, mushroom clouds — all good for short moss

Sphagnum reproduces with a bang that compensates for life so close to the ground

July 23, 2010 at 2:31 pm

MOSS BLASTS A sequence of frames from a video filmed at 10,000 frames per second shows a capsule of a sphagnum moss launching its reproductive spores in a roiling vortex ring that lofts them higher than a straightforward up-blast ever could. The video captured, for the first time, evidence of a moss creating such a vortex ring. Clara Hard, Joan Edwards and Dwight Whitaker

ONE DOWN A spore capsule of Sphagnum fimbriatum has blown its top and sent spores flying upward, after building up intense internal pressure. Johan L. van Leeuwen

It’s not Armageddon, just moss reproduction.

On sunny days, scientists now realize, mushroom-shaped clouds routinely explode out of beds of sphagnum moss. Researchers captured the very tiny, very fast clouds of rising spores in action by videotaping moss at speeds of 10,000 frames per second.

Biologists have known that animal motions, such as jellyfish propulsion, can generate mushroom-cloud turbulence in water or air. Vortex rings, as these doughnut-shaped blasts are called, are a surprise coming from a moss.

At a meeting in January, Joan Edwards of Williams College in Williamstown, Mass., reported that at least some of the species of sphagnum mosses create vortex rings when they shoot out their spores. But the actual videos of sphagnums in action just became available this week as she and Dwight Whitaker of Pomona College in Claremont, Calif., publish their analysis in the July 23 Science.

Vortex rings boost the moss’s launch of its spore offspring, the researchers say. At moss level on the ground, air doesn’t move much. Spores wafting in this zone would probably just fall back down into the territory already occupied by moss. For a good chance at colonizing new territory, spores need to soar more than 100 millimeters above ground into higher, friskier air. And to do that, the mosses are using more than simple ballistics, according to Whitaker’s calculations.

If the plant just shot out one of its tiny and light spores in a straightforward path like a bullet, the researchers calculate that the particle would rise only a few millimeters. Whitaker and Edwards measured spores typically soaring 114 millimeters, and up to 166 millimeters, high enough to catch a breeze to new horizons.

Explosion of a Sphagnum palustre capsule from Science News on Vimeo.

(Recorded at 10,000 fps with a 0.097 ms exposure; displayed at 15 fps.) A clear “mushroom cloud” vortex ring forms and reaches a nearly fixed width within 0.5 ms. Spores are carried upward within the vortex ring and its wake. At this exposure, the mushroom cloud has some motional blurring compared to the images shown in Fig. 1D. and Movie S3, which were captured with a shorter exposure time.

Credit: Clara Hard, Joan Edwards, Dwight Whitaker