A type of moss called ‘Sphagnum’ has a peculiar way of launching its spores so that they rise high enough in the air to be dispersed by winds. As the spores are released from the moss’ capsules—only about a centimetre above the ground—they form tiny mushroom clouds, helping them rise at least 10 cm up in the air, a study in Science finds (doi: 10.1126/science.1190179).
Sphagnum moss, covering about 1% of the Earth’s surface, is thought to be able to store more carbon than any other plant, which makes it important in the planet’s carbon cycle. The researchers say that with over 285 species of Sphagnum, “long-distance spore dispersal by wind is critical in maintaining this genus”.(Science Brevia).
In order for the spores to reach the turbulent air layer that disperses them; they have to be released explosively—by a sort of ‘air-gun’ mechanism. In fact, their journey cannot be explained by regular mechanics—so called ‘ballistic trajectories’. The study found that it is instead the generation of turbulent, so-called ‘vortex rings’ that carries the spores to the observed heights, and enables their spread over large distances.
So how does this blast happen? As wet, spherical Sphagnum capsules dehydrate in the sun, the pressure inside them increases and they start to become more cylindrical. At some point, the pressure causes the capsules to burst—a process that only takes a fraction of a second. Since the capsule is cylindrical and pressurized at this critical moment, the spores get launched vertically.
This type of vortex-ring movement is usually not associated with plants, but it is similar to how squid and jellyfish move in water.
The research is based on high-speed videos—up to 100,000 frames per second—of the Sphagnum-spore capsules as they burst. The videos could be used to calculate the trajectories and terminal velocities of the spores.
Video 1: courtesy of Nora Mitchell, Joan Edwards
Video 2: courtesy of Clara Hard, Joan Edwards, Dwight Whitaker