As you read this, forceful explosions are rocking the planet, covering it in mushroom clouds. Thankfully, nuclear winter isn’t going to befall us quite yet. These explosions are caused by biological cannons rather than man-made bombs and the clouds they produce are mere millimetres high. They are the means by which peat mosses disperse their spores.
There are over 285 species of peat moss, all belonging to the genus Sphagnum. They are among the most common plants in the world, growing in the cold, moist parts of the Earth and covering about 1% of its land. They rely on the wind to disperse their spores and all of them face a similar problem. They grow in flat mats, which hug the ground at a level where the air is relatively still. Ideally, they need to get their spores into the ‘turbulent boundary layer’ – a zone up to 10cm off the ground, where swirls of air and sideways currents can carry the spores over long distances.
Most species place their spores at the end of stalks, but even these only stick out a centimetre off the ground. To get the spores even higher, the peat mosses shoot them out, using their stalks as mini-cannons. Each one has anywhere from 20,000 to 250,000 spores loaded into a round capsule at its tip. On sunny days, the capsule dehydrates and collapses inwards, transforming from a sphere into a cylinder and squashing the air inside it. Soon, the pressure becomes too great and the capsule literally blows its top, shooting out both spores and air.
The whole process takes less than a hundredth of a millisecond. The spores are ejected at around 30 miles per hour at around 32,000 times the force of gravity. And they reach a height of around 10 centimetres, more than enough to reach the turbulent layer of air above the moss.
But this isn’t the whole story. Despite its forceful launch, the spore shouldn’t be able to reach that height. Given its initial speed, it should get to a maximum height of no more than 7 millimetres in less than half a millisecond. In reality, it goes far higher for much longer. After 5 milliseconds, the average spore has already risen by 4 centimetres, with only vague signs of slowing down.
To solve this mossy mystery, Dwight Whitaker from Pomona College and Joan Edwards from Williams College filmed the firing of the spore cannons. The action is so quick that they had to use ultrahigh-speed cameras that shoot up to 100,000 frames every second. These videos revealed that each launch is accompanied by a tiny mushroom cloud. These clouds are rolling haloes of air called ‘vortex rings’. It’s these rings that give the spores the extra boost they need to rise above it all.
Vortex rings are produced when a ball of fluid (in this case, the air trapped in the capsule) moves through larger mass of fluid (in this case, the atmosphere). As the ball of capsule air explodes outwards, its leading edge pushes the molecules of the surrounding air apart. But remember that air has friction – because of this, the outer layer of the ball is pulled outwards only to roll back in on itself later. The result is a moving doughnut of air – a vortex ring. You can see this happening more clearly in the animations on this page.
Squid and jellyfish commonly produce vortex rings behind them to push themselves along. Humans make them whenever we blow smoke rings. Dolphins produce them sometimes by blowing water rings. But this is the first time that anyone has found a plant that can create its own vortex rings. The fact that the low-lying peat mosses can shoot their spores to such a height is a crucial element of their success.
Reference: Science http://dx.doi.org/10.1126/science.1190179
Images by Clara Hard, Dwight Whitaker and Joan Edwards
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