Science News

Spores Travel on Mushroom Clouds

Sun, 25th Jul 2010

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Moss spores get extra lift from the same process that generates smoke rings and mushroom clouds, according to new research published in the journal Science this week.

Dwight Whitaker at Pomona College in California and Joan Edwards at Williams College, Massachusetts, realised that the spores of Sphagnum moss couldn’t possibly reach as high as they do by ballistic propulsion alone, and set out to find the moss’ trick.

Habit shot of the low-growing Sphagnum moss showing reproductive heads with capsules raised above the mat by pseudopodia.

Habit shot of the low-growing Sphagnum moss showing reproductive heads with capsules raised above the mat by pseudopodia. Some capsules are round and not yet exploded, and some have recently exploded (cylindrical). Sphagnum acts like a sponge - holding water by capillary action between its leaves and in specialized water-holding cells. It acidifies the water creating anoxic acidic areas where decomposition is slow. Over the millennia, Sphagnum has sequestered more carbon and any other plant genus - thus it plays a critical role in the global carbon cycle. © Image courtesy of Joan Edwards

Sphagnum moss is thought to store more carbon than any other plant genus, and covers around 1% of the Earth’s surface.  In order to distribute its spores, it fires them upwards into a turbulent patch of air, where they can be picked up and transported on eddies and breezes.  To take advantage of the breeze, the spores have to be very light, and have a terminal velocity of just 5 millimetres per second.  This low terminal velocity creates a problem when getting through a layer of still air directly above the moss, where the spores will rapidly decelerate.

The spores develop in the top half of a 2mm spherical capsule, the bottom half of which is hollow.  Each capsule contains from 20,000 to 250,000 spores.  When the conditions are right, these capsules dehydrate and become cylindrical – in the process vastly increasing the air pressure inside.  Eventually, the capsule fails, and the internal pressure fires both the spore, and a puff of air, upwards.

This puff of air creates a vortex – a self sustaining ring of rising air like a smoke ring or mushroom cloud - which is strong enough to carry the spore far further than it would otherwise go, and up high enough to catch the turbulent air and be carried away.  High speed imaging confirmed their hypothesis.

We know of a few species that take advantages of vortices like this – bees and jellyfish for example – but this is the very first example of vortex use in plants.

 

Sequential still frames from a video filmed at 10,000fps. Each frame is 1/10,000 of a second. The mushroom cloud with a trailing wake is clearly visible.

Sequential still frames from a video filmed at 10,000fps. Each frame is 1/10,000 of a second. The mushroom cloud with a trailing wake is clearly visible. © Image courtesy of Clara Hard, Joan Edwards and Dwight Whitaker

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