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Flying for Free the Horsetail Spore Way

The views expressed are those of the author and are not necessarily those of Scientific American.


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In spite of their sedentary reputations (putting down roots being, perhaps, the ultimate symbol of stability), plants are capable of a surprising range of movements, and not just the Venus flytraps of the world.

Observe:

At :36, the spores appear to scuttle about like dozens of itsy bitsy spiders, and at 1:17 they launch themselves from a cluster of spores like the spiderlings in Charlotte’s Web bidding Wilbur farewell. Their robust scrambling and leaping — they can achieve heights of 1 centimeter, about 20 times their own body length — certainly seems worthy of an animal. And if their sweet dance moves weren’t extraordinary enough for a plant, there’s also the fact that they manage to burn zero calories in the process. Take that, silly animals.

Horsetails are ancient plants, having evolved about 350 million years ago in the early Carboniferous. Many were understory plants, but some were great trees. Dinosaurs lived among them, and they have at least two major mass extinctions under their belt. The plants are easy to recognize but diminutive; they have a jointed, whorled appearance and usually a rough, glassy exterior. There’s a reason for that — they are coated in silica, a major component of glass, and rumor has it pioneers used them to scrub pots, a use which resulted in another common name, “scouring rushes”.

The plants look like this. They are almost all stem; their leaves appear as little vestigial black strips in bands at the joints.

Equisetum telmateia by Rror. CC by 3.0, via Wikimedia Commons. Click image for source and license

The spores come out of cones (technically, strobili) that look like this. You can see their little black vestigial leaves better here too.

Equisetum hymenale by OpenCage. CC by 2.5, via Wikimedia Commons. Click image for source and license.

Scientists have long known horsetail spores are wrapped by four long strips called elaters that fit together around nascent spores like the peels of an orange. These elaters, when mature, will unfurl when the relative humidity drops.  Though the marshes and bogs where horsetails prefer to live are humid, this might happen on a sunny or windy day.

"Les fougères de pleine terre et les prêles, lycopodes et sélaginelles rustiques, Octave Droin éditeur, 1896". Public domain. Click image for source.

A team of French scientists (who also made the film you watched) set out to study the dynamics of these movements, and how horsetail spores might use them to motor around. They published their results in the Proceedings of the Royal Society B in November.

They found that when horsetail elaters refurl as the humidity rises, they never quite return to that neat compact state you seen in the upper right of this image, like a map than you can never quite refold correctly. Instead, the arms of the elaters tangle. This tangling is the key to their movement, as the random distribution of their points of contact with whatever surface they rest upon affects where the spores will move when the elaters next unfurl. This was the random walking you observed in the itsy spiders.

If the spore is very lucky, the right configuration of tangled elaters will produce a jump the next time it dries, because two elaters sometimes get stuck. Friction between the stuck elaters stores energy as the humidity drops, and when this stored energy exceeds the frictional force, the spore will spring upward, propelled by the elater touching the ground.

This can really turn into a really good day for the spores if they happen to catch a breeze during their aeronautics. The elaters may act like sails, boldly carrying them to glades and meadows where no spore may have gone before. If the spores never left the ground, their odds of doing this would be greatly reduced. As tundra plants know, the closer you stay to the ground, the calmer the wind. In a wind tunnel test with a Beaufort number of 3 (“gentle breeze”) that these scientists performed, the spores that jumped caught a ride on the air. Those that remained on the ground stayed put at a wind velocity of 5 m/s (~11 mph) or less, and just scooted along the ground at higher speeds.

The reason that horsetail spores can do this without spending a calorie is the structure of their elaters. They are made of two layers — a dense cellulose microfibril layer topped by a low-density cellulose absorptive layer. As the humidity increases, this low-density layer expands and the elaters curl closed. As the air dries, the absorptive layer shrinks, pulling the elater open. Since the power source is changing humidity, the fuel is free. The net result is that horsetail spores can sally forth repeatedly, their wanderlust limited only by the whims of the weather.

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A few blogger notes:

I did a short video spot about my recent piece on tobacco ringspot virus and honeybees on this month’s Best of the Blogs video over at Carin Bondar’s PsiVid blog. You can check that out here.

Moselio Schaechter over at the excellent microbiology blog Small Things Considered put up a nice post recently highlighting his favorite microbiology blogs and he was kind enough to say some nice things about me and my blog. Thank you, Elio!

Reference

Marmottant P., Ponomarenko A. & Bienaime D. (2013). The walk and jump of Equisetum spores, Proceedings of the Royal Society B: Biological Sciences, 280 (1770) 20131465-20131465. DOI:

What they did not understand so well was how the system worked. A team of French scientists set out to study the dynamics of these movements, and how horsetail spores might use them to get around. They published their results in the Proceedings of the Royal Society B in November.
Jennifer Frazer About the Author: Jennifer Frazer is a AAAS Science Journalism Award-winning science writer. She has degrees in biology, plant pathology/mycology, and science writing, and has spent many happy hours studying life in situ.
Nature Blog Network
Follow on Twitter @JenniferFrazer.

The views expressed are those of the author and are not necessarily those of Scientific American.





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