Horsetail plant spores use 'legs' to walk and jump
Researchers in France have discovered a strange new type of movement in plants - tiny spores that walk and jump.
The researchers used high speed cameras to find out how horsetail (Equisetum) spores dispersed.
This revealed that the microscopic spores' "legs" curl and uncurl when the moisture levels change, causing them to appear to crawl around or even to spring from the ground.
The findings are published in the Royal Society journal Proceedings B.
Lead researcher Dr Philippe Marmottant from Joseph Fourier University, Grenoble, explained that the motion of plants provides natural inspiration for self-propelled devices.
"If you think of a carnivorous plant, [like a Venus flytrap] their motion can be quite fast," Dr Marmottant told the BBC.
"So I was interested in finding new types of motion in plants, and a friend who is a biologist told me about these very special plant spores that have a motion driven by humidity."
When Dr Marmottant examined the spores under the microscope, he saw their movement. But it was only when he combined the microscope with a high-speed camera that he revealed that the plants were not only moving, but walking and jumping.
Their motion is driven by four moisture-sensitive legs, or elaters, which curl when the moisture level changes.
"It's very much like how human hair can curl when it's humid," explained Dr Marmottant.
But the horsetail spores' special layered structure makes this curling and uncurling sufficiently fast and powerful to move them around.
"The legs are layers of of two kind of materials," explained Dr Marmottant.
"One layer is made of stiff material, while the other layer is a softer, sponge-like material, that deflates when it gets dry.
"Because the two layers are bonded, the shrinkage of one layer and not the other results in a change of shape of the legs, when humidity drops."
Jump into the wind
Before this study, it was not clear what the function of the spores' leg structures was.
"People assumed they were like wings, so they would help dispersal into the wind," explained Dr Marmottant.
"But here we show they actually induce motion on the ground.
"And more importantly, they also enable jumps, which means [the spores] can enter the wind currents.
"And once you're in the wind current, you can travel long distances, which is an evolutionary advantage, because it means you can disperse your [spores] very widely."
Horsetail plants, which grew when dinosaurs roamed the Earth, are still found all over the world today. But only now have researchers discovered the strange secret of their mobility, which has helped them disperse so effectively and become so prevalent.
Dr Marmottant and his colleagues are now in the process of designing new self-propelled objects based on the horsetail spores. These, they say, could be used in agricultural settings, for example, using changes in humidity to power environmental monitoring probes or seed-dispersing devices.
Prof Monique Simmonds is director of the Innovation Unit at the Royal Botanic Gardens, Kew. She said the work was "very interesting" and an example of how much more we have to learn about the natural world around us.
Prof Simmonds added that some "classical" technological breakthroughs had been inspired by plants, including Velcro and the very water-repelling surfaces that were developed after the discovery of the surface structure of the lotus leaf.
"But we have so much more to learn from nature," she told BBC News.
"Plants are like chemical factories that have to interact with one another and with their environment to get their food without moving.
"A better understanding of how nature communicates could even help companies to work together better."