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Moss is a master of mechanical engineering

How this desert moss might help engineers build water collections systems. (Video: The Washington Post)

Almost nothing in this world is as good at living as moss.

It thrives where almost no other living creature can dwell: on wind-whipped mountaintops and rusty tin roofs and glacier-carved caves and cracks in the sidewalk, on frigid Antarctic rocks and desiccated desert sands. Millions of years ago, it was one of the first living things to make the leap out of the relatively safe and comfortable oceans to the barren and exposed earth.

To achieve all this, moss has mastered feats of engineering that still mystify human scientists. Take, for example, Syntrichia caninervis, a tiny, delicate desert moss whose leaves are capped with spindly white hairs called "awns." These plants have a water collection system so effective it can suck water vapor straight from the air, rather than absorbing it from the ground via roots. Understanding how moss achieves this could help human water system engineers do the same.

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"It really is incredible," said Tadd Truscott, an author of a recent study in the journal Nature Plants about the moss's upside down water collection strategy.

Truscott, who researches fluid dynamics at Utah State University, grew up in the arid areas of the West. He has a deep appreciation for the creatures that manage to eke out a marginal existence in the driest places on Earth. But he generally looked at water from a mechanical point of view — particularly the desalination process. Most of his admiration for moss was from afar.

It was actually one of his students, Zhao Pan, who brought Syntrichia caninervis to his attention. Pan, a PhD candidate at Brigham Young University, had heard about the moss from one of his classmates and thought his professor would be interested.

Pan brought the classmate and her moss research to Truscott's lab and asked him to take a look.

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"The day they showed up was really exciting," Truscott said, recalling how he and his colleagues watched in awe as water droplets formed on the moss's spindles, seemingly from nowhere, before being sucked down into the plant. "We were totally enthralled by it."

The process starts on the millimeter-long awns, which natural selection has winnowed and perfected to be the ideal surface for water collecting. Each spindle is covered in a network of barbs and grooves that can catch a single water droplet and hold it, like a tiny, glistening diamond. On cool desert mornings, water vapor in the air condenses onto this network, much the same way it coats the windshields of cars left out overnight. The awns are just as good at snagging droplets out of passing waves of fog and collecting raindrops that splash down from above.

The awn's conical shape guides the water droplet down to a capillary-like network of structures that soaks it up and sends it to the plant's cells. A thin film it leaves behind makes the awn an even better spot for condensation, making water collection yet more efficient.

When enough water has accumulated, the moss's leaves — which are brown and compact during dry spells — begin to unfurl. They extend their thin tendrils toward the sky and start to photosynthesize, creating a spot of vivid green in the parched desert.

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"Mosses reclaim the land in a lot of ways," Truscott said. "You'll find a lot of things growing around it: sagebrush, juniper. . . . In general it comes in first, and these other plants follow" — just as they did when moss first colonized the continents 470 million years ago.

Other desert plants have adopted similar strategies — many cacti, for example, collect water on their spines. But none do it so effectively as moss. The aboveground water collection system is so efficient that moss doesn't absorb water from the soil at all; the roots are just used to anchor it to the earth.

"It sheds a lot of light for mechanical engineers who want to build similar things . . . like fog nets," Truscott said, referring to the fine mesh structures used in places such as Peru to collect water from the air.

Figuring out how to collect water cleanly and efficiently is "one of the major engineering challenges we're facing," Truscott said — and moss has mastered it.

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