A moss that grows in harsh deserts throughout the Northern Hemisphere uses its leaves, not its roots, to collect the water it needs to survive, reports a paper published online this week in Nature Plants. The study shows that Syntrichia caninervis extracts water directly from the air rather than the soil, a mechanism that could inspire nanotechnological innovations to improve water transport and collection systems in dry environments.

Desert plants, such as cacti, have evolved specialized mechanisms to make the most of sparsely available water. However, Tadd Truscott and colleagues find that, unlike cacti, S. caninervis uses its roots only to anchor itself to the ground, and instead captures all the water it needs with tiny hairs (awns) on its leaves that are 0.5-2 millimetres long and less than 50 micrometres in diameter.

They observed the ways in which S. caninervis takes in water by closely monitoring its leaves with an environmental scanning microscope and cameras. They show that the awns can catch water as raindrops, but can also harvest fog by providing nanoscale sites where water vapour can condense into liquid water, a process called nucleation. They find that the miniscule droplets are then herded together by a microscopic system of barbs and grooves on the surface of the awns, forming droplets big enough to move down onto the leaf proper, where the water is then absorbed. Finally, the authors find that a film of water left behind on the awns and leaf enables quicker capture of further droplets, maximizing collection efficiency when water is scarce.