You have a problem. You are a plant, you live in a desert, and you have no roots – at least not any that can absorb water, which is a bit of an issue out here. What do you do?
If you’re the moss Syntrichia caninervis, you solve this problem by turning your water collection system upside-down, according to the authors of a new paper in Nature Plants. In a strategy that echoes that of the lichens who can survive on the moisture from fog, this moss has evolved moisture-trapping antennae called awns at the tips of its leaves. Their structure harnesses physics so powerful that even if the plant itself is inverted, the awns can hoist water against gravity. The principles of the system are reminiscent of the wind traps described in Frank Herbert’s Dune. Planetary ecologist Liet-Kynes would no doubt approve.
Here is one of the lead authors discussing the findings and some excellent videos of the plant’s abilities:
S. caninervis is diminutive, standing just three to eight millimeters high. But what it lacks in stature, it makes up for in toughness. It lives in some of the harshest deserts of the world, from the sizzling, Joshua-tree spiked Mojave of the U.S. southwest (the driest in North America) to the remote Gurbantunggut Desert of China, which contains the point farthest from any marine coastline in the world.
There, the moss’s shoots huddle together for dear life, covering the soil as part of the celebrated “biological soil crust” which stabilizes desert soil and protect the many other life forms who live there (If you’ve spent any time in America’s deserts, you’ve likely been admonished, “Don’t bust the crust”). When dry, the moss appears dark brown or black, but when moistened, seems to magically green within seconds.
The moss has complementary ways of catching water at four different scales. First, there are nanogrooves with a shape that catalyzes the condensation of water vapor into liquid when humidity is high. Next, the awns also possess larger but still tiny microgrooves whose shape and size are optimized to catch the pre-condensed water droplets of fog or cloud. Third, the antennae are conically-shaped and so funnel droplets that have reached sufficient size through either of the above methods toward the leaf at their base. Finally, a network of flexible awns and leaves works to absorb the energy of impact and suppress splashing when a raindrop hits the plant. Splashing that does occur tends to travel less than it would upon hitting the ground and to therefore hit nearby awns and leaves, where it is quickly absorbed.
In addition, barbs that line the awn and cluster randomly along it seem to act as staging areas for droplets accumulating along nanogrooves or microgrooves until the drops reach a size large enough to be funneled to the leaf by the conical shape of the awn.
When they finally do so, they are sucked down quickly, as you saw. Meanwhile, the barb cluster starts growing a new droplet.
Of course, assets can also be liabilities. Awns greatly improve the moss’s ability to harvest water when it’s moist, but they also speed water loss when things are drying. But for a moss, that’s no problem. They are one of the few plants that possess poikilohydry: the ability survive dessication. If you’ve ever noticed what happens to a houseplant you allow to dessicate, you know this is a super-power indeed.
Still, a plant in suspended animation in a place where water is rare needs a way to quickly capitalize on moisture when it does appear. For this moss, awns are vital to tapping the fleeting supply, and milking it for all it’s worth. That a key to survival in such a withering environment is a delicate tendril of tissue is one of life's beautiful ironies.
Pan, Zhao, William G. Pitt, Yuanming Zhang, Nan Wu, Ye Tao, and Tadd T. Truscott. "The upside-down water collection system of Syntrichia caninervis." Nature Plants 2 (2016): 16076.