Wednesday, August 24, 2016

Ballistic Fungi Use Surface Tension to Create Extraordinary Accelerations

Put a droplet of water on the table. Wet your finger and then, observing closely, touch your finger to that water droplet, and watch as the water on the table joins the droplet on your fingertip. It's a mundane process, but this humble mechanism is powerful enough to create some of the strongest accelerations on Earth.

Seed and spore dispersal is one of the main challenges faced by living organisms, and has been for practically the entire history of life on Earth. For a big, leafy tree's offspring to flourish, it just won't do to have the next generation sitting in their mother's shade—they need space and sunlight. Conversely, a parent doesn't usually want a brood of offspring hanging around close to home, sucking up resources. (Kids living in their folks' basements is apparently a universal problem.)

The ancient and ubiquitous nature of this challenge has made for an amazing variety solutions—grasses grow tall so that, when their brittle stalks are stepped on or blown over, the seeds can land far from their parents. The filaree, otherwise known as the red-stem stork's bill, has a sort of elastic catapult that can fling seeds more than a foot away when the pod is ripe. Fruiting plants, of course, take the game to a whole new level, employing animals to do the seed dispersal for them and offering a tasty reward as payment in advance. None compare, however—in terms of ingenuity—to the ballistospore fungi.

Ballistospore is a designation that refers to over 30,000 different species of fungus, all of which share a common method of spore dispersal. As you might have guessed from the name, it's a rather violent one, launching spores with an acceleration of up to 10,000 g—one of the most immense values known in biology. So how does a totally stationary organism like a fungus create 400 times the G-force necessary to kill a human? The answer involves a clever use of surface tension, taking advantage of water's unique properties to create a microscopic catapult with no moving parts.

Image Credit: Pringle, et al.
When a mushroom's spore is mature and ready to set out on its own, a condensation process begins. A small amount of water accumulates on the side of the spore, which grows at the tip of a conical structure called a basidium, located among the gills of a mushroom's cap. At the same time, another droplet—called Buller's drop—forms, also attached to the tip of the basidium. When these two drops grow large enough, they come into contact, and it's this contact that launches the spore.

This is where our little desktop experiment from earlier comes into play. The motive force that flings the spore free of its perch—the surface tension of these droplets—is the same one that pulls the table's drop up to wet your fingertip. So how does such a seemingly small force create such an incredible acceleration? The answer is in Newton's 2nd law of motion.

The law, as you might remember, comes in the form of a simple equation: F=m*a: force is equal to mass times acceleration. So, although the force of surface tension in these droplets seems small to you and me, it's extraordinary compared to the mass of a spore—which usually weighs on the order of 17 billionths of a gram. With a payload that small, even a modest amount of force is enough to create truly incredible accelerations.

Unfortunately for any human parents fed up with their homebound offspring, that means the process doesn't scale—it'll take more than a few drops of water to catapult those kids to independence.

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