On mushrooms, spores grow along the gills on the underside of the caps. The size varies, but a typical spore is about 10 microns, or 1/2,500th of an inch, in width, and it is attached at the end of a stalk called a sterigma. In a single day, a mushroom releases billions of spores.
If the spores were merely dropped, many of them would waft back into the parent mushroom and get stuck. “When a spore launches, it has to go far enough that it clears its apparatus,” said Anne Pringle, a professor of botany and bacteriology at the University of Wisconsin and a collaborator on the new research.
So a mushroom fires the spores away from the vertical gill — but not so far that they fly into the next gill over. The speed is not that fast — less than 10 miles per hour — and the distance is usually just a few hundred microns before air friction slows down the microscopic spores. But the acceleration is explosive, exerting thousands of times the force of gravity.
Scientists call spores launched in this manner ballistospheres.
At the same time they are traveling away from the gills gravity pulls them down and the spores catch a ride on air currents to spawn into new mushrooms elsewhere.
The energy for propelling the spores turns out to come from the surface tension of water — the forces that cause a drop of water to roll up into a bead on a water-repellent surface.
In his early observations, Buller noticed a tiny droplet next to a spore.
“There’s a point at the top of the sterigma, and it has one of the most poetic names in biology,” Dr. Pringle said. “It’s called the punctum lacryman, which means the point that cries. Something about it, either its texture or its chemistry, means that it accumulates water from the surrounding environment.”
Buller hypothesized that when the tiny sphere of fluid — it’s now called the Buller’s drop — touched the liquid on the spore, the two merged, releasing the surface tension energy and launching the spore.
“He had it very, very close,” Dr. Money said.
But the launching was so fast that no one knew for sure. Other scientists offered other ideas like squirty sterigmata, bursting bubbles and electrostatic repulsion.
More than a decade ago, Dr. Pringle and Dr. Money turned to ultrahigh-speed video cameras, capturing 100,000 frames a second, to fill in some of the blanks. Even that was not quite fast enough to capture all of the details of what was going on.
In the new experiment, polystyrene spheres were sliced in the shape of a spherical cap, mimicking the shape of a spore. A lens-shaped drop of water, with some ethanol mixed in to make it sticky enough to stay on the surface, was added on top of the flat side. Drips from an inkjet printer created the Buller’s drop next to it until it touched the liquid on the plastic spore.
The merging was still fast — less than a thousandth of a second — but slow enough to be studied. “When they coalesce, they actually get this bounce, which is precisely what we see in nature,” Dr. Money said.
The researchers also used computer simulations to show how the merging launched the spore at a right angle to the surface — the perfect direction for the spore to safely ride air currents.
“It’s gratifying, after so many years,” Dr. Chen said. “We finally saw how to explain this century-old puzzle of directionality.”
Mycologists now have a tool to study the process more exactly, varying the shape of the spore or the relative size of the Buller’s drop.
It could conceivably even have practical uses. Imagine a surface that cleans itself, flinging away any dirt particles that land on it.
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