How basidiomycete fungi hurl millions of spores into the atmosphere has intrigued scientists for almost a century. Sitting atop a microscopic structure, the sterigma, each spore is immobile until a tiny drop of water,known as Buller's drop, condenses on it. Within seconds the spore is catapulted into the air, but how the droplet sends its spore flying wasn't clear. Physicist Xavier Noblin from the Université de Nice-Sophia-Antipolis explains that various theories had been suggested: maybe the droplet was actually a bubble and the spore blasted off when the bubble exploded; or maybe the droplet suddenly fused to the spore by surface tension,launching them both into the air. But no one had ever calculated the droplet and spore's energetics to find out how the droplet launches the spore on its way. Having joined Jacques Dumais' plant biomechanics lab in Harvard in 2004,Noblin was in the perfect place to apply his knowledge of the physics of droplets to the puzzling question of how fungi launch spores(p. 2835).

Knowing that the microscopic spores were fired off at a speed of about 1 m s–1, Dumais, Noblin and Sylvia Yang realised that they would have to use a super-fast high-speed camera to have a chance of seeing what happened to the droplet as the spore tore free of its launch site. Fortunately, Dumais had access to two cameras that could record movies at a remarkable 250,000 frames s–1, but it still wasn't clear if this would be fast enough to capture the droplet's movements. `We knew we would get the spore motion,' says Noblin, `but we weren't sure we'd get the droplet motion'. And the team had no control over when, and in which directions, the spores would take off, so having chosen a spore that looked as if it might be about to fly, the team would set the camera rolling and hope that they captured the moment of take off.

Amazingly, they did. After weeks of filming, the team's patience was rewarded when they captured the instant when the expanding droplet suddenly touched the banana-shaped Auricularia auricula fungus spore and fused to the depression in the spore's surface as it took off. The launch was over in less than 8 μs (two frames in the movie) and the trio could clearly see the spore tumbling as it flew through the air. Next the team filmed Sporobolomyces yeast spores, and saw exactly the same process as the droplet initially grew before contacting the spore and fusing to its surface to send the spore flying.

But how is a single water droplet able to send a spore rocketing from its launch site? Noblin and Dumais suspect that as surface tension fuses the droplet to the surface of the spore, surface tension energy is released, which powers the droplet's movement, smashing it into the spore, sending both of them flying. Noblin adds that the spore essentially `jumps off' the sterigma,powered by surface tension energy as the droplet collides with the spore.

So rather than develop complex catapult systems to hurl their spores into the atmosphere, some fungi have evolved a simple ballistic system that only requires water to produce one of the planet's most amazing microscopic ballistic performances.

Noblin, X., Yang, S. and Dumais, J. (
). Surface tension propulsion of fungal spores.
J. Exp. Biol.