Spores fired at 180,000 g

Pilobolus grows on the dung of herbivores, but its spores have to get away from that place. Grass right beside dung is avoided by grazing animals. If the sporangia fell next to the fruiting body, the life cycle would quickly break. The fungus solves the problem like a microscopic catapult: it aims the sporangium towards the light, builds up pressure and fires the package of spores onto fresh vegetation.

Measurements with high-speed cameras have shown initial velocities from a few up to about 25 metres per second. The highest calculated accelerations reached 180 thousand times the acceleration due to gravity, and the acceleration phase lasted a fraction of a millisecond. The record applies to a very small mass over a short distance, so it does not mean an enormous energy comparable to a bullet. What it does show is how abruptly pressure built up in cells can be released.

Beneath the transparent vesicle of the sporangium sits a swollen structure filled with fluid. When the boundary between its parts ruptures, the fluid propels the sporangium, and a sticky layer helps it adhere to a plant. Spores may fly more than two metres, thousands of times the length of the fruiting body itself. They are later eaten along with the grass, pass through the animal's gut and return to fresh dung.

This is a case where the point of a record becomes clear only in the ecology. Pilobolus does not "shoot" for the sake of speed. The whole construction combines phototropism, turgor pressure, aerodynamics and stickiness in one dispersal mechanism. The acceleration sounds cosmic, but the goal is a very earthly journey: from one tuft of grass into the digestive tract of a cow or a horse. The figure of 180 thousand g was the maximum within the range measured, not a constant property of every discharge. Giving the full range protects a record from turning into a false rule. Other microscopic fungi use different mechanisms, so the Pilobolus result cannot be attributed to all spores.

What the evidence actually shows

The 180,000-g value does not belong to Pilobolus, despite frequent captions saying so. Yafetto and colleagues used ultra-high-speed video on four dung-inhabiting fungi. They measured launch speeds of roughly 2–25 m/s and accelerations from 20,000 to 180,000 g; the maximum was recorded for the ascomycete Ascobolus immersus. Pilobolus kleinii, by contrast, launches an entire sporangium using pressure stored in its swollen stalk and orients the shot toward light. Such acceleration lasts only a microscopic fraction of a second and acts on an object of minute mass, so comparisons with human g-force tolerance are vivid but physiologically misleading. The biological purpose is practical: the launch carries spores through still air and away from the dung surface, improving their chance of landing on vegetation that may be eaten by another herbivore.