Ballistospore Discharge: Buller's Drop Catapult

Discharge Parameters

Spore radius 2.5 µm

Surface-energy → KE efficiency 20%

Gill gap width 450 µm

Gravity 1.0 g

Live Readouts

Launch velocity v₀

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Peak launch accel.

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Horizontal reach x∞

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Discharge success

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Adjust the sliders, then watch spores launch from the hymenium.

How it works

Gilled fungi (Basidiomycetes) fire their spores using Buller's drop: a hygroscopic droplet condenses at the spore's hilar appendix. When it touches the spore surface the two coalesce, and the surface energy released acts as a catapult — no muscle, no pressure pump.

Surface energy released:  E = γ · 4πr_d²
Launch kinetic energy:    KE = η·E
Launch speed:             v₀ = √(2·KE / m)
Post-launch drag (Stokes, low Re):
  τ = m_s / (6πμr_s)         (velocity time-constant)
  x(t) = v₀τ(1 − e^(−t/τ))   (horizontal, decays to x∞ = v₀τ)
  v_y(t) = gτ(1 − e^(−t/τ))  (vertical, driven by gravity)

Spore radius — mass grows as r³ but the drop's released energy only grows as r_d², so bigger spores get proportionally less launch speed: a real upper limit on ballistospore size.
Efficiency — the fraction of surface energy that becomes kinetic energy (the rest heats the drop/spore or is lost to internal oscillation).
Gill gap — the spore must clear roughly half the gap (x∞) without hitting the opposite gill, and must travel far enough to escape the sub-micron no-slip air layer clinging to the gill surface — set it too narrow and launches collide with the neighboring gill.
Gravity — the horizontal catapult stage is gravity-independent, but the *fall* through the gap and out of the cap is not: drop gravity to zero and spores drift forever without ever leaving the hymenium, which is why gills must hang vertically for spore release to work at all.

Real basidiospores launch at roughly 0.5–2 m/s in about 2 microseconds — tens of thousands of g of acceleration — then lose almost all horizontal speed within ~100 µs to air drag, landing only a fraction of a millimeter away before gravity pulls them clear.

Successful discharge Failed / collided Spore print

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