How mushrooms launch their spores at 12,000 G using nothing but surface tension — and why the mechanism may be helping to seed the very rain clouds that grow the forests they live in.
There is a moment, happening right now in forests across the world, that most people will never see. Beneath the cap of a mushroom, in the narrow spaces between its gills, thousands of microscopic spores are preparing to launch themselves into the air. They have no muscles. They carry no stored chemical energy. They cannot jump, flap, or spring. And yet each one will achieve an acceleration greater than a fighter jet — powered entirely by the physics of water.
This is Buller's drop: one of the most elegant and counterintuitive mechanisms in all of biology, first described by the mycologist Arthur Henry Reginald Buller in the early twentieth century, and only fully understood in the last two decades thanks to high-speed cameras and fluid dynamics modelling.
What follows is a complete breakdown of the mechanism — from the first hygroscopic sugar molecule to the moment the spore clears the gill and begins its journey into the atmosphere. And at the end, a finding so surprising that it reframes the relationship between fungi and the weather itself.
"Each spore achieves an acceleration greater than a space shuttle launch — powered entirely by the physics of water."
Who Was Buller, and What Did He Find?
Arthur Henry Reginald Buller (1874–1944) was a British-Canadian mycologist whose seven-volume series Researches on Fungi, published between 1909 and 1950, remains one of the most comprehensive works in the history of mycology. Among his many contributions, Buller was the first to systematically document and describe the hygroscopic droplet that forms at the base of fungal ballistospores — the spores that are actively discharged rather than passively released.
He observed that these spores, found across thousands of species of gilled mushrooms (Basidiomycota), are not simply shed by the mushroom. They are launched. And the launching mechanism, he noted, appeared to involve a small droplet of liquid at the spore's attachment point. The droplet was later named Buller's drop in his honour.
What Buller could not fully explain — because the technology did not yet exist — was precisely how that droplet powered the launch. That answer took another century to arrive.
Step 1: The Sugar Signal
Each ballistospore — the technical term for a spore discharged by this mechanism — sits on a slender stalk called a sterigma, attached to the gill surface of the mushroom. The spore's surface is not uniform. Two distinct patches are coated in hygroscopic compounds, primarily the sugar alcohol mannitol, along with smaller amounts of glycerol and other solutes.
These compounds are hygroscopic — they actively attract and absorb water molecules from the surrounding air. In the humid microenvironment between mushroom gills (relative humidity typically exceeds 95%), this process begins almost immediately after the spore matures.
The two patches are separated by a narrow hydrophobic strip — a water-repelling region that prevents the forming droplets from merging prematurely. This strip is critical: it is the biological equivalent of a safety pin on the catapult, holding the mechanism in a loaded state until the precise moment of release.
Step 2: The Sixty-Second Build
As the hygroscopic patches absorb moisture, two distinct droplets form. The first — Buller's drop — grows at the hilar appendix, the point where the spore connects to the sterigma. The second, called the adaxial drop, forms on the flat upper face of the spore.
Both drops grow simultaneously, but slowly. Research by Pringle et al. (2005) and subsequent high-speed imaging studies established that this growth phase typically takes around 60 seconds — a remarkably long preparation time for what will become an extraordinarily brief event.
The hydrophobic strip between the two patches keeps the drops separated throughout this entire period. The spore is, in effect, a loaded spring — accumulating surface energy with every additional water molecule, held in check by a single hydrophobic barrier.
"For about sixty seconds, the drops slowly grow. And then they touch."
Step 3: The Snap — Surface Tension as a Catapult
When Buller's drop finally grows large enough to bridge the hydrophobic strip and make contact with the adaxial drop, the event that follows happens in microseconds.
The two drops coalesce — merge — and in doing so, release the surface energy stored in their combined surface area. Because the coalescence is asymmetric (the spherical Buller's drop merging with the flatter adaxial drop), the energy release is directional. The merged water mass rapidly shifts position on the spore, moving the spore's centre of mass in a fraction of a millisecond.
This centre-of-mass shift is the catapult. The spore is flung away from the sterigma at approximately 1.2 metres per second, achieving a peak acceleration of 12,000 G — a figure first calculated by Nicholas Money in 1998 and later confirmed by high-speed imaging. For context, a fighter jet pilot experiences around 9 G in a sharp turn.
The launch distance is deliberately short — just 0.1 to 2 millimetres. This is precisely enough to clear the gill surface and enter the air current flowing between the gills, where the spore can be carried upward and outward. A longer launch would risk the spore hitting the opposite gill wall; a shorter one would leave it trapped. This mechanism is calibrated to extraordinary precision.
The Mind-Blow: Mushrooms and Rain
The story of Buller's drop does not end at the gill surface. After launch, the spore enters the air current and begins to rise. As it does, something remarkable happens to the mannitol coating that powered its launch.
The water that formed the two drops evaporates — but the mannitol remains, now spread across the entire spore surface. This coating continues to be hygroscopic. As the spore rises into the atmosphere, it continues to absorb moisture from the air around it.
In a landmark 2015 study published in PLOS ONE, Hassett, Fischer, and Money demonstrated that mushroom spores — because of this hygroscopic coating — can act as exceptionally powerful cloud condensation nuclei (CCN). CCN are the microscopic particles around which water vapour condenses to form cloud droplets, and eventually, raindrops.
The authors found that mushroom spores were particularly effective CCN — more so than many other biological particles — because of the hygroscopic compounds on their surface. Fungi release an estimated 50 million tonnes of spores into the atmosphere every year. If even a fraction of these are acting as CCN, the contribution to global rainfall patterns could be significant.
The implication is almost poetic: mushrooms grow in forests. Their spores help seed the rain clouds that water those forests. The forests shelter and feed the fungi. A closed loop, powered by surface tension and mannitol, operating at scales from the microscopic to the planetary.
"Seeding the very rain that grows the forests that grow the mushrooms."
Why This Matters Beyond the Lab
Buller's drop is not merely a curiosity of mycology. It is a window into the broader intelligence of the fungal kingdom — a kingdom that has been using complex engineering mechanisms since time memorial.
The Buller’s drop mechanism requires no nervous system, no muscles, no active energy expenditure. It is a passive system that harvests energy from the environment — specifically, from the humidity of the air — and converts it into kinetic energy with extraordinary efficiency. Engineers studying micro-scale propulsion systems have taken note.
For those of us interested in functional fungi — in what the mushroom kingdom has to offer beyond spore dispersal — Buller's drop is a reminder of the depth of biological sophistication that exists in organisms we are only beginning to understand. The same kingdom that builds the Wood Wide Web underground, that has been used in traditional wellness practices for thousands of years, that is now at the frontier of materials science and medicine, also invented a microscopic catapult powered by water. Mushrooms had this figured out long before we had the physics to explain it.
Scientific References
All factual claims in this article are supported by peer-reviewed scientific literature. References are listed in order of relevance to the article's narrative.
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Buller, A.H.R. (1909–1950). Researches on Fungi (7 volumes). Longmans, Green & Co., London. The foundational work describing the hygroscopic drop mechanism, later named 'Buller's drop' in his honour.
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Money, N.P. (1998). More g's than the Space Shuttle: ballistospore discharge. Mycologia, 90(4), 547–558. https://www.tandfonline.com/doi/abs/10.1080/00275514.1998.12026942 Established the extraordinary acceleration values (up to 12,000 G) achieved during ballistospore launch.
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Pringle, A., Patek, S.N., Fischer, M., Stolze-Rybczynski, J., & Money, N.P. (2005). The captured launch of a ballistospore. Mycologia, 97(4), 866–871. https://www.jstor.org/stable/3762235 First high-speed photographic capture of the ballistospore launch event, confirming the surface tension catapult mechanism.
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Noblin, X., Yang, S., & Dumais, J. (2009). Surface tension propulsion of fungal spores. Journal of Experimental Biology, 212(17), 2835–2843. https://journals.biologists.com/jeb/article/212/17/2835/18550/Surface-tension-propulsion-of-fungal-spores Detailed biophysical analysis of the surface tension forces involved in spore launch, including velocity measurements of ~1.2 m/s.
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Stolze-Rybczynski, J.L., Cui, Y., Stevens, M.H.H., Davis, D.J., Fischer, M.W.F., & Money, N.P. (2009). Adaptation of the spore discharge mechanism in the Basidiomycota. PLOS ONE, 4(1), e4163. https://pmc.ncbi.nlm.nih.gov/articles/PMC2612744/ Examined how the Buller's drop mechanism has adapted across diverse basidiomycete species.
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Fischer, M.W.F., Stolze-Rybczynski, J.L., Cui, Y., & Money, N.P. (2010). How far and how fast can mushroom spores fly? Physical limits on ballistospore size and discharge distance in the Basidiomycota. Fungal Biology, 114(8), 669–675. https://www.sciencedirect.com/science/article/pii/S1878614610000875 Established the physical constraints on launch distance (0.1–2 mm) and the relationship between spore size and dispersal.
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Liu, F., Chavez, R.L., Patek, S.N., Pringle, A., Feng, J.J., & Chen, C.H. (2017). Asymmetric drop coalescence launches fungal ballistospores with a twist. Journal of the Royal Society Interface, 14(132), 20170083. https://royalsocietypublishing.org/rsif/article/14/132/20170083/64833 Revealed that asymmetric coalescence of the two drops imparts a rotational component to the launch trajectory.
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Hassett, M.O., Fischer, M.W.F., & Money, N.P. (2015). Mushrooms as rainmakers: how spores act as nuclei for raindrops. PLOS ONE, 10(10), e0140407. https://pmc.ncbi.nlm.nih.gov/articles/PMC4624964/ Demonstrated that mushroom spores, coated in hygroscopic compounds including mannitol, can act as giant cloud condensation nuclei — potentially contributing to rainfall.
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Ingold, C.T. (1939). Spore Discharge in Land Plants. Clarendon Press, Oxford. Early foundational text identifying surface energy transfer as the key step in ballistospore release.
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Money, N.P. (2023). Progress in understanding the mechanism of ballistospore discharge. Fungal Biology Reviews, 44, 100293. https://www.sciencedirect.com/science/article/pii/S1878614623000016 The most recent comprehensive review of the ballistospore discharge mechanism, summarising advances in understanding from high-speed imaging and fluid dynamics modelling.