A Closer Look at Spore Dispersal

In the infographic I made about fungi, I mentioned that mushrooms could eject their spores at a whopping 30,000 G. These accelerations that aren't seen this side of the arguably more exciting railgun. The question of how the mushroom achieves this puzzled researchers for a long time, going back to the mycologist Buller in the 1930's. This problem was in part due to the fact that the process happens so quickly can't easily be caught on camera.  Let's compare:

Using the traditional 24 frames per second videography, the spore might as well have disappeared. But that's okay! As scientists, we have access to much more advanced technology. Click here for another attempt, this time at 20,000 frames per second.

That was just another disappearing act, so to see what's going on, you need an even higher speed: 100,000 frames per second. This is finally fast enough to see what's happening. To break it down, what you're seeing is the spore resting on the sterigma, a pointy appendage of the basidium (the club-shaped cell that makes spores). Below the spore is a droplet of water, named the Buller's drop after Arthur Henry Reginald Buller, the mycologist who first observed it.

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The Buller's drop is balanced on a small projection of the spore called the hilar appendage. In order to understand what happens next, we need two facts: first, that water droplet is a substantial fraction of the spore's weight. Second, the spore is coated in sugars that are very hydrophilic, or water-attracting. It's hard to catch even at these high speeds, but what's happening in that tiny fraction of a second before the spore takes off is that the Buller's drop is just grazing the outside of the spore and coming into contact with those hydrophobic sugars.  These sugars pull the droplet off the hilar appendage, causing it to gain momentum and move toward the top of the spore. This sudden change in balance sucks the spore off the sterigma and launches it into space.

One way to think of what's going on in this video is to think of the Buller's drop as being like a rubber band under tension by a pair of fingers. Releasing the rubber band at one end causes it to contract toward its other end, but the force of the contraction causes the whole band to spring forward. Spores are so elegantly constructed that this contraction happens in an incredibly short period of time, multiplying the magnitude of the affect. 

Sadly, however, even though the spore accelerates at such an impressive rate, its mass is much too low for it to punch a smoking hole in its parent mushroom and exit the atmosphere (if they could, fungi probably wouldn't reproduce this way). Air resistance kicks in almost immediately and slows the spore back down so it can peacefully exit the cap and be picked up by the surrounding air currents.

The inspiration for this article and the high-speed videos of spore dispersal are both from Pringle et al.'s The captured launch of a ballistospore. More videos are available here.