Ballistospore basics

After more than 30 years working as an educator I still derive great pleasure from learning new and interesting things. Let me be more precise, what I meant was that I derive great pleasure from figuring out new things. I find freakish pleasure in reading a piece of primary literature which, at first go, makes little sense and to then take the time to rework the thing until I understand it. While doing some research in support of my previous post about the life cycle of Basidiomycete fungi, I came across a couple of fascinating papers concerning the ballistic discharge of fungal spores. Perhaps you have not thought much about the dispersal of fungal spores; they are very small, averaging just one micron in length (a µm is equal to 1/1,000,000 of a meter). At this scale particles are influenced more by surface tension and less so by gravity. [There’s nice mathematical proof of this scaling effect which we’ll forego for the moment.] So what? you may ask. Well, because they are so small and so prone to the influence of surface tension, when spores are released they tend to stick to other spores or to the adjacent surface of the fungus rather than disperse into the atmosphere … unless, of course, they are released explosively. Take a look at this video clip which demonstrates the surface tension of a water droplet being cut with a hydrophobic (water repelling) knife.

Connections between and among water molecules at the surface of the drop are tenacious and able to resist the knife edge for quite some time until they give way. The phenomenon of surface tension is a fascinating one. Picture a single water molecule at the center of a drop of water, because this molecule is surrounded by a number of other water molecules it shares its cohesive forces equally among them. Water molecules at the outside of the drop however are not surrounded by other molecules beyond the limits of the drop and because these have fewer molecules with which to share their cohesive forces they share proportionally more force with a smaller number of molecules – each connection is therefore stronger at the skin of the drop than on the inside. These tenacious forces at the surface of a liquid film or droplet are responsible for the phenomenon of surface tension. Each fungal ballistospore, as it is called, is perched on top of a pedestal called a sterigma and the thin connection between this and the spore is called the hilum. A second or two before dispersal, a droplet called Buller’s drop forms at the very bottom of the spore. At the same time a film of water forms higher up on the surface of the spore itself. As Buller’s drop gets larger its increase in weight has the effect of lowering the center of gravity of the spore and this puts the hilum under tension as the connection resists compression. Buller’s drop then merges with the film. As coalescence occurs the cohesive forces of the droplet and the film combine to drive the fluid mass across the spore. This happens so quickly that the momentum generated is enough to break the hilum and send the spore flying into the distance. The scientists who described this process (Noblin, Yang, and Dumais) did a fine job of drawing an analogy between these actions and those associated with jumping. One bends at the knees to lower the center of gravity and to allow the legs to work on the substrate. As the legs unfold, forces of rotation about the joints are resisted by the substratum and this provides the impulse required to accelerate the body. [In the interest of accuracy and correctness I should report that one of my most dedicated followers, Elke, responded with two important points (see comments to this post). First, that the ultimate source of energy in our jumper is, of course, the cellular bond energies of ATP. And second, that the propulsive force of the spore is derived from the energy released as the droplet and film coalesce thus reducing the total surface of water in comparison to starting conditions. Thanks Elke for keeping me honest!] I’ve included some figures from the research which describes these findings, the one on the left shows the stages of spore activity and the parallel of each to the phases of a jump. The one on the right is a series of images showing the launch of a spore taken at 250,000 frames per second. Analyses determined that spores are ejected at a speed of 1 meter per second and an acceleration of 12,000 g. Don’t be impressed … this rate of acceleration is all but eclipsed by the 40,000 g recorded for the cnidarian nematocyst. Stay tuned.

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