Author’s note: This is the latest post in the Wonderful Things series. You can read more about this series here.
One of the more under-appreciated and ingenious machines evolved by plants is the cavitation catapult of leptosporangiate ferns. If that sounds exciting and mysterious, that's because it is.
This is a leptosporangium, where the fern makes its reproductive cells called spores:
"ONF Fig 02 - Sporangium of Polypodium vulgare" by Lucien Marcus Underwood - Our Native Ferns and their Allies Edition 6, 1900. Licensed under Public Domain via Wikimedia Commons.
You'll notice this one has been broken open and some of the spores formed inside have fallen out. Note also the little striated ring around the upper right, called the "annulus". It gives the spore capsules the appearance of wearing a Roman Centurion's crested helmet.
The bottom of this structure is attached to the underside of a fern frond, and it is usually found in a cluster of dozens just like it. Here's the way those clusters look on the underside of one fern:
"Fern spores P1180804". Licensed under CC BY-SA 3.0 via Wikimedia Commons.
If you are a stationary being such as a plant or a fungus, it's in your best interest to devise a method of evicting your offspring in the most expedient and efficient way possible. For ferns with leptosporangia, that means launching them in the manner of a boulder addressed to the nearest crusader castle. Just have a look:
The narrator of this video, a co-author of a 2012 study in Science describing the same findings, mentions that the catapult is launched by a phenomenon called cavitation that deserves a little more explanation. When bubbles of gas form spontaneously due to pressure changes in a liquid, that is cavitation. If you've watched a lot of submarine movies like me, you know that cavitation is a bad thing because a cavitating propeller makes noise that can give away your position to the Russkies. That's the swift movement of the propeller blades produces very low water pressure along their edges, creating cavitation bubbles in poorly-designed propellers or propellers driven badly or too quickly. These bubbles collapse once the low-pressure zone passes and they experience normal ocean pressure. The stream of collapses produces noisy shock waves that can be picked up by passive sonar.
In plants, the water pressure usually reaches the low levels that can cause cavitation when plants are thirsty but there's nothing to drink. In wilting or drought-stressed plants that can't draw more water from the soil, this can cause big problems. As the water pressure reaches critically low levels inside the plant's water-conducting vessel cells, they cavitate, forming bubbles in the cells that impede further flow of fluids. The cavitation inside trees may be so noisy that you can hear it with microphones on hot, dry days.
In the ridged compartments of a leptosporangium's annulus, evaporation through the thin transparent walls between the rings also produces the low pressure environment conducive to cavitation. But because a fern leptosporangium is a single-use device, cavitation here is not only not destructive, it is desirable and essential. When the bubbles burst into being, the annulus suddenly relaxes like a bow string loosed by an archer, providing the burst of acceleration necessary to carry spores into adjacent zip codes.
But the spores would just get slammed into the bottom of the sporangium by the annulus without some way to halt its motion mid-swing. The narrator points out that every respectable medieval catapult contained a crossbar for stopping the arm and launching the payload. You have no doubt noted that the fern leptosporangium lacks such a device. So what halts the rebound of the annulus?
The answer is viscosity. The paper explains the effect using a lesser-known relation called "Darcy's Law". The walls of the annulus are especially thick and spongy. Water moving through the walls as the annulus springs back is subject to a lot of viscous drag. This drag is created by the difference in the speed of water moving next to walls (slow) and that moving farther away from them (faster), which induces internal friction in the water molecules moving at different speeds.
Because there are so many tiny pores (and hence more wall surface area) in the cellulose of the annulus, the viscous drag is great, dissipating the energy of the rebound and halting the catapult arm. The spores, encountering no similar resistance to their motion, continue off into the wild blue yonder, with any luck landing on a patch of fertile, fern-free ground.
I'll leave you with a music video featuring firing fern sporangia so you can observe this fascinating phenomenon some more. To me, it looks a bit like the fern is engaged in a spore-snowball fight with itself. Note that stray spores that fall from the sporeballs as they go sailing past get stuck to the plant and cover of the sporangia. It's good, clean, messy botanical fun.
Noblin X., J. Westbrook, C. Llorens, M. Argentina & J. Dumais (2012). The Fern Sporangium: A Unique Catapult, Science, 335 (6074) 1322-1322. DOI: http://dx.doi.org/10.1126/science.1215985