The fastest known catapult in the plant kingdom is probably the fern's spore dispersal system. The parallels between the fern's catapult (a pod called the sporangium) and siege engines of yore go beyond the superficial. There are three physical requirements of a catapult – be it a medieval onager (a siege engine catapult) or a sporangium – for it to be effective. First, there must be a way of storing energy in the catapult, loading the ‘spring’. Second, there must be a trigger: something to release all of the stored energy simultaneously. And finally, there must be a brake: something to stop the ‘arm’ of the catapult at a specific point to send the projectile on its desired trajectory and prevent the arm from crashing into the catapult frame.
Humans can design each of these three features in a siege engine catapult. But how does a fern build such a good catapult without the advantages of engineering? Coraline Llorens at the Nice Sophia Antipolis University, France, and a group of international colleagues used the comparison with a siege engine to explain the mechanisms meeting each of the three requirements of the fern's sporangium.
In an onager, the arm is loaded using torsion: the operators twist rope, storing energy in the twists. If the rope were too difficult for the operators to load, the catapult would not work: there must be correspondence between the force applied in loading and the stiffness of the material being loaded. In a fern, the sporangium is loaded by deformation of a row of cells called the ‘annulus’ along the midline of the capsule. Evaporative water loss causes the annulus to bend backwards, storing energy in the bend. Llorens and her colleagues found that the annulus is precisely tuned to deform in response to negative pressure generated by evaporative changes in volume.
The trigger in a fern comes in the form of virtually simultaneous implosion of the annulus cells. This occurs when evaporation causes the negative pressure inside the cells to pass a threshold. This ‘cavitation’ partially closes the sporangium and casts out spores in mere microseconds. But cavitation must be coordinated; if the cells implode too early (before the annulus is loaded) or independently, the catapult would fail. Llorens and colleagues demonstrated that as each cell in the annulus nears threshold pressure, it causes the pressure in the neighboring cells to drop; thus, multiple cells reach the threshold pressure simultaneously.
With an energy storage mechanism and a trigger, a sporangium can launch spores. But it also needs a brake if it wants to avoid spores crashing back into the sporangium itself – and the fern does. The motion of water within the porous annulus cell walls dissipates any remaining stored energy. The movement of water takes considerably more time than the cavitation trigger process, so the sporangium only slows to rest after the spores have been successfully hurled at an appropriate trajectory.
Even though medieval catapults and the sporangium are built in completely different ways, they still share the same underlying mechanical principles. By considering the fundamental requirements of the sporangium, Llorens and colleagues have shown the neat integration of multiple biomechanical systems, to hurl spores with unparalleled speed.