Many of these mechanisms
use the same physical principles as man-made devices
but often achieve better performance. One
such dispersal mechanism is the cavitation-triggered
catapult of fern sporangia. The sporangia open
when dehydrating and use the stored elastic energy
to power a fast closure motion that ultimately
ejects the spores. The beauty of this dispersal mechanism
and its similarity with medieval catapults
have not escaped notice (1). All man-made catapults
are equipped with a crossbar to stop the motion
of the arm midway. Without it, catapults would
launch their projectiles into the ground. This
crossbar is conspicuously missing from the sporangium,
suggesting that it should simply speed up
to its closed conformation without ejecting the
spores. We show that much of the sophistication
of this ejection mechanism and the basis for its
efficiency lie in the two very different time scales
associated with the sporangium closure