Annulus All Bent Out of Shape

Fern Roof (Kathrin Marks)

Imagine that you're a spore nestled on a leaf in a sleepy forest. It's a dry, sunny day. All of a sudden - within 0.00001 seconds, you are flung into the air with an acceleration of 100,000 times the force of gravity. What happened?

The play by play of this extraordinary voyage is now explained:

The whole annulus is thus bent out of shape, much like an accordion in the hands of a musician.

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.

It's as if inside each fern is a spore sack mimicking a medieval catapult (or is it the catapult mimicking the spore?) to launch each spore with maximum velocity to begin the next generation.

A dozen cells placed in a row can fulfill all the functions of a medieval catapult, including the motive force for charging the catapult (water cohesion), energy storage (annulus wall), triggering mechanism (cavitation), and returning motion arrest (poroelastic behavior of the annulus wall).

Pretty clever!