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American Association for the Advancement of Science

Timing is key to fern's spore-throwing catapult

The sporangium of Polypodium aureum during sporangium opening.
[Image courtesy of Science/AAAS]

If you've ever been hiking, chances are you've seen fern plants in the woods. Nestled under fern leaves are tiny capsules chock-full of spores, tiny life vessels which, like seeds, are used for dispersal. Fern plants launch their spores with tiny catapults. Once in the air, wind and air currents can take the spores around the world.

A new study appearing in the March 16 issue of the journal Science takes a closer look at the mechanics of the fern's catapult structure, showing that the precise timing of the catapult's closure allows the ferns to launch their spores at high speeds.

First, the capsules containing the spores open as they dry out. A row of about a dozen cells called the annulus wraps around the capsule. (Under a microscope, the annulus looks like a coiled worm.) As it continues to dry, the annulus snaps forward and launches the spores in a manner similar to a catapult.

Yet, there is one mystery to the fern's catapult mechanism. All man-made catapults are equipped with a crossbar to stop the motion of the arm midway; without it, catapults would launch their ammunition right into the ground.

In their experiment Xavier Noblin and colleagues show how ferns successfully launch their spores without a crossbar. They found that the sponge-like structure of the annulus wall leads to two different closing time scales. The first time scale of the closing catapult is inertial, meaning that the elastic energy stored in the annulus wall is converted into kinetic energy in just a few tens of microseconds.

The first snap occurs so quickly that the water permeating the wall has not conformed to the annulus' new shape. The flow of water through the small pores in the wall makes up the second, much longer time scale (tens of milliseconds).

These two distinct time scales result in a sudden braking of the catapult midway in the closing process, thus allowing the spores to be ejected at a velocity of 10 meters per second. That's fast!