Natures wonders: Ferns catapult spores (Introduction)

by David Turell , Wednesday, December 31, 2014, 19:57 (3613 days ago) @ David Turell

They use a very complex mechanism that does not require the usual stopping arm or bar seen in Mediaeval catapults:-http://blogs.scientificamerican.com/artful-amoeba/2014/12/31/wonderful-things-ferns-eject-their-spores-with-medieval-style-catapults/-"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."