The annulus of a fern is a special row of cells that bisects a stalked structure called the sporangium, similar to a spine. The purpose of this structure is reproduction, and it functions by acting as a catapult, flinging spores away from the parent plant. The annulus acts as the spring of the catapult.
Because drag is difficult to overcome for small particles in flight, the sporangium must fling its spores with a tremendous amount of force. The inner walls of an annulus cell are heavily reinforced, while its thinner outer walls are permeable to water. This water is drawn out of the annulus as the sporangium dries, causing the cells to shrink and buckle. This shortens the curved surface of the sporangium and pries it open, exposing the spores inside.
The thick inner walls of the annulus resist collapse as water evaporates, and eventually the water pressure is low enough for a gas bubble to form inside the cell. The formation of one gas bubble sets off a similar reaction in its neighboring cells. The air inside the annulus cells brings them back to their pre-evaporated shape; when this happens, they snap back into their original positions, flinging spores in all directions.