As the indusium dries, it shrinks and the spore carrying structures, the sporangia, are exposed. Each sporangium can be seen to be surrounded by a ring (the annulus) of cells. As drying out continues, the sporangia lose water; this causes a tension to build up in the annular cells. Ultimately, the annulus ruptures and the spores inside are "shot" out.

If you leave a ripe fern frond, with its lower side down on a piece of paper for a couple of hours and then carefully remove the frond, you will often see the scattered spores in a ghostly fern frond pattern on the paper.

Alternatively, view the ripe sporangia under a stereo microscope and watch them explode, an exciting experience!