The first fronds produced in spring are sterile-that is, they lack reproductive structures. The fertile fronds arise later in the season. The fertile regions on fronds that bear sporangia are known as receptacles , and the group of sporangia on a single receptacle is called a sorus. In many species, but by no means in all, the sorus is covered during development by a flap of tissue called an indusium . The shape and arrangement of the sori are different in different species.
Spores arise by meiosis from sporocytes.
As the spores inside each sporangium mature, they typically get darker, until they are a deep brown or black. Not all spores mature to a deep brown or black, however. The indusium, if present, ultimately shrivels when the spores have matured, exposing the sporangia. The annulus breaks near the base of one side, tearing the sporangium apart, and arches backward. Then the annulus snaps forward abruptly, catapulting the spores into the air.