When you flip the mature blade of a fern over to observe the underside (abaxial side), you often find some brown patches. Each patch is a sorus (plural form: sori) which is a cluster of sporangia.

The sorus is sometimes covered and protected by an indusium, a flap-like tissue arising from the leaf blade. In addition to its protective function for the developing sporangia, indusia may function to control the release of spores.