The reproductive structures of the ferns and fern allies. They are miniature sacks or capsules that produce the dustlike spores that are the "seeds" by which ferns are propagated. Several sporangia grouped together are called a Sorus. The arrangement of sporangia varies greatly, most ferns having their sporangia on the underside of the frond, arranged in an organized pattern usually associated with veins in the pinnule (leaf). Many times (but not always) the ferns provide a protective covering for the Sorus called an Indusium.