What are the structures on the underside of fronds called in which the spores of ferns are reproduced?

The structures on the underside of fronds where spores are produced are called sori (singular: sorus). Each sorus typically contains clusters of sporangia, which are the containers that produce and release spores. These structures often appear as small, brownish dots or patches on the frond's surface. In some ferns, sori are protected by a thin, membranous cover called an indusium.