Sori

Sori are clusters of sporangia on the undersides of fern fronds. They are the main spore-producing structures of ferns and function in their life cycle by generating and dispersing spores that give rise to the gametophyte generation.

A sorus (singular) usually consists of many sporangia grouped in a defined patch. Many sori are shielded by a protective covering called an indusium; depending on species, the indusium is umbrella-like, kidney-shaped, or cup-shaped, and may be absent in others. In some ferns the edge of the leaflet rolls over the sorus to form a false indusium.

During development, each sporangium undergoes meiosis to produce haploid spores. When ripe, the sorus dehisces and releases spores, which are dispersed by air currents. If a spore lands in a suitable moist environment, it germinates into a small, heart-shaped gametophyte called a prothallus, which contains the male and female reproductive organs. Fertilization occurs on the prothallus and produces a new sporophyte, the fern frond.

Sori arrangement, size, and the presence or absence of an indusium vary widely among ferns and are used in taxonomic identification. They can appear as discrete patches along veins or along the leaf margins, and may color the frond before or after release of spores.

Sori are a defining feature of ferns, distinguishing them from other plant groups that reproduce by seeds or cones.