Sorus

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Sori redirects here. For the commune in Italy, see Sori, Italy.

Sori on the underside of a curling fern.

The underside of a fertile frond of Dicksonia antarctica. Each dot is an individual sorus.

Sori with indusia at different stages of development.

In ferns, a sorus (pl. sori) is a cluster of sporangia on the edge or underside of a fertile frond. In many species, they are protected by an umbrella-like cover called the indusium.

Sori occur on the sporophyte generation, the sporangia within producing haploid meiospores. As the sporongia mature, the indusium shrivels so that spore release is unimpeded. The sporangia then burst and release the spores.

The shape, arrangement, and location of the sori are often valuable clues in the identification of fern taxa. Sori may be circular or linear. They may be arranged in rows, either parallel or oblique to the costa, or randomly. Their location may be marginal or set away from the margin on the frond lamina. The presence or absence of indusium is also used to identify fern taxa.

In fungi and lichens, a sorus is a cluster of sporangia surrounded by an external layer