Sorus
From Freepedia
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 sprongia mature, the indusium shrivels so that spore release is unimpeded. The sporangia then burst and release the spores.
The shape of the sori is often a valuable clue in the identification of fern taxa. Sori may be circular or linear, and marginal or set away from the margin on the frond lamina.
