Concentrations of sporangia as found on the leaves of ferns.

Sori can be found as clusters on the underside of fern sporophyte leaves during certain times of the year. They appear as fuzzy dots.

The large size of ferns, a consequence in part of their being vascular plants, can be interpreted in part in terms of the advantage of a single sporophyte being able to display a great many sporangia, which happen to be arranged into sori.