Sori are clumps of sporangia that hold the reproductive spores. They are found on the underside of fern fronds.
Dr Leon Perrie
The sporangia is the structure which produces spores. In ferns, the sporangia are usually aggregated into particular bigger structures. If you turn over a fern frond and you might see lines, and those are aggregations of the sporangia.
Video: © University of Waikato