Sporangium

The reproductive structures on the underside of the frond. Each sporangium is a capsule that contains spores. They are usually aggregated into clusters called sori. The position and arrangement of the sporangia are very important for the identification of ferns. Fronds that have sporangia on their underside are fertile, and those that don't are sterile.

Image acknowledgement: Museum of New Zealand Te Papa Tongarewa