Fern sorus. Fluorescence light micrograph of a sorus on the underside of a fern frond. A sorus is a cluster of spore-producing receptacles called sporangia. Each sporangium is packed with spores, which are the fern's reproductive cells. A rim of thick-walled cells, the annulus, functions to open and rapidly close each sporangium when the spores are mature, releasing them into the environment.