Fern sorus and spore cases, fluorescence light micrograph
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 (blue and green). Each sporangium is packed with spores (brown), which are the fern's reproductive cells. A rim of thick-walled cells, the annulus (green/yellow), functions to open and rapidly close each sporangium when the spores are mature, releasing them into the environment.
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