Fern sporangia in sorus, light micrograph. Sporophyte is a dominant phase of life cycle in vascular seedless plants like ferns. Spore producing process takes place in sporangium. Sporangia are born on lower side of fern leaves and are arranged in clusters called sori (sing. sorus). In mature sporangium, special layer of cells with unevenly thickened walls called annulus (orange) helps in sporangium dehiscing (splitting of plant structure along in built-weakness) and spore releasing. This sample has been imaged using autofluorescence under ultraviolet (UV) light. Magnification: x65 when printed 10 centimeters wide.