The annulus of the fern sporangium helps in spore dispersal. The annulus is a specialized ring of cells that contracts and expands as it dries and absorbs moisture. This motion causes the sporangium to rupture and release the spores, which are then carried away by wind or other means to grow into new ferns. In summary, the annulus is an essential structure for the dissemination of spores, which is necessary for ferns' reproduction and survival.