In each sporangium there is a single layered jacket that encloses 12-16 diploid spore mother cells. A marginal row of jacket cells are differentially thickened to form annulus. The remaining marginal cells constitute stomium. The diploid spore mother cells divide meiotically to form haploid spores. With the maturity of spores, the indusium shrivels. The exposed sporangia dehisce in the region of stomium due to differential contraction of annulus. The spores are thrown away. They are dispersed by air currents.