Explanation

In ferns, sporangium consists of stalk and capsule, later is filled with sporocytes, which undergo meiosis to produce haploid spores. The one layered wall of the capsule is thin and has a strip of cells called annulus. The cells of annulus have thickenings on the inner and radial walls but in some regions, its cells are thin walled. These regions are called stomium. Both annulus and stomium help in spore dispersion.