operculum

1. The membranous cap covering the *peristome in the undehisced capsule of many moss sporophytes. It is displaced either by the pressure that builds up within the capsule or by the swelling of the cells of the annulus.
2.  A lid covering the aperture in many pollen grains, which is pushed aside by the emerging pollen tube.
3.  A cap covering the ostiole in certain ascomycete fungi.