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The Lid or Operculum

The upper part of the spore-case is in the leaf-bearing mosses usually thrown off as a lid (operculum). In order that the upper portion of the spore-case may be separated from the lower, either one of two things happens: The walls of a zone of cells of the spore-case wall, in the exterior layer, separates from the adjoining walls when the spore-case is mature, or a zone of cells consisting of one or more rows has the cell-walls modified so that when they are distended by absorbed moisture, the zone of cells is displaced as a ring or annulus and so frees the outer layer of cells in the upper part of the spore-case to form a lid.