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Teeth or Peristome

When the lid falls, as a rule, one or two rows of teeth are discovered. They are the remains of the cell-walls lying just within the layer which separated as a lid. (See diagrams on page 52.)

If the outer walls of this layer of cells become thickened and split from the summit downward, but one row of teeth will be formed; if the inner walls as well, become thickened, and only the side walls break down, then an inner row of teeth or cilia will be formed. All the varied forms of teeth are determined by the portions of the cell-walls which remain.

In the genus Georgia, after the outer layer of the upper part of the spore-case has fallen away as a lid, the whole inner layer splits into four triangular valves which form the teeth.

In Fontinalis antipyretica the inner peristome forms a lattice work due to the breaking down of the inner cell-wall faces and the retention of the side walls.

The columella of many mosses shrivels up and disappears when the spores are ripe; in some cases it remains attached to the base of the spore-case when the lid falls; in some cases it is severed from the base and remains attached to the lid.

In Polytrichum mosses it remains standing in the spore-case and retains at its summit a plate of cells (epiphragm) in the form of a thin membrane to the rim of which are attached the tips of the teeth.

The structure of the peristome in the Polytrichum mosses is entirely different from that of other mosses. The teeth are composed of bundles of thickened fibrous cells arranged in crescent form. The ends of the crescent point upward and are united with the adjacent ends of the bundle next to it. On the inner face of each tooth is a growth of cells extending inward as a thin blade; the tip of each tooth is connected with a thin, papery membrane which covers the opening of the spore-case. From the under surface of this membrane, processes like little curtains hang down and in the young stages of the spore-case reach the basal membranes, so that the case is completely closed. Later, when the spores are mature, the "curtains" shrink away from the basal membranes and leave little holes between the teeth for the exit of the spores.

In the species of Catharinea the epiphragm is not attached to the apices of the teeth, but hangs from them by processes which at first exactly line the inner surfaces of the upper parts of the teeth. As the spores within the spore-case mature they exert a pressure upon the under surface of the epiphragm which causes it to rise and at the same time to peel upward the lining of each tooth. As the linings of the teeth are torn away, they curve upward and inward until they lie against the under surface of the epiphragm, which then appears to rest upon the tips of the teeth. When in this position, the tiny spaces between the teeth open into the spore-case and through them spores may escape.