The peristome is located around (peri-) the mouth (-stome) of the moss capsule, a structure which contains the spores. For the past 200 years, peristome characteristics have played an important role in defining major groups of mosses. The peristome shown here has a unique morphology and is an identifying feature for the Timmiaceae. This scanning electron micrograph of Timmia megapolitana has been colored to highlight the two layers of the peristome. The endostome (inner layer, colored in orange) consists of a membrane that is topped by 64 filaments, while the exostome (outer layer, colored in yellow) consists of 16 large teeth. These teeth have the ability to move in response to humidity, thus opening and closing the mouth of the moss capsule. This movement facilitates the release of spores (colored in green) under optimal dispersal conditions.
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