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peristome
Peristome (from the Greek ''peri'', meaning 'around' or 'about', and ''stoma'', 'mouth') is an anatomical feature that surrounds an opening to an organ or structure. Some plants, fungi, and shelled gastropods have peristomes.
==In mosses==
In mosses, the peristome is a specialized structure in the sporangium that allows for gradual spore discharge, instead of releasing them all at once.
Most mosses produce a capsule with a lid (the operculum) which falls off when the spores inside are mature and thus ready to be dispersed. The opening thus revealed is called the ''stoma'' (meaning "mouth") and is surrounded by one or two peristomes. Each peristome is a ring of triangular "teeth" formed from the remnants of dead cells with thickened cell walls. There are usually 16 such teeth in a single peristome, separate from each other and able to both fold in to cover the stoma as well as fold back to open the stoma. This articulation of the teeth is termed arthrodontous and is found in the moss subclass Bryopsida. In other groups of mosses, the capsule is either nematodontous with an attached operculum (as in the Polytrichopsida), or else splits open without operculum or teeth.
There are two subtypes of arthrodontous peristome.〔Edwards, S. R. 1984. "Homologies and inter-relationships of moss peristomes", pages 658-695 ''in'' R. M. Schuster (Ed.) ''New Manual of Bryology''. (Japan: The Hattori Botanical Laboratory). .〕 The first is termed haplolepidous and consists of a single circle of 16 peristome teeth. The second type is the diplolepidous peristome fround in subclass Bryidae. In this type, there are two rings of peristome teeth—an inner endostome (short for ''endoperistome'') and an exostome. The endostome is a more delicate membrane, and its teeth are aligned between the teeth of the exostome. There are a few mosses in the Bryopsida that have no peristome in their capsules. These mosses still undergo the same cell division patterns in capsule development, but the teeth do not fully develop.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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