The word
peristome is derived from the
GreekGreek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...
peri, meaning 'around' or 'about', and
stoma, 'mouth'. It is a term used to describe various anatomical features that surround an opening to an organ or structure. The term is used in
plantPlants are living organisms belonging to the kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The scientific study of plants, known as botany, has identified about 350,000 extant species of plants, defined as seed plants,...
s and
invertebrateAn invertebrate is an animal without a vertebral column. The group includes 95% of all animal species — all animals except those in the Chordate subphylum Vertebrata ....
animals, such as in describing the shells of gastropod molluscs, i.e.
snailThe word snail is a common name for almost all members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word snail is used in a general sense, it includes sea snails, land snails and freshwater snails. Snails lacking a shell or having only a very small one are...
s.
In mosses
In
mossMosses are small, soft plants that are typically 1–10 cm tall, though some species are much larger. They commonly grow close together in clumps or mats in damp or shady locations. They do not have flowers or seeds, and their simple leaves cover the thin wiry stems...
es, the peristome is a specialized structure in the
sporangiumA sporangium is a plant, fungal, or algal structure producing and containing spores. Sporangia occur in angiosperms, gymnosperms, ferns, fern allies, bryophytes, algae, and fungi...
that allows for gradual
sporeIn biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions...
discharge, instead of releasing them all at once.
Most mosses produce a capsule with a lid (the
operculumIn botany, operculum may be used to describe any of the following:*A flap of the sporangium of a moss, covering the peristome .*The cap of the ascus in certain ascomycetous fungi....
) which falls off when the
sporeIn biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions...
s 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
peristomeThe word peristome is derived from the Greek peri, meaning 'around' or 'about', and stoma, 'mouth'. It is a term used to describe various anatomical features that surround an opening to an organ or structure. The term is used in plants and invertebrate animals, such as in describing the shells of...
is a ring of triangular "teeth" formed from the remnants of specially thickened
cell wallA cell wall is a tough, flexible and sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to act as a...
s. 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
BryopsidaThe Bryopsida constitute the largest class of mosses, containing 95% of all moss species. It consists of approximately 11,500 species, common throughout the whole world....
. 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. 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
BryidaeBryidae is an important subclass of Bryopsida. It is common throughout the whole world. Members have a double peristome with alternating tooth segments.-Classification:The classification of the Bryidae.Superorder: Bryanae...
. 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.
In pitcher plants
In
pitcher plantPitcher plants are carnivorous plants whose prey-trapping mechanism features a deep cavity filled with liquid known as a pitfall trap. It has been widely assumed that the various sorts of pitfall trap evolved from rolled leaves, with selection pressure favouring more deeply cupped leaves over...
s, the peristome is a reflexed ring (or partial ring) of tissue that surrounds the entrance to the digestive tube in these plants. It often (for example in
CephalotusCephalotus is a genus which contains one species, Cephalotus follicularis, a small carnivorous plant. The pit-fall traps of the modified leaves have inspired the common names for this plant, which include Albany Pitcher Plant, Western Australian Pitcher Plant, fly-catcher plant or mocassin plant.-...
) possesses sharp, overhanging 'teeth' which aid in prey retention. It is often studded with nectar secreting glands, hence its popular name, 'nectar roll'.
In shelled gastropods (snail shells)
The peristome is the margin of the
apertureThe aperture is an opening in certain kinds of mollusc shells: it is the main opening of the shell, where part of the body of the animal emerges for locomotion, feeding, etc....
of a
gastropod shellThe gastropod shell is a shell which is part of the body of a gastropod or snail. It is an external skeleton or exoskeleton, which serves not only for muscle attachment, but also for protection from predators and from mechanical damage...
. It is the edge of the lip of the shell. This part is sometimes reflected (turned back) or thickened once the snail reaches adult size, and these qualities of the peristome can be diagnostic features of the shell which may aid in identification of the species.