Moss is a type of simple plant or nonvascular plant, of the Division Bryophyta, that have rhizoids instead of true roots.
Aside from lacking a vascular system, they have a gametophyte-dominant life cycle, i.e. the plant's cells are haploid for most of its life cycle. Sporophytes (i.e. the diploid body) are short-lived and dependent on the gametophyte.
The life of a moss starts from a haploid spore, which germinates to produce a protonema, which is either a mass of filaments or thalloid (flat and thallus-like). This is a transitory stage in the life of a moss. From the protonema grows the gametophore ("gamete-bearer") that is differentiated into stems and leaves ('microphylls'). From the tips of stems or branches develop the sex organs of the mosses. The female organs are known as archegonia (singular archegonium) and are protected by a group of modified leaves known as the perichaetum (plural perichaeta). The archegonia have necks called venters which the male sperm swim down. The male organs are known as antheridia (singular antheridium) and are enclosed by modified leaves called the perigonium (plural perigonia).
Mosses can be either dioicous (compare dioecious) or monoicous (compare monoecious). In dioicous mosses, both male and female sex organs are borne on different plants. In monoicous (also called autoicous) mosses, they are borne on the same plant. In the presence of water, sperm from the antheridia swim to the archegonia and fertilisation occurs, leading to the production of a diploid sporophyte. The sperm of mosses is biflagellate, i.e. they have two flagella that aid in propulsion. Without water, fertilisation cannot occur. After fertilisation, the immature sporophyte pushes its way out of the archegonial venter. It takes about a quarter to half a year for the sporophyte to mature. The sporophyte body comprises a long stalk, called a seta, and a capsule capped by a cap called the operculum. The capsule and operculum are in turn sheathed by a haploid calyptra which is the remains of the archegonial venter. The calyptra usually falls off when the capsule is mature. Within the capsule, spore-producing cells undergo meiosis to form haploid spores, upon which the cycle can start again. The mouth of the capsule is usually ringed by a set of teeth called peristome. This may be absent in some mosses.
In some mosses, green vegetative structures called gemmae are produced on leaves or branches, which can break off and form new plants without the need to go through the cycle of fertilisation. This is a means of asexual reproduction.
Classification of Mosses
The mosses are classified as a class Musci in the division (or phylum) Bryophyta within the Kingdom Plantae. There are seven subclasses of mosses:
Andreaeidae are distinguished by the biseriate (two rows of cells) rhizoids, multiseriate (many rows of cells) protonema, and sporangium that splits along longitudinal lines. Most mosses have capsules that open at the top.
The Sphagnidae, the peat-mosses, comprise a single genus Sphagnum. These form extensive acidic bogs in peat swamps. The leaves of Sphagnum have large dead cells alternating with living photosynthetic cells. The dead cells help to store water. Aside from this character, the unique branching, thallose (flat and expanded) protonema, and explosively rupturing sporangium place it apart from other mosses.
The Tetraphidae are unique as their name implies, in having only four large peristome teeth surrounding the opening of the capsule.
Polytrichidae have leaves with lamellae, which are flaps on the leaves that look like the fins on a heat sink. These help it retain moisture. They differ from other mosses in other details of their development and anatomy too.
The Buxbaumiidae are called 'bug mosses' because they usually have a very small and reduced gametophore and the whole plant is mostly the sporophyte capsule. The shape reminds one of a bug, which is the reason for its common name.
Most (>95%) mosses belong to the Bryidae.
The Archidiidae are distinguished by their extremely large spores and the way the sporangium develops.
Mosses are found chiefly in areas of low light and high water content. Mosses are common in wooded areas and at the edges of streams. They require moisture to survive because of the small size and thinness of tissues, lack of cuticle (waxy covering to prevent water loss), and the need for liquid water to complete fertilisation.