Moss
Mosses are small, non-vascular flowerless plants in the taxonomic division Bryophyta sensu stricto. Bryophyta may also refer to the parent group bryophytes, which comprise liverworts, mosses, and hornworts. Mosses typically form dense green clumps or mats, often in damp or shady locations. The individual plants are usually composed of simple leaves that are generally only one cell thick, attached to a stem that may be branched or unbranched and has only a limited role in conducting water and nutrients. Although some species have conducting tissues, these are generally poorly developed and structurally different from similar tissue found in vascular plants. Mosses do not have seeds and after fertilisation develop sporophytes with unbranched stalks topped with single capsules containing spores. They are typically tall, though some species are much larger. Dawsonia superba, the tallest moss in the world, can grow to in height. There are approximately 12,000 species.
Mosses are commonly confused with liverworts, hornworts and lichens. Although often described as non-vascular plants, many mosses have advanced vascular systems. Like liverworts and hornworts, the haploid gametophyte generation of mosses is the dominant phase of the life cycle. This contrasts with the pattern in all vascular plants, where the diploid sporophyte generation is dominant. Lichens may superficially resemble mosses, and sometimes have common names that include the word "moss", but they are fungal symbioses and not related to mosses.
The main commercial significance of mosses is as the main constituent of peat, although they are also used for decorative purposes, such as in gardens and in the florist trade. Traditional uses of mosses included as insulation and for the ability to absorb liquids up to 20 times their weight. Mosses are keystone species and benefit habitat restoration and reforestation. Mosses absorb 6.4 billion tons of carbon annually and provide various other environmental benefits, including controlling pathogens dangerous for humans, stopping land erosion, improving microclimate, filtering air pollution, and cooling cities.
Physical characteristics
Description
Botanically, mosses are non-vascular plants in the land plant division Bryophyta. They are usually small herbaceous plants that absorb water and nutrients mainly through their leaves and harvest carbon dioxide and sunlight to create food by photosynthesis. With the exception of the ancient group Takakiopsida, no known mosses form mycorrhizae, but bryophilous fungi are widespread among mosses and other bryophytes, where they live as saprotrophs, parasites, pathogens and mutualists, some of them endophytes. Mosses differ from vascular plants in lacking water-bearing xylem tracheids or vessels. As in liverworts and hornworts, the haploid gametophyte generation is the dominant phase of the life cycle. This contrasts with the pattern in vascular plants, where the diploid sporophyte generation is dominant. Mosses reproduce using spores, not seeds, and have no flowers.Moss gametophytes have stems which may be simple or branched and upright or prostrate. The early divergent classes Takakiopsida, Sphagnopsida, Andreaeopsida and Andreaeobryopsida either lack stomata or have pseudostomata that do not form pores. In the remaining classes, stomata have been lost more than 60 times. Their leaves are simple, usually only a single layer of cells with no internal air spaces, often with thicker midribs. The nerve can run beyond the edge of the leaf tip, termed excurrent. The tip of the leaf blade can be extended as a hair point, made of colourless cells. These appear white against the dark green of the leaves. The edge of the leaf can be smooth or it may have teeth. There may be a distinct type of cell defining the edge of the leaf, differing in shape and/or colour from the other leaf cells.
Mosses have threadlike rhizoids that anchor them to their substrate, comparable to root hairs rather than the more substantial root structures of spermatophytes. Mosses are known to absorb water through their rhizoids, and some species may also take up nutrients this way. They can be distinguished from liverworts by their multi-cellular rhizoids. Spore-bearing capsules or sporangia of mosses are borne singly on long, unbranched stems, distinguishing them from the polysporangiophytes, which include all vascular plants. The spore-producing sporophytes are usually capable of photosynthesis, but are short-lived and dependent on the gametophyte for water supply and most or all of their nutrients. Also, in the majority of mosses, the spore-bearing capsule enlarges and matures after its stalk elongates, while in liverworts the capsule enlarges and matures before its stalk elongates. Other differences are not universal for all mosses and all liverworts, but the presence of a clearly differentiated stem with simple-shaped, non-vascular leaves that are not arranged in three ranks, all point to the plant being a moss.
Life cycle
Vascular plants have two sets of chromosomes in their vegetative cells and are said to be diploid, i.e. each chromosome has a partner that contains the same, or similar, genetic information. By contrast, mosses and other bryophytes have only a single set of chromosomes and so are haploid. There is a period in the moss life cycle when they do have a double set of paired chromosomes, but this happens only during the sporophyte stage.The moss life-cycle starts with a haploid spore that germinates to produce a protonema, which is either a mass of thread-like filaments or thalloid. Massed moss protonemata typically look like a thin green felt, and may grow on damp soil, tree bark, rocks, concrete, or almost any other reasonably stable surface. This is a transitory stage in the life of a moss, but from the protonema grows the gametophore that is structurally differentiated into stems and leaves. A single mat of protonemata may develop several gametophore shoots, resulting in a clump of moss.
From the tips of the gametophore stems or branches develop the sex organs of the mosses. The female organs are known as archegonia and are protected by a group of modified leaves known as the perichaetum. The archegonia are small flask-shaped clumps of cells with an open neck down which the male sperm swim. The male organs are known as antheridia and are enclosed by modified leaves called the perigonium. The surrounding leaves in some mosses form a splash cup, allowing the sperm contained in the cup to be splashed to neighboring stalks by falling water droplets.
Gametophore tip growth is disrupted by fungal chitin. Galotto et al., 2020 applied chitooctaose and found that tips detected and responded to this chitin derivative by changing gene expression. They concluded that this defense response was probably conserved from the most recent common ancestor of bryophytes and tracheophytes. Orr et al., 2020 found that the microtubules of growing tip cells were structurally similar to F-actin and served a similar purpose.
Mosses can be either dioicous or monoicous. In dioicous mosses, male and female sex organs are borne on different gametophyte plants. In monoicous mosses, both 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 flagellae that aid in propulsion. Since the sperm must swim to the archegonium, fertilisation cannot occur without water. Some species keep their antheridia in so called 'splash cups', bowl-like structures on the shoot tips that propel the sperm several decimeters when water droplets hit it, increasing the fertilization distance.
After fertilisation, the immature sporophyte pushes its way out of the archegonial venter. It takes several months 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.
Most mosses rely on the wind to disperse the spores. In the genus Sphagnum the spores are projected about off the ground by compressed air contained in the capsules; the spores are accelerated to about 36,000 times the earth's gravitational acceleration g.
It has recently been found that microarthropods, such as springtails and mites, can effect moss fertilization and that this process is mediated by moss-emitted scents. Male and female fire moss, for example, emit different and complex volatile organic scents. Female plants emit more compounds than male plants. Springtails were found to choose female plants preferentially, and one study found that springtails enhance moss fertilization, suggesting a scent-mediated relationship analogous to the plant-pollinator relationship found in many seed plants. The stinkmoss species Splachnum sphaericum develops insect pollination further by attracting flies to its sporangia with a strong smell of carrion, and providing a strong visual cue in the form of red-coloured swollen collars beneath each spore capsule. Flies attracted to the moss carry its spores to fresh herbivore dung, which is the favoured habitat of the species of this genus.
In many mosses, e.g., Ulota phyllantha, 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 fertilization. This is a means of asexual reproduction, and the genetically identical units can lead to the formation of clonal populations.