Mosses (Division Bryophyta) are small plants found in various ecosystems, including areas with short growing seasons and tropical rain forests. There are approximately 12,000 recognized species that display a wide range of morphological diversity.
Due to different lineages and character loss, a unifying macroscopic definition for mosses is impossible. Most mosses have a sporophyte with a robust stalk elevating the capsule. However, this characteristic is absent in peat mosses and ephemeral taxa from seasonally dry ecosystems. Capsules often have one or two rings of teeth in their mouth. The vegetative (gametophyte) body consists of a terete axis with sessile leaves, distinguishing them from hornworts and some liverworts.
While associations with fungi are common in mosses, there is a lack of evidence confirming a symbiotic relationship that benefits the moss.
As an important group of nonvascular plants, mosses showcase a unique and fascinating vegetative architecture. The vegetative body is constructed from blocks of cells assembled into axes, with multiple axes joining to form a stem or branch system ( Newton et al. 2007). This type of modular growth contributes to mosses being highly adaptable to different environmental conditions, making them prevalent in various habitats.
Growth in the moss gametophyte involves the accumulation of successive modules, with each module extending from the surface to the center of the axis ( Newton et al. 2007). This process leads to various branching patterns, directly impacting the resulting moss body shape and size.
These diverse forms are attributed to the variation in the mode of branching, polarity of branch development, the direction of growth (orthotropic or plagiotropic), and the density of branching determined by the dormancy of successive initials ( La Farge-England 1996).
One well-known genus of mosses, Sphagnum, is easily identified by its fasciculate branches, which highlight how mosses utilize branching to create specific adaptations for their growth and survival ( Newton et al. 2007). Other moss genera exhibit different branching patterns and leaf development forms, highlighting the complexity and adaptability of the moss group.
Leaves are a key feature of mosses, originating from leaf-initials and exhibiting a wide range of shapes and arrangements ( Newton et al. 2007). Heteroblastic leaf development is prominent in mosses, allowing them to adapt to different environments and structural configurations.
This feature also influences the overall appearance of the moss plant. Various types of cells and surface ornamentations can be observed within a single leaf, adding to the morphological diversity of mosses.
The curious dimorphism found in Sphagnum and Leucobryum is an excellent example of the unique cellular structures mosses can exhibit. In these genera, the leaves comprise small, photosynthetic cells and large, hyaline, dead cells that function in water storage and carbon acquisition. This kind of cellular differentiation allows the plants to optimize their growth and maintain their metabolic processes efficiently.
Reproductive biology is another fascinating aspect of mosses, with these plants showcasing different sexual configurations in either monoicy (presence of both sexes in a single plant) or dioicy (one sex per plant).
Gametangia, the reproductive organs housing egg or sperm cells, are surrounded by differentiated leaves that protect the delicate sex organs and the young embryo ( Newton et al. 2007).
These specialized structures and cells further demonstrate the adaptability and uniqueness of mosses in the plant kingdom, as they are superbly tailored to thrive in various environments and habitats. Overall, mosses’ intriguing structural and reproductive diversity highlights their significance and contribution to Earth’s rich tapestry of plant life.
The moss sporophyte possesses a simple structure similar to liverworts, consisting of an unbranched seta anchored into the maternal gametophyte by the foot and carrying a single terminal sporangium.
The architecture of the sporophyte is unique among bryophytes due to the presence of a new meristem below the apex ( Garner & Paollilo 1973). The seta serves to elevate the capsule above the perichaetial leaves in order to protect the developing sporophyte.
Stalks, solid and composed of parenchyma cells, may contain long, thin-walled water-conducting cells that form an axial strand in some taxa, even though conducting cells may be absent in the gametophyte ( Vitt 1981).
Stomata in the capsule are often kidney-shaped and used to regulate gas exchange. Like in vascular plants, stomatal crypts are found in mosses growing in xeric conditions. The peristome, a unique attribute of mosses but absent in all species, comprises one or two concentric rings of teeth exposed after the loss of the operculum ( Boudier 1988).
Peristome teeth help to control the release of spores as they gradually disperse over time. Mosses exhibit a variety of peristome architectures that are crucial in diagnosing major lineages. In operculate taxa, the shedding of the operculum, loosening of the annulus or the presence of a nematodontous peristome assist in releasing spores.
In order for spore release, dehiscence occurs through the loss of an apical lid, longitudinal lines or irregular breakdown of the capsule wall ( Newton et al. 2007). Spore dispersal is usually passive and gradual, although entomophilous Splachnaceae relies on insect-facilitated dispersal.
The sporophyte relies on the maternal gametophyte to obtain nutrients and water to complete its development. The enveloping hood, or calyptra, provides critical taxonomic characters and is essential for the protection and development of the sporophyte.
Overall, the moss sporophyte displays a unique architecture, and structures like stomatal guards, peristomes and calyptras play key roles in regulating gas exchange and spore dispersal. Understanding these structures aids in the identification of species and lineages within mosses.
Seedless plants, including mosses and other bryophytes, face challenges in sexual reproduction, as their flagellated sperm cells need to travel through water to reach the egg ( Cronberg et al. 2006a). In species with separate male and female gametophytes, fertilization can be limited if they are not in close proximity.
Some mosses overcome this hurdle by recruiting tiny arthropods as sperm vectors or using raindrops to disperse sperm cells. As many as 50% of mosses are dioicous and rely on asexual means to maintain local and regional populations. Asexual reproduction is also observed in monoicous species for rapid expansion after establishment.
Various asexual propagules have been observed in mosses, differing in shape, size, mode of abscission, longevity, and origin on the plant ( Newton and Mishler, 1994;  Pohjamo, M. & Laaka-Lindberg, S. (2003)
A species may produce multiple diaspore types ( Duckett & Ligrone 1992). Mosses can be propagated easily from fragments in vitro, facilitating the conservation of rare and endangered species ( Duckett et al. 2004). Ground-dwelling mosses can regenerate from detached leafy shoots, even after passing through the digestive tract of bats ( Parsons et al. 2007).
In nature, programmed fragmentation is rare, and specialization is observed through caducous leaf tips or leaves, plates of leaf blades, or small-leafed branches.
Various asexual propagules include cauline or foliar gemmae, generally multicellular structures, and rhizoidal tubers, which serve as perennating structures in seasonal habitats (e.g., Bryum rubens;  Duckett & Ligrone, 1992). Tubers, however, are not found in pleurocarpous mosses. Mosses have developed diverse reproductive strategies to adapt to their environments and ensure survival.
Mosses interact with various fungi from the three main lineages: Glomeromycota, Ascomycota, and Basidiomycota ( Davey & 2006). The nature of these interactions may vary, with some being mutualistic while others can be parasitic or pathogenic. Some endophytic fungi may increase the host moss’s tolerance to environmental stress or resistance to pathogens ( Davey & Currah 2006).
In addition to fungi, mosses also associate with cyanobacteria, which can grow as epiphytes on leaves and stems or even inside certain structures called hyalocysts in the case of Sphagnum ( Solheim & Zielke 2002). This relationship provides mosses with a valuable source of nitrogen, which is often a growth-limiting nutrient for plants. Cyanobacteria benefit from the shelter the moss provides ( Adams & Duggan 2008).
Occasionally, green algae may be found inside the cells of mosses ( Reese 1981); however, these algae do not benefit the host moss. Mosses are also colonized by heterotrophic prokaryotes, particularly methylobacteria, which consume organic molecules emitted by the host plant.
These bacteria produce cytokinins and other phytohormones that promote plant growth ( Hornschuh et al. 2002). In submerged Sphagnum, methanotrophic bacteria oxidize methane into CO2, providing the moss with an additional carbon source (Raghoebarsing et al. 2005). This illustrates how moss gametophytes can form complex miniature ecosystems with extensive nutrient recycling.
Classification and macroevolution
Mosses are diverse plant species with unique morphological and evolutionary characteristics. Understanding their phylogenetic relationships can be challenging due to the significant morphological differences among taxa and the possibility of a rapid succession of early cladogenic events ( Newton et al. 2007). Molecular data, such as DNA sequences, have corroborated the evolutionary history and divergence of moss classes, but certain nodes in the phylogeny remain ambiguous ( Duckett et al. 2004).
Estimates of divergence times date the origin of mosses to around 380 million years ago, but the radiation of major lineages took much longer ( Newton et al. 2007). Molecular data have revealed the phylogenetic relationships of certain genera like Oedipodium and Timmia and taxa with reduced morphologies such as Ephemerum ( Goffinet & Cox 2000).
Despite the ambiguity in the phylogenetic tree, some character transformations in mosses, like the appearance of costae, stomata, operculum, and peristome, can be determined. However, the polarity and timing of some transformations remain unclear ( Budke et al. 2007).
The classification of mosses is continuously refined based on phylogenetic inferences. Most affected are familial delineation within the pleurocarpous mosses, which account for nearly 50% of moss diversity. Their rapid diversification may be linked to the rise of angiosperms, increasing habitat diversity ( Newton et al. 2007).
Biogeography and ecology
Mosses are found on all continents and grow on a wide range of substrates. They have high turnover of specialist species, contributing to their success in colonizing almost all available habitats.
This specificity, known as niche conservatism, is evident at a high taxonomic level ( Bates 2009). For example, Sphagnum dominates ombrotrophic bogs ( Vitt & Wieder 2009), and Calliergonaceae and Amblystegiaceae families dominate rich fens ( Hedena ̈s 2001). All 50 species of Andreaea are restricted to hard rocks with low calcium concentration ( Heegaard 1997).
Mosses tend to exhibit a more generalist strategy than the highly specific angiosperm flora that characterizes harsh habitats. For instance, moss species composition along a salinity gradient does not show the zonation pattern that characterizes vascular plant vegetation in salt marshes ( Zechmeister 2005).
Adaptations to extreme habitats in mosses occur but are not the rule. Metal-contamination in soils may be revealed by Scopelophila cataractae, but does not exclude other moss species from surrounding, non-polluted soils from becoming established. Some moss species, such as Mnium hornum and Campylium stellatum, display considerable salinity tolerance (Garbary et al. 2008). Mosses may exhibit a broader, albeit hidden, ecological amplitude than is revealed by their typical realized niche, eliminating the need for specialization.
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