WHY IS WATER IMPORTANT FOR MOSSES? - BIOLOGY - 2025

Water is of great importance for mosses because these plants do not have vascular tissues or specialized organs for absorption. On the other hand, they are unable to regulate water loss and depend on it for sexual reproduction.

Mosses belong to the bryophytes, considered the first group of plants to colonize the terrestrial environment. The gametophyte forms the vegetative body and the sporophyte is dependent on it.

Drops of water on a moss. Author: publicdomainpictures.net

These plants have a very thin cuticle and do not have stomata that regulate perspiration. They are very susceptible to changes in humidity, so they can become dehydrated very quickly.

Water absorption can occur throughout the plant or through the rhizoids. Conduction can be by capillarity, apoplastic or simplistic. In some groups there are cells specialized in the transport of water (hydroids).

Male gametes (sperm) are flagellated and require the presence of water to reach the egg cell (female gamete).

Many mosses have a great ability to recover from dehydration. Herbarium samples of Grimmia pulvinata have proven viable after 80 years of drying.

General characteristics of mosses

Mosses belong to the group of bryophytes or non-vascular plants, characterized by not having specialized tissues for conducting water.

The vegetative body corresponds to the gametophyte (haploid phase). The sporophyte (diploid phase) is poorly developed and depends on the gametophyte for maintenance.

In general, mosses do not reach a large size. They can range from a few millimeters to 60 cm long. They have a foliose growth, with an erect axis (caulidium) that is attached to the substrate by small filaments (rhizoids). They have leaf-like structures (filidia).

Vegetative body of the gametophyte

The caulidium is erect or creeping. The rhizoids are multicellular and branched. The filidia are helically configured around the caulidium and are sessile.

The body of mosses is practically made up of parenchymal tissue. Stomatal-like pores may be present in the outermost tissue layers of some structures.

The filidios are flattened. It generally presents a layer of cells, with the exception of the central zone (coast) where they can present several.

Reproductive structures

Sex structures are formed on the vegetative body of the gametophyte. Mosses can be monoecious (both sexes on the same foot) or dioecious (sexes on separate feet).

The antheridium constitutes the male sexual structure. They can be spherical or elongated in shape and the inner cells form sperm (male gametes). Sperm have two flagella and need to move through water.

The female sexual structures are called archegonia. They are shaped like a bottle with a widened base and a long narrow part. Within these, the oocell (female gamete) is formed.

Sporophyte

When the fertilization of the egg occurs in the archegonium, an embryo is formed. This begins to divide and forms the diploid body. It consists of a haustorium attached to the gametophyte, whose function is the absorption of water and nutrients.

Then there is a pedicel and the capsule (sporangium) in apical position. When mature, the capsule produces the archespore. Its cells undergo meiosis and spores are formed.

The spores are released and dispersed by the wind. Later they germinate to originate the vegetative body of the gametophyte.

Vegetative structure of mosses and their relationship with water

Bryophytes are considered the first plants that colonized the terrestrial environment. They did not develop supporting tissues or the presence of lignified cells, so they are small in size. However, they have some characteristics that have favored their growth out of water.

Protective fabrics

One of the main characteristics that has allowed plants to colonize the terrestrial environment is the presence of protective tissues.

Terrestrial plants have a fatty layer (cuticle) that covers the outer cells of the plant body. This is considered to be one of the most relevant adaptations to achieve independence from the aquatic environment.

In the case of mosses, a thin cuticle is present on at least one of the faces of the filidia. However, its structure allows the entry of water in some areas.

On the other hand, the presence of stomata has allowed terrestrial plants to regulate water losses through transpiration. Stomata are not present in the vegetative body of the gametophyte of mosses.

For this reason, they cannot control water losses (they are poikilohydric). They are very sensitive to changes in humidity in the environment and are not able to retain water inside the cells when there is a water deficit.

Stomata have been observed in the sporophyte capsule of several species. They have been associated with the mobilization of water and nutrients towards the sporophyte and not with the control of water loss.

Water absorption

In vascular plants, water absorption occurs through the roots. In the case of bryophytes, the rhizoids generally do not have this function, but rather that of fixing to the substrate.

Mosses present two different strategies for absorbing water. According to the strategy they present, they are classified into:

Endohydric species: the water is taken directly from the substrate. The rhizoids take part in the absorption and later the water is conducted internally to the whole body of the plant.

Exhydric species: water absorption occurs throughout the plant body and is transported by diffusion. Some species may have a woolly cover (tomentum) that favors the absorption of water present in the environment. This group is very sensitive to desiccation.

Endohydric species are capable of growing in drier environments than exhydric species.

Water conduction

In vascular plants the water is conducted by the xylem. The conducting cells of this tissue are dead and the walls are highly lignified. The presence of xylem makes them highly efficient in the use of water. This characteristic has allowed them to colonize a large number of habitats.

In mosses, there is no presence of lignified tissues. Water conduction can occur in four different ways. One of these is cell-to-cell movement (the simplistic pathway). Other ways are the following:

Apoplastic: water moves through the apoplast (walls and intercellular spaces). This type of driving is much faster than the simplistic. It is more efficient in those groups that present thick cell walls, due to its higher hydraulic conductivity.

Capillary spaces: in ectohydric groups the mobilization of water tends to be by capillarity. Capillary spaces are formed between the filidia and the caulidium that facilitate the transport of water. Capillary ducts can reach lengths of up to 100 µm.

Hydroids: in endohydric species the presence of a rudimentary conduction system has been observed. Cells specialized in water conduction called hydroids are observed. These cells are dead, but their walls are thin and very permeable to water. They are arranged in rows one above the other and centrally located in the caulidium.

Water-dependent sexual reproduction

Mosses have flagellated male gametes (sperm). When the antheridium matures, the presence of water is necessary for it to open. Once dehiscence occurs, the sperm remain floating in the film of water.

For fertilization to occur, the presence of water is essential. Sperm can remain viable in the aqueous medium for approximately six hours and can travel distances of up to 1 cm.

The arrival of the male gametes to the antheridia is favored by the impact of the drops of water. When they splash in different directions, they carry a large number of sperm. This is of great importance in the reproduction of dioecious groups.

In many cases, the antheridia are shaped like a cup, which facilitates the dispersal of sperm when the impact of water occurs. Mosses with a creeping habit form more or less continuous layers of water through which the gametes move.

Moss tolerance to dehydration

Some mosses are aquatic obligate. These species are not tolerant to desiccation. However, other species are capable of growing in extreme environments, with marked dry periods.

Because they are poikilohydric, they can lose and gain water very quickly. When the environment is dry, they can lose up to 90% of the water and recover when the humidity increases.

The Tortula ruralis species has been stored with a moisture content of 5%. By being rehydrated, she has been able to regain her metabolic capacity. Another interesting case is that of Grimmia pulvinata. Herbarium samples over 80 years old have proven viable.

This tolerance to dehydration of many mosses includes strategies that allow them to maintain the integrity of cell membranes.

One of the factors that contributes to maintaining cell structure is the presence of proteins called rehydrins. They intervene in the stabilization and reconstitution of the membranes damaged during dehydration.

In some species, the vacuole has been observed to divide into many small vacuoles during dehydration. As the moisture content increases, they merge and form a large vacuole again.

Plants tolerant to long periods of desiccation present antioxidant mechanisms, due to the fact that oxidative damage increases with the time of dehydration.

References

1. Glime J (2017) Water relations: Plant strategies. Chapter 7-3. In: Glime J (ed.) Bryophyte Ecology Volume I. Physiological Ecology. Ebook sponsored by Michigan Technological University and the International Association of Bryologist. 50.pp.
2. Glime J (2017) Water relations: Habitats. Chapter 7-8. In: Glime J (ed.) Bryophyte Ecology Volume I. Physiological Ecology. Ebook sponsored by Michigan Technological University and the International Association of Bryologist. 29.pp.
3. Green T, L Sancho and A Pintado (2011) Ecophysiology of Desiccation / Rehydration Cycles in Mosses and Lichens. In: Lüttge U, E Beck and D Bartels (eds) Plant Desiccation Tolerance. Ecological Studies (Analysis and Synthesis), vol 215. Springer, Berlin, Heidelberg.
4. Izco J, E Barreno, M Brugués, M Costa, J Devesa, F Fernández, T Gallardo, X Llimona, E Salvo, S Talavera and B Valdés (1997) Botánica. McGraw Hill - Interamericana from Spain. Madrid Spain. 781 pp.
5. Montero L (2011) Characterization of some physiological and biochemical aspects of the moss Pleurozium schreberi related to its ability to tolerate dehydration. Thesis to apply for the title of Doctor of Agricultural Sciences. Faculty of Agronomy, National University of Colombia, Bogotá. 158 pp.