BRYOPHYTES: CHARACTERISTICS, PHYLOGENY, CLASSIFICATION, REPRODUCTION - BIOLOGY - 2025

The bryophyte or bryophytes, also known as mosses, are nonvascular small plants in wet environments and on various surfaces such as rocks, soil, trunks, among others. Some 24,000 species are known and thanks to their ability to tolerate wide ranges of temperatures they are distributed in arctic, tropical and even deserts.

Historically, this group has been classified into three main groups: liverworts, hornworts, and mosses. At present, this classification is considered paraphyletic, since anthocerotes are more related to vascular plants than to the rest of bryophytes.

Source: By Marie Čcheidzeová, from Wikimedia Commons

Bryophytes, like plants, contain a number of pigments such as chlorophyll, carotenes, and xanthophylls. Its life cycle includes a process of alternation of generations, called gametophyte and sporophyte.

Each generation differs from the other in terms of chromosome number, shape, and function. They also have asexual reproduction through budding and fragmentation processes.

Due to their sensitivity, bryophytes are useful in conservation areas, as they function as an environmental indicator of air pollution.

Many times, due to their morphological similarity, some algae or lichens are incorrectly called "mosses". Similarly, the term "moss" and "bryophyte" has been used very loosely in the literature. Strictly speaking, mosses or bryophytes are a clade that does not include liverworts and anthocera.

General characteristics

Botanists divide their study organisms into two large groups of terrestrial plants: bryophytes or non-vascular plants and tracheophytes or vascular plants.

Bryophytes are characterized by small sizes and grow in highly packed structures reminiscent of a cushion or cushion. We find them on various types of rocks and logs on the ground and as epiphytes on forest trees.

All bryophytes are ecologically persistent and the phase that performs the photosynthetic processes is the gametophyte, which is haploid. The sporophyte phase is diploid and grows as an unbranched stem and a terminal sporangium. It is characterized by an ephemeral existence and living anchored to the gametophyte for nutritional reasons.

Morphologically, its structures may resemble those of a vascular plant. In the gametophyte, rhizoids and small sharp "leaves" can be distinguished. However, they differ from them by certain characteristics.

Although bryophytes lack true vascular tissues responsible for the transport of sugars and other nutrients, they present homologous structures called hydroids. These plants never form xylem, the lignified conductive tissue responsible for the transport of salts and water in vascular plants.

Habitat

Bryophytes have a relatively broad environmental tolerance level. They can live and develop in warm and temperate environments, either humid or shady. They can also be found in swamps.

A particular species is the genus Sphagnum or peat moss that covers 1% of the surface of the entire world. Among its peculiarities is the retention of immense amounts of water, from 20 to 30 times its weight.

Phylogeny

The phylogeny that includes plants with stomata is divided into two large branches, one leads to plants with incipient stems - anterophytes - while the other leads to plants with very efficient conducting systems.

This last group is known as hemitracheophytes and includes bryophytes or mosses, with a rudimentary conduction system, and tracheophytes, which includes vascular plants that have true conducting vessels.

As the phylogeny of bryophytes has changed over the years, we will make a description based on a time scheme:

Historical perspective

The three known lineages of bryophytes are liverworts, hornworts, and mosses. The relationship between them remained unknown for many years and was one of the most important questions in the evolutionary biology of plants.

Many of the hypotheses that were raised included different arrangements of the tree, commonly considering the bryophytes as a degree of the three monophyletic lineages mentioned above.

Some authors suggested that liverworts were the sister group of the other embryophytes, and others proposed anthocerotes as a sister group.

Bryophytes were previously considered as a single phylum, which was in an intermediate position between algae and vascular plants.

Current phylogenies

Molecular biology and the existence of powerful computer programs have revolutionized the reconstruction of phylogenies, allowing a massive amount of data to be analyzed. Thus, the phylogenies obtained using morphological characters can be supported.

Currently different conclusions have been reached. It is now accepted that the three groups of bryophytes mentioned comprise three evolutionarily separate lineages.

Using structural features of the genome and sequence data it was found that anthocerotes are the closest relative to tracheophytes.

Classification

Bryophyte species are classified into three phyla: Marchantiophyta (liverworts), Bryophyta (mosses) and Anthocerotophyta (anthocerotes). As discussed, they do not form a monophyletic group - a group containing the most recent common ancestor and all of its descendants - so they represent a degree in the evolution of embryophytes.

Of the three groups, we find the greatest diversity in mosses, with more than 15,000 species recognized so far.

Hepatic

Liverworts usually inhabit tropical areas of America. Its size is small, although some species can reach 30 cm. The protonema is globose, the gametophyte is a simple or air-chambered thallus.

The "leaves" are arranged in three columns and divided into more than two lobes, without a middle vein. They do not have stomata and have special organelles called oil bodies.

Anthocerotes

They are characterized by a globose protoneme, the shape of the gametophyte is simple thallus. They present a plastid and a pirinoiodes.

Mosses

Mosses are a cosmopolitan group divided into three orders: Bryales, Sphagnales, and Andraeales. The protonema is filamentous and the arrangement of the "leaves" is spiral and with the presence of a middle vein. It does not present special organelles.

Unlike the previous groups, the rhizoid is brown and composed of several cells. Stomata are present in the sporophyte capsule, which is complex with an operculum, a theca, and a neck.

Reproduction

Bryophytes have a life cycle that involves two generations: the gametophyte and the sporophyte. The first cell of the gametophyte is the spore that, when germinating, becomes a structure with a filamentous, laminar, globose appearance, among others called protonema.

The protoneme

The protonema is attached to the ground by appendages that lack chlorophyll called rhizoids. A sprout originates from the protonema which, in turn, produces a complex gametophyte.

This structure is the haploid phase of the life cycle and is characterized by having a small, flattened or foliose thallus. In some cases morphologically reminiscent of a filamentous alga.

In the first case, the thallus is a lobed ribbon that branches in two and uses the rhizoids to anchor itself to the substrate. In contrast, if the thallus is foliose, the structure consists of an axis that resembles a stem and leaves are born from this. As in flattened thalli, foliose are attached to the substrate by means of rhizoids.

Although there are structures that resemble the stems, leaves and roots of vascular plants, in bryophytes there are no vessels and these organs are simpler.

Another difference is related to the chromosomal endowment, the gametophyte is haploid while in plants the leaves, roots and others are diploid.

The gametophyte

The gametophyte produces structures asexually, although it also has sexual organs. Asexual reproduction occurs through buds or fragments of the thallus. If these structures are located in regions with favorable environmental conditions, they will be able to develop a protoneme and a new gametophyte.

Similarly, the sex organs are known as archegonia (bottle-shaped female organ) and antheridia (globose male organs) and can be differentially located.

In thaloid gametophytes, the sex organs are found inside the plant. Some bryophytes can be monoecious and others can be dioecious.

Male sex organs produce a type of cell with two flagella called anterozoids. The presence of water is essential for fertilization to occur since sperm are able to use their flagella to swim short distances. This is how sexual reproduction occurs.

The sporophyte

As the oosphere develops, the cells located in the neck of the archegonium disappear and their content is released by the rupture of the apex. The anterozoids are expelled and only one of them can open the covering of the oosphere. At this point the first diploid structure is formed: the sporophyte.

The sporophyte develops by cell division until a foot forms and the other cells form the sporophyte organs. The cells of the archegonium's belly give rise to a structure called the caliptra.

Compared with the gametophyte, the sporophyte is short-lived and the structure is not as interesting and attractive as the gametophyte.

The life cycle previously described is quite similar in the three groups of bryophytes, with the exception that some structures vary in terms of their morphology and arrangement.

References

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