Botany - B.Ye. Yakubenko 2017

Part Three. Kingdom Plantae
Chapter VII. Systematics of Higher Plants
7.2. Division Bryophyta

Bryophytes comprise about 25,000 species, found primarily in the Northern Hemisphere. They include both leafy and thalloid forms. Lacking true roots, they absorb Water and soil nutrients and anchor to the substrate via rhizoids or their entire body surface. The gametophyte (sexual generation) is the dominant phase in The life cycle of bryophytes, which assigns them a unique position in the plant kingdom. Representing an evolutionary blind alley, they have survived from the mid-Paleozoic to the present day, occupying habitats free from competition with evolutionarily younger and more advanced plants (such as waterlogged forests, bogs, and occasionally meadows and rocks).

The gametophyte of bryophytes is a dioecious or monoecious perennial herbaceous plant that bears female and male sex Organs—archegonia and antheridia, respectively. Archegonia are typically flask-shaped, multicellular, and enclosed by a single-layered wall. The expanded lower portion is called the venter, where the egg Cell matures, while the narrowed neck contains neck canal Cells that play a specific role during Fertilization.

Antheridia appear as slightly flattened sac-like or spherical structures. They are filled with spermatogenous tissue that gives rise to numerous biflagellate spermatozoa. Fertilization takes place early in spring; spermatozoa travel to the female gametophytes exclusively via water and fuse with the egg cell. A sporophyte, known in Bryophytes as a sporogonium, develops from the zygote. The sporogonium develops on the female gametophyte and is nourished by it, consisting of a haustorium, a seta (stalk), and a capsule. The capsule varies in Structure among bryophytes. In some species, it is covered by a calyptra, which represents the remains of the archegonium wall. The most vital part of the capsule is the urn, capped by an operculum. The Base of the urn transitions into the apophysis, which in turn connects to the seta. Inside the urn, sporangia filled with sporogenous tissue are formed. During spore maturation, the cells of this tissue undergo Meiosis to produce haploid spores. Once mature, the spores are dispersed upon the opening of the capsule and, upon reaching moist soil, germinate to form a protonema. The protonema is filamentous or thalloid in shape, resembling an algal thallus. The protonema subsequently gives rise to a male or female gametophyte. Mosses originate from Algae, sharing many features with them: the absence of roots, Vascular Tissues, and woody forms, the requirement of water for fertilization, and the dominance of the sexual generation in the life cycle.

Bryophytes are divided into three classes: Anthocerotopsida (hornworts), Marchantiopsida (liverworts), and Bryopsida (true mosses).

Hornworts (Anthocerotopsida) represent the most primitive group of mosses, comprising 5 genera and over 300 species, with only 3 species found in Ukraine. These are plants of relatively simple structure. Their body is a dorsiventral thallus shaped like a small leaf or rosette with rhizoidal cell outgrowths on the underside. The thallus has a simple internal structure, consisting of uniform parenchymal cells containing granular METABOLISM/14.html">Chloroplasts and pyrenoids. The sex organs—antheridia and archegonia—are embedded within this tissue. The sporogonium appears as a horn-like, dark green capsule with two Valves and a bulbous FOOT (absorptive organ). Inside the sporogonium, There is a central columella surrounded by sporogenous cells that produce spores along with elaters.

Hornworts grow on sandy soils along river banks and have no practical significance.

Liverworts (Marchantiopsida) are a more advanced group that includes both thalloid and leafy forms. The Class encompasses 8,500 species across 235 genera, distributed almost worldwide, with over 200 species occurring in Ukraine. Thalloid forms feature a dorsiventral plate-like thallus with unicellular rhizoids on the lower surface. Leafy forms are tiny plants whose gametophyte shoots bear small leaves of various shapes lacking a midrib.

Antheridia and archegonia develop On the surface of the thallus or on shoots of the same or different individuals. In some cases, the sex organs are embedded within the thallus tissue, while in others they sit on its surface or are elevated on specialized stalked receptacles.

In addition to spores, the sporophyte capsule produces elaters—spiral structures that aid in spore dispersal. The capsule opens via valves.

This class is divided into two subclasses: Jungermanniopsida, which includes leafy forms, and Marchantiopsida, which comprises thalloid forms.

A representative thalloid species is Marchantia polymorpha, a plant quite common in Ukraine that grows in forest clearings along streams, as well as in damp and shady places. It is a dioecious plant whose gametophytes are distinguished by the shape of the receptacles that bear the sex organs. The female gametophyte bears a receptacle shaped like a multi-rayed disc, with archegonia attached to its underside. In the male gametophyte, the disc has a slightly wavy margin, and antheridia are embedded in cavities on its upper surface. By the time of fertilization, the Cells of the disc above the antheridia undergo mucilaginous degeneration, forming a channel through which spermatozoa are released into the external environment. Carried by water to the female gametophytes, the neck canal cells of the archegonium dissolve into mucilage, clearing a pathway to the egg cell. From the zygote on the female gametophyte, a sporophyte (sporogonium) develops, consisting of a spherical capsule and a seta whose lower part acts as a haustorium. Spores are formed from sporogenous tissue via meiosis; the capsule splits upon spore maturation, and when reaching favorable conditions, a spore germinates into a thalloid protonema, from the buds of which a gametophyte eventually grows. Marchantia can also reproduce vegetatively through gemmae produced in specialized gemma cups.

The subclass Jungermanniopsida comprises leafy plants common in the tropics and temperate regions, numbering about 7,800 species. They represent the next evolutionary grade of liverworts, occupying an intermediate position between Marchantiopsida and true mosses. The gametophytes of Jungermanniopsida are small plants with two rows of phyllids (leaf-like structures); their shoots lack a central vascular bundle, and their rhizoids are smooth. Archegonia are formed at the apex of the gametophyte, while antheridia develop in the axils of the phyllids. The sporophyte forms at the apex of the main SHOOT and consists of a seta and a capsule that opens by valves. The spores possess elaters, and the protonema is thalloid.

Leafy mosses (Bryopsida) are characterized by a higher degree of complexity, featuring well-developed vegetative organs: a "stem" with differentiated Anatomical Structure, spirally arranged phyllids, and rhizoids. The sporogonium has a complex structure. Mosses with leaves are divided into three subclasses (sometimes treated as orders in certain textbooks): Andreales, Sphagnales, and Bryales (true or hypnoid mosses).

The subclass Andreales comprises about 120 species of tiny rock-dwelling plants distributed in the Arctic and mountains of both hemispheres, with vegetative organs colored reddish-brown. Their perennial stem is sympodially branched and bears phyllids. Rhizoids are multicellular and cylindrical. Phyllids lack a midrib. The sporogonium consists of a capsule and a foot (a gametophytic structure). The mature capsule is reddish-brown, elevated above the apex of the female gametophyte on a pseudopodium, and opens by longitudinal slits. The protonema initiates its development inside the spore before emerging to form branched green plates.

The subclass Sphagnales includes a large group of plants growing predominantly in oligotrophic peat bogs, forming cushion-like turf mats of various sizes. The subclass includes a single order and a single family with over 350 species, of which 32 species are widespread in Ukraine.

Their stems are branched, lack rhizoids, and are covered by a multi-layered epidermis (hyalodermis) whose cells feature thickenings and pores that conduct water. These are followed by elongated prosenchymatous cells providing mechanical support, while the core of the stem consists of parenchymal cells. The phyllids vary in shape from spatulate to triangular; they are single-layered and lack a midrib. Their cellular structure contains Two Types of cells: narrow chlorophyllose (green) cells and dead, colorless hyaline (water-storing) cells. During rain or snowmelt, hyaline cells absorb and retain water for extended periods, thereby promoting waterlogging. Plant remains and the lower parts of Sphagnum do not fully decay; instead, they become progressively compressed, eventually transforming over time into a semi-mineralized organic mass known as peat.

Sphagnum mosses are dioecious or monoecious plants, with male and female sex organs located on separate shoots. Branches bearing antheridia are club-shaped, swollen, and often brightly colored, accompanied by small floral bracts. Antheridia are situated singly in the axils of these bracts. Archegonia develop at the apices of large-leaved, shortened branches.

The sporogonium features a calyptra enclosing a spherical, dark brown capsule with an operculum, lacking a peristome. The sporangium is dome-shaped and positioned above the upper part of the columella. The spore germinates into a thalloid protonema.

The subclass Bryidae represents the most advanced group of plants among leafy mosses, encompassing approximately 14,500 species, with over 580 species found in Ukraine. Their gametophytes are perennial plants featuring a robust stem densely covered with phyllids of diverse shapes, while the underground portion is covered with multicellular rhizoids. The stem exhibits a complex anatomical structure: externally covered by an epidermis, beneath which lies the cortex, followed by a vascular strand. The outer part of the strand consists of thin-walled phloem with sieve elements, followed by one or two layers of starch-storing cells, with xylem occupying the center. The phyllids possess a central midrib. The vast majority of true mosses are dioecious plants bearing antheridia and archegonia at their apices. Male gametophytes often bear modified reddish leaves (perichaetium) at the apex surrounding the antheridia, inside which biflagellate spermatozoa are produced. On female plants, archegonia develop within apical green leaves (perianth) and sterile filaments known as paraphyses. Fertilization occurs exclusively in the Presence of water. A sporophyte develops from the zygote, consisting of a seta, a haustorium, and a capsule covered by a calyptra. The capsule comprises an urn and an operculum; the urn is covered by a delicate membrane called the epiphragm, with openings between the membrane and the urn that are regulated by hygroscopic Teeth known as the peristome. In dry weather, the peristome teeth curl outward, allowing spores to fall out of the capsule. Conversely, during rain, the peristome teeth absorb moisture and swell, sealing the urn to protect against moisture and preventing the spores from germinating inside the capsule. The urn is divided by a columella into four sections, in each of which sporangia attached to the urn walls by trabeculae develop. The spores lack elaters and are dispersed by the wind. Spores germinate into a filamentous protonema, from the buds of which male or female gametophytes develop.

The most common Representatives of the subclass include genera such as Polytrichum (Hair-cap moss), Leucobryum, and others.

Bryophytes are distributed globally, most frequently occupying habitats with elevated soil moisture, and often forming a continuous carpet in pine and mixed forests while playing a vital ecological role. They aid in moisture retention and maintain the hydrological balance of regions. However, they can occasionally contribute to the waterlogging of forests; upon dying, they form peat deposits that retain value as fuel, fertilizer, and a substrate for ornamental plants. Oligotrophic Sphagnum bogs harbor rare plants, including the widely distributed bog cranberry (Vaccinium oxycoccos). Consequently, the drainage of these bogs has recently been prohibited in Ukraine.