Bryophyte

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About Bryophytes

The term bryophyta, (Gk. bryon : moss; phyton : plant), is used as a collective name to represent a group of plants that includes liverworts, hornworts and mosses growing predominantly in moist environment. The group, therefore, goes well with the name of amphibians of plant kingdom owing to the amphibious habitat of plants.

Characteristics of Bryophytes

The plants are characterized by the presence of conspicuous, green, well developed, nutritionally independent gametophytes to which are always attached physically and nutritionally dependent sporophytes. The gametophyte constitutes the dominant phase of life cycle that exhibits sharply defined alternation of generation.Lack true roots, stems or leaves. Unicellular or multicellular rhizoids are present. They are first embryophyta. Some cells of sporophyte undergo meiosis to produce haploid spore, which germinate to produce gametophyte.Diploid zygote is produced, which do not undergo immediate reduction division.

Fertilization takes place with the help of water.

Male gamete is biflagellate.Sex-Organ is multicelluar, male sex organ is antheridium and female sex organ is archegonium, which is flask shaped.Gametophyte is photosyntheticMain plant body is gametophyte and sporophyte is dependent on it. Gametophyte (The plant Body).The plants show two morphologically distinct heteromorphic generations, i.e., gametophytic and sporophytic generations.Gametophytic generation is the dominant phase of life cycle and in general the term plant body is used to represent this phaseThe gametophytes are well developed, green and autotrophic to which the sporophytes are not only attached but are also physically and physiologically (nutritionally) dependent.The plant body of primitive forms, e.g., Riccia and Marchantia is thalloid but in mosses. It is foliose and is differentiated into root like (rhizoids), stem like (axis) or cauloid and leaf like (phylloid) structures.The thalli of primitive forms are found attached to the substratum by unicellular unbranched rhizoids but in higher forms as in mosses these are attached by means of branched multicellular rhizoids. Rhizoids are absent in aquatic forms. In Marchantiales (e.g., Riccia, Marchantia etc.) often multicellular scales are present in addition to rhizoids. Scales/amphigastria take part in capillary action and protection.

Bryophytes lack vascular tissues. The plant body consists of simple parenchymatous cells. Xylem, phloem and lignified cells are completely absent. The parenchymatous cells may be differentiated into several types e.g., chlorophyllous cells, storage cells, rhizoids, etc. to perform various functions, as in Riccia and Marchantia. Few thick walked cells called stereom are present in mosses. Mosses retain moisture like sponges.

REPRODUCTION of Bryophytes

Bryophytes reproduce by vegetative and sexual methods.

Vegetative Reproduction of Bryophytes

Bryophytes largely multiply by means of vegetative reproduction which is accomplished by fragmentation, adventitious branches, tubers, persistent apices, buds, gemmae, rhizoids, primary protonema, secondary protonema, etc.

Sexual Reproduction of Bryophytes

The sexual reproduction is invariably advanced oogamous type.

Sex organs are multicellular and jacketed with sterile jacket. They may be embedded type, e.g., Riccia, Anthoceros or projected type, e.g., Marchantia, mosses, etc.

The male reproductive organ is called antheridium. It consists of a central mass of androcytes enclosed by a single layer of sterile jacket. Each androcyte produces a single biflagellate spermatozoid or antherozoid.

The female reproductive organ is called archegonium. Each archegonium is a multicellular and flask shaped structure. The basal swollen portion is called venter whereas slender and elongated upper portion is called neck. Wall of the neck is single layered made up of 6 rows of 6-8 cells each. Venter has 1-2 layered wall. The neck of archeogonium is filled with 6-10 neck canal cells (4-6 in Riccia) and the venter has a large egg cell and small venter canal cell. The egg is large and non-motile.

Fertilization of Bryophytes

 

Water is indispensable for fertilization (zooidiogamy).

Many antherozoids swim to the neck to archegonium.

All the neck canal cells and venter canal cell disorganise to form mucilage, carbohydrate, proteins, K+, etc. These chemicals not only provide the medium for swimming of antherozoids but also chemotract them. Many antherozoids enter into the venter but only one, the most active one, fuses with egg to form diploid zygote (oospore).

With the formation of diploid zygote, the gametophytic generation ends and the sporophytic generation starts.

The Sporophyte

The zygote, immediately after fertilization, divides repeatedly without undergoing any resting period. The first division of zygote is transverse and the embryo proper develops from the outer cell. This type of embyrogeny is commonly known as exoscopic embryogeny.The embryo is not liberated but is retained within the archegonium where it develops into a sporophyte. The sporophyte in bryophytes is called sporogonium because it is dependent and mainly meant for producing spores. The sporophyte consists of foot, seta and capsule. In a few cases only seta is absent as in Corsinia whereas in Riccia both foot and seta are absent. Wall of the venter proliferates to form a covering around the growing embryo. It is called calyptra.In a capsule, the spores are formed after meiosis. These spores (meiospores), that are all of one kind make the plants homosporous.

The Young Gametophyte

The spore is the mother (first) cell of the gametophytic generation. .

The spores are cutinized and non motile. They are exclusively wind disseminated. The individual spores separate from the tetrad before they are discharged from the capsule.

The spores germinate directly into the new gametophytic plants (e.g., Riccia and Marchantia), but in mosses they germinate into filamentous protonema from which are produced buds that give rise to a new plant. The protonema represents the juvenile stage of plant body.

Thus, bryophytes have heteromorphic or heterologous alternation of generation.

CLASSIFICATION of Bryophytes

Bryophytes are divided into three classes: Hepaticae (Liverworts), Anthocerotae (Hornworts), Musci (Mosses). However, ICBN recommended names of these classes as Hepaticopsida, Anthocerotopsida and Bryopsida.

The recent classification of Bryophyta is a as follows:

Liverworts

Present in moist and shady habitats, such as banks of streams, marshy ground, damp soil, bark of trees and deep in woods. E.g., Marchantaia, Riccia.

Liverworts are thalloid and thallus is dorsiventral which is closely appressed to the substratum.

Some members like Pelia and Porella is leafy having tiny leaf like appendages in rows on the stem like structure.   

Marchantia: Female thallus, Male thallus

Asexual reproduction by fragmentation of thallus or by specialised  structures called gemmae.

Gemmae are green, multicellular, asexual buds, produced in small cup like receptacles called gemma cups, present on upper suface.

When gemmae detaches from parent thallus, it germinates to form new individual.

Sex organs are produced on same thallus or on different  thallus.

Sporophyte is differentiated into foot, seta and capsule.

Anthocerotopsida or Hornworts 

Anthoceros, Notothylas (members of ANthocerotae) are called as hornworts, as they possess elongated ‘horn like’ sporophytes. Capsules of these members contain spores and pseudoelaters (elater like structures without spiral thicknenings). Anthoceros cells have one chloroplast with one pyrenoid, so resembles algal thallus. 

Mosses

THE SPOROPHYTE of Funuria

The mature sporophyte is a complex and highly elaborated structure differentiated into the foot, seta and capsule.

Foot: It is a poorly developed, small, dagger like conical basal part of the capsule embedded in the apical tissue of the female shoot. It functions both as an absorbing and anchorage organ.

Seta: It is a long, slender, tough and twisted reddish brown stalk like structure. It bears capsule at the top.

Capsule: The capsule is highly organized, erect or pendent, pear shaped structure situated at the tip of long seta. It is chiefly concerned with the formation and dispersal of the spores. Externally it is differentiated into three well marked regions (i) sterile basal region the apophysis (ii) central fertile region, the theca and (iii) upper region the operculum enclosing peristome.

Apophysis: It appears as an expanded part of the seta. It constitutes the basal zone of the capsule that gradually narrows below. It is characterized by the presence of stomata. Each stomata has an annular guard cell with an opening in the middle that opens into a small air space. It content chloroplast therefore it is the site of photosynthesis. The central tissue of this zone that consists of vertically elongated achlorophyllous cells and is collectively called as central strand. 

Theca: The central zone of the capsule situated in between the apophysis and the operculum is called theca. The wall of this zone is several cells thick and highly differentiated. A large number of wide, cylindrical air spaces occurring within the capsule wall are formed in between transversely oriented strands of narrow, green elongated cells the trabeculae which connect the innermost wall of the capsule with the outer most wall of the spore sac. Spore sac is a barrel shaped structure open at both the ends. It is situated in between the wall of the capsule and the columella and contains numerous spores. The central part of the theca, the columella looks like a solid cylinder composed of delicate, colourless compact parenchymatous cells. The columella is expanded above whereas its narrow basal part is connected with the central strand of the apophysis.

Operculum: The upper most part of the capsule is highly modified in relation to spore dispersal. It looks like an obliquely placed conical cap and is known as operculum. 

Immediately underneath the operculum attached below the edge of the rim, lies the peristome consisting of two rings of long conical teeth one within the other. Each ring of peristome possesses 16 teeth. The teeth of inner ring of peristome are colourless and delicate with their bases covered by the outer peristomial teeth that are placed opposite to that of inner ones. The outer teeth are conspicuous, red in colour and possess thick transverse bands. The tapering distal ends of the outer peristome teeth and joined to a centrally placed disc of tissue. The opening of the spore sac is closed by these elaborately sculptured and highly hygroscopic teeth

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