Structure, Reproduction, Life Cycle, Importance microbiologystudy

Systematic Position of Marchantia (Source: USDA, NRCS. 2024. The PLANTS Database)

Kingdom- Plantae

Division- Hepaticophyta (Liverworts)

Subdivision- Hepaticae

Class- Hepaticopsida

Order- Marchantiales

Family- Marchantiaceae

Genus- Marchantia

Habitat and Distribution of Marchantia

It is one of the most common liverworts found in moist, shady, cool areas with abundant moisture. It grows in large mats. Damp soil, streams, springs, wet rocks are the favorable places for its growth. There are about 65 species of Marchantia and are found all over the world. The most common species of Himalayan region are M. palmata, M. polymorpha and M. nepalensis.

Morphology of Marchantia (Gametophyte Phase)

• Thallus is dark-green, fleshy, flat, dichotomously branched with dorsiventral symmetry. Each thallus lobe is traversed by a central midrib. 
• It also has a notch at the apex which is called as “apical notch.” 
• The upper surface of the thallus possesses rhomboidal or polygonal shaped areolae. Each areola has a tiny dot in the centre and that is the air pore that helps in aeration of the thallus. 
• The small cup-like structures called gemma cup are seen on the upper surface of the thallus which is responsible for vegetative reproduction.
• When the thallus attains maturity, they bear umbrella shaped structures at the apices of certain lobes. These are called gametophores and are of two types- antheridiophore and archegoniophore.
• The antheridiophore bears antheridia and archegoniophore bears archegonium.
• From the lower surface (ventral surface) of the thallus arise the rhizoids. These serves as an organ of anchorage and absorption. These are of two types- smooth walled and tuberculate.
• Besides the rhizoids, the ventral surface bears purplish flattered scales. These are usually arranged in two or four rows on either side of the thallus.

Anatomy of Marchantia (Internal Structure)

The internal structure of Marchantia is differentiated into three distinct layers:

• The epidermal region
• The photosynthetic region
• The storage region

The epidermal region

• It consists of well-defined upper and lower epidermis. The upper epidermis forms the layer over the photosynthetic region. 
• It consists of single layer of thin walled cells with slightly thickened outer walls. The epidermal cells are protective in function but they also contain few chloroplasts. Embedded in the epidermis, there are special barrel-shaped air pores. It facilitates gaseous exchange necessary for photosynthesis and respiration.

The photosynthetic region

• Beneath the upper epidermis is the air chambers which are arranged in single horizontal layer. The chambers are bounded by one cell thick partitions. From the floor of each chamber arise short, simple or branched filaments of green cells which are known as assimilatory or photosynthetic filaments. 
• The photosynthetic filament contains numerous chloroplasts. It is the principal centre of photosynthesis in the thallus. Photosynthesis is at its maximum rate in dim light.

The storage region

• Just beneath the photosynthetic region lies the storage region of the thallus. It consists of uniform tissue made up of relatively large, colorless, thin-walled polygonal parenchymatous cells that are compactly arranged. 
• They contain starch, protein grains and some of the cells contain oil-bodies. 
• The lowermost layer of the storage region is composed of cells similar to that of upper epidermis. This layer is the lower epidermis. From this layer, rhizoids and scales arise.

Reproduction in Marchantia

Marchantia reproduces both vegetatively and sexually. 

Vegetative Reproduction in Marchantia

The methods of vegetative reproduction are as follows:

1. Fragmentation- The cells in the older portions die of old age and eventually disorganize. When the death and decay reaches dichotomy, the young lobes becomes separated. Each of these grows into a new thallus.
2. Adventitious branches– In some species of Marchantia, special adventitious branches arise from the ventral surface of the thallus. In M.palmata, development of adventitious branches from the stalk and disc of female gametophore has been reported. They get detached from the parent thallus by the decay of the connecting tissue and form new plants.
3. Gemma formation– Gemmae are small bud like structures that are produced in large numbers. They are detached and carried by wind, water and eventually grows into new individuals in new habits. In Marchantia, gemma is a cup-shaped structure with fringed margins that is why it is known as gemma cup and it contains many buds. Each gemma stands on a short stalk and possesses mucilage hairs. The gemma are detached from the parent plant due to absorption of water and carried out by water current and each gemma eventually germinates into a new individual.

Sexual Reproduction in Marchantia

The sexual reproduction is oogamous type. It takes place only once during the growing season in high humidity when the days are long. 

Position and distribution of sex organs

The sex organs are borne on the vertical branches that are highly specialized for this purpose. The sexual branches are apical or terminal in position. Each upright sexual branch is called gametophore or gametangiophore. The gametangiophore in Marchantia are unisexual or dimorphic or heterothallic. The one bearing Antheridia is called antheridiophore and the one bearing archegonia is called archegoniophore. The antheridiophore and archegoniophore are borne on different thalli.

Antheridiophore- It consists of a stalk bearing disc at the terminal region. The stalk is long and cylindrical. The male receptacle is flattened lobed disc. Generally, it consists of eight lobes. The antheridial chamber lies deep inside the upper surface of each lobe. They are arranged in acropetal order. The antheridial chamber opens by a narrow channel called ostiole. 

Antheridium- The mature antheridium is ovoid object raised on a short, multicellular stalk. The stalk attaches to the antheridium to the floor of antheridial chamber. The body of antheridium has a jacket layer of sterile jacket which encloses androcyte mother cells. Each androcyte mother cell divides to form sperm cells. 

Dehiscence- Presence of moisture is necessary for the dehiscence of sperms. Water enters from the ostiole into the antheridial chamber. The cells at the apex of this chamber absorb water and disintegrate to form a pore. The sperms are then extruded out through the pore and swim freely in thin film of water to reach archegonia.

Development of antheridium–

• Each antheridium develops from a cell called antheridium initial that lies in the antheridiophore of the thallus.
• It matures and divides transversely to form upper cell and lower cell. The upper cell develops into upper primary antheridial cell and the lower one develops into primary stalk cell. Both undergoes transverse cleavage and form four cells. 
• The primary stalk cell undergoes few divisions to form stalk.
• The primary antheridial cell divides by parallel; cleavage to form a row of two or three cells. Each of these further divides by two successive vertical walls at right angles to each other. 
• Periclinal division takes place that separates outer tier of cells and inner tier of cells.
• The outer tier forms the jacket initials which later form the jacket and the inner tier forms primary androgonal cells.
• The primary androgonal cells further undergo division to form sperm mother cells or androcyte mother cells. They divide diagonally to form sperm cells which later give rise to sperm

Spermatogenesis

• Androcyte mother cells divide diagonally to form androcytes. Each androcyte has a large nucleus and dense chloroplast. 
• There exists a small granular extra-nuclear body, near the periphery of the androcyte which is known as blepharoplast. 
• The androcyte along with the blepharoplast and nucleus becomes elongated and the blepharoplast forms a cord like structure. At this stage, androcyte become comma shaped and the blepharoplast coils up to the periphery.  
• The nucleus becomes sickle-shaped and moves to the periphery and from the end of the blepharoplast two flagella arise.

Archegoniophore- It consists of a stalk bearing disc at the terminal region. The stalk is long and cylindrical and resembles that of antheridiophore, however the stalk is usually longer. The female disc is an inconspicuously eight lobed object. The growing apices of these bend downward. From the margin of the disc grows long cylindrical processes called the rays. The archegonia are developed in acropetal order and covered by thin layer called perichaetium.

Archegonium- The archegonium is a flask- shaped structure that consists of a swollen portion called venter and a slender neck. 

The venter forms the swollen part. The stalk is short and helps in attachment of archegonium to the receptacle. 

Next to this, there exists a vertical row of four cells, the neck canal cells surrounded by a layer of sterile cells forming a protective jacket. 

The tip of the neck consists of four specialized large cells, known as the cover cells.  The venter also has a jacket of sterile cells which makes up the venter wall. The venter cavity consists of two cells, the larger one is the egg cell and is situated at the lower side and the upper one is ventral canal cell which is smaller in size.Dehiscence- When the archegonium reaches maturity, the ventral canal cells and the neck canal cells degenerate to form mucilage. This mucilage imbibes water and swells which leads to the opening up of four cover cells thus making a passage for sperms.

Archegonium development- 

• The archegonium develops from archegonial initial that lies on the female receptacle.
• It divides transversely to form upper cell and lower cell. The lower cell takes no part in archegonium formation. The upper cell acts as archegonial mother cell. It enlarges and divides by three eccentric vertical walls and forms three peripheral initials surrounding a middle fertile cell (primary axial cell). 
• Each of these three peripheral initials divides longitudinally and gives rise to jacket initials. 
• These jacket cells undergoes transverse division and undergoes differentiation to form two tiers of six cells each. The upper tier functions as neck initials and the lower tier functions as venter initials. Neck initial forms neck and venter initial forms the venter.
• Meanwhile, the primary axial cell divides transversely to form one large lower central cell and upper primary cover cell. The central cell forms upper primary neck canal cell and a lower ventral cell. Primary neck canal cell divides to form neck canal cells. The ventral cell forms ventral canal cell and egg cell.

Fertilization in Marchantia

Fertilization takes place in presence of water. The male and female receptacles are borne in different plants, thus it is important for both the plants to grow together. It usually takes place by splash cup mechanism. 

In this method, the sperms splashed on the grounds by the rain drops from the male receptacle swim through the water and reach the archegonia. 

However, sperms are also discharged by small insects, mites etc. In the meantime, the neck canal cells disintegrate to form mucilage and absorb water to swell which leads to opening of cover cells. Thus a passage is formed. The mucilage in archegonium contains chemical substances that attract the sperms and due to chemical interaction, they enter the venter and the most compatible one penetrates the egg and unites to form zygote.

Sporophyte

It comprises of zygote, embryo, and sporogonium.

Zygote

• It is a unicellular structure formed by the fusion of male and female gametes and is the pioneer structure of the sporophytic phase. 
• It secretes a wall around it and enlarges in size. 
• The zygote is retained within the venter and has a diploid nucleus with cellulose cell wall around it. 

Embryo

• The zygote undergoes repeated division and cell enlargement.
• A spherical mass of undifferentiated cell is formed which is known as embryo.
• The venter expands as a close envelope, two cell layers thick and forms the calyptra over the developing embryo.

Sporogonium

• The sporophyte of Marchantia consists of foot, seta and capsule.

The zygote divides by a horizontal wall at right angles to the axis of archegonium that leads to the formation of epibasal and hypobasal regions. The capsule is formed from epibasal region. Again, further cell division takes place to form quadrant stage and octant stage respectively. The following changes can be seen in the surrounding tissue as well- 

• The ventral cell undergoes periclinal division to form calyptra.
• Perigynium eventually forms cylindrical sheath.
• Perichaetium and rays further develops forming a protective covering around whole group of archegonia.
• The four epibasal cell by repeated cell division and cell differentiation forms the capsule.
• The hypobasal cells give rise to foot and seta.
• The cells in the capsule region by periclinal division give rise to different layers- 
  • The outer layer of cells is called ampithecium and the inner one is called endothecium. 
  • The cells of ampithecium are undergoes division to form capsule wall. 
  • The cells of endothecium give rise to sporogenous tissue called archesporium. 
  • This archesporium matures and divides to form spore mother cells. 
  • Each spore mother cell divides meiotically to form four spores that are arranged in tetrahedral manner and also known as spore tetrad. 
  • However, few cells differentiate and elongate to form elaters.
  • In a mature spore, there are two spore coats, the outer one is exine and the inner one is intine. 
  • At maturity, this layer separates to release spores.
• The young sporogonium is protected by three sheaths i.e. perigynium, the calyptra and the perichaetium.

Life Cycle of Marchantia

The life cycle of Marchantia includes two different generations, the sporophyte and the gametophyte. 

Sporophytic generation is diploid and depends completely upon gametophytic generation. 

Gametophytic generation is haploid and is the dominant phase in life cycle of Marchantia. Both the generations are morphologically dissimilar to one another so this type of alternation of generation is called heterologous or heteromorphic alternation of generation. 

These two individuals occur one after the other generation after generation.

Economic Importance of Marchantia

• Soil Formation and Stabilization

Marchantia plays a critical role in soil formation by breaking down rocks and organic matter, which contributes to the creation of new soil. It also helps in stabilizing soil in moist environments, preventing erosion and maintaining soil structure.

Marchantia species are sensitive to environmental changes, making them valuable bioindicators. They can be used to monitor ecosystem health and detect pollution levels, providing early warnings of environmental degradation.

Marchantia contains compounds with antibacterial, antifungal, and anti-inflammatory properties. These medicinal properties have been recognized in traditional herbal medicine and are currently being explored for potential pharmaceutical applications. Roig Y. Mesa (1945) mentioned that Marchantia polymorpha is used to cure pulmonary tuberculosis and afflictions of the liver.

Marchantia supports biodiversity by providing habitat and food for various microorganisms and small invertebrates. This contribution is essential for maintaining ecological balance and promoting nutrient cycling within their ecosystems.

• Biotechnological Applications

The liverworts have played an important role at research tools in various phases of botany such as genetics, morphology and physiology. Marchantia polymorpha is a prominent model organism in plant biology and genetics. Its simple structure and ease of cultivation make it ideal for studying plant development, gene function, and responses to environmental stress.

References

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