Sexual reproduction in Marchantia

Sexual reproduction in Marchantia

Marchantia is a dioecious plant. The male reproductive organ and female reproductive organ are developed in the separate plant. The male reproductive organ is known as antheridia and the female reproductive organ is known as archegonia. The reproductive organs are developed in a special structure known as the receptacle. The stalk of the receptacle is known as gametophore. The gametophore bearing male receptacle with antheridia is known as antheridiophore. The gametophore bearing female receptacle with archegonia is known as Archegonophore.

Antheridiophore and antheridium

Antheridiophore is developed from the apical notch of the thallus. It is 1 to 3 cm long which bears male receptacle at its tip. The male receptacle is disc-shaped slightly convex 8 lobed structure. The internal structure of antheridiophore and a male receptacle is similar to the thallus but in a male receptacle, the antheridial chamber alternates with the air chamber. The air chamber contains photosynthetic filament and opens outside by air coat. The antheridial chamber contains single antheridium and opens outside by a pore called ostiole. The older antheridia are present towards the center and the younger antheridia are present towards the periphery. Such type of development is called acropetal development or succession.

Antheridium is oval shaped structure and it has the multicellular stalk. The antheridium is covered with single layered sterile jacket layer inside which there is the development of androcytes. Each androcyte metamorphoses into antherozoid. Antherozoid is male gamete which is rod-shaped coiled bi-flagellated structure. The antheridial wall absorbs water and disintegrates. The antherozoid comes out from the ruptured wall of antheridium.

Archegoniophore and archegonium

Archegoniophore develops from the apical notch of the thallus. It is 2 to 5 cm long and bears umbrella shaped female receptacle at its tip. The female receptacle is 8 lobed structure which bears ray between the lobes. Archegonia lies on the ventral surface of the female receptacle. The group of archegonia is covered with involucre called perichaetium. Each archegonium is covered with a sheath called perigynium. Archegonia are also developed in acropetal succession.

Archegonium is the female reproductive organ which is flask-shaped structure. It is divided into the vessel, bulbous venter, and elongated neck. Venter contains egg cell and venter canal cell. At the tip of the neck, there are four cover cells which cover the neck.

Fertilization

Water is essential for fertilization. During fertilization, the venter canal cells and neck canal cells degenerate and produce mucilaginous substance. Mucilaginous substance absorbs water, swells up and gives pressure on the cover cell and cover cell separates apart. In this way, there is the formation of passage for the entry of antherozoids. Antherozoids are attracted towards archegonium by the chemicals coming out from the archegonium. Such type of movement of antherozoids towards the archegonium in response to chemicals is known as the chemotactic movement. Many antherozoids enter inside the archegonium but only one fuses with the egg cell to form a diploid zygote.

Sporophyte

A zygote is the first cell of sporophytic generation. It develops into the embryo and then in sporophyte. It can be differentiated into three parts. They are:

• Foot
• Seta
• Capsule
  

  

fig:L.S of Sporophyte of Marchantia

The foot is basal, bulbous part of sporophyte which is made up of parenchymatous cells. It is attached to gametophyte and helps in absorption of nutrients from the gametophyte for developing sporophyte.

Seta

It is the elongated structure which connects foot and capsule. It also helps in conduction.

Capsule

It is oval or globular structure covered with single layer capsule wall. Capsule contain diploid spore mother cells which divide meiotically and produce haploid spores. The capsule also contains sterile diploid hygroscopic brown color cell known as elaters which help in rupturing of capsule wall and liberation of spores. It is covered with three protective coverings calyptra, perigynium, perichaetium. At maturity, seta elongates which pushes the capsule and help in rupturing of coverance. Spore liberate out from the ruptured wall of a capsule with the help of elaters.

Germination of spores

Spore is the spherical, haploid structure. Spore contains two layered wall outer layer is known as exine and the inner layer is known as intine. After coming out from the capsule if spores get moist, it can divide into new gametophytic thallus of Marchantia.

Alternation of generation in Marchantia

Marchantia itself is a gametophyte, which is followed by another structure called sporophyte, which grows independent in the moss plant and is the sporophyte. The lifecycle of Marchantia shows distinct alternation of generation.
The Marchantia plant (gametophyte) which develops from the haploid spores consists dorsoventrally differentiated thallus. Its dorsal surface bears male sex organ antheridia and female sex organ archegonia which produce male gamete and female gamete or egg respectively.
The antherozoids swim in water and reach the egg. Out of them, only one fuses with egg forming the diploid zygote during fertilization that represents the first stage of sporophytic generation. The diploid zygotes develop into sporogonium or sporophyte. The sporophyte is differentiated into the foot, seta, and capsule containing numerous sporogenous cells, which by meiosis cell divisions forms the tetrad of haploid spores that represent the first gametophytic stage. It is an asexual reproduction. Each spore germinates and produces haploid gametophytic thallus of Marchantia.
Here, the haploid gametophytic stage forms the diploid stage of sporophyte after fertilization and in turn, the diploid sporophyte forms the haploid gametophyte by an asexual method involving meiosis, that are regularly alternate to each other. This is known as alternation of generation.