Q. 2. Give briefly an illustrated account of the sexual reproduction in Marchantia.
Ans.2. The sexual reproduction in Marchantia is of highly advanced oogamous type and it takes place by definite male and female reproductive organs which are known as antheridium and archegonium respectively. The male and female reproductive organs always remain present of the gametophytic thallus and are borne on special stalked erect structures which are raised some distance above the vegetative thallus.
These special structures are known as antheridiophores and archegoniophores respectively. Marchantia is usually heterothallic i.e., antheridia and archegonia are borne on separate thalli.
Male reproductive organs or Male receptacle
The male reproductive organs are the antheridium. The antheridia are borne on the disc raised above the thallus on slender stalks called antheridiophores. Thus, the antheridiophore consists of stalk, one to three cm, long bearing at its apex a lobed disc, which is usually eight lobed. These antheridiophores are modified upright branches of the thallus. The antheridial receptacle of Marchantia is a flat structure with eight rounded lobes.
A number of marginal growing points are formed at the tips of lobes, from each of which an acropetal series of antheridia extends towards the centre. The antheridia are located in flask-shaped cavities-antheridial chambers on the surface of receptacle shows the same structures as that of the vegetative tissue.
The mature antheridium is a club-shaped structure consisting of a short stalk and a globular body. Jacket of body consists of a single layer of thin walled cells side with a large number of androcytes. The androcytes get metamorphosed into antherozoids which are bicilliate.
Development of antheridial receptacle or male reproductive organs
Any cell from the dorsal surface of thallus near midrib functions as antheridiophores initial. It increases in size and divides by several divisions so as to form a multicellular elongated stalk or antheridiophores. The apical position of the stalk gets swollen and due to checking of growth at various portion of swollen part a 8 lobed disc is formed known as antheridial disc.
Now a cell in each antheridial disc’s lobe functions as antheridial initial cell and develops into a mature antheridium just in the similar way as in Riccia.
Female Reproductive Organs or Female Receptacle
The female reproductive organs are known as archegonium. The archegonium is borne on the discs raised above the thallus on slender stalks, called archegoniophores. Thus the archegoniophores (Carpocephallum) consists of stalk and archegonial disc. The disc of archegoniophore becomes a lobed structure as follows.
While the archegoniophore is still very young, its apex forks and becomes swollen and dichotomy is repeated in quick succession, as a result of which the young archegoniophore ultimately becomes a rosette eight lobed disc. Now the apical cells of each lobe of disc, cut-off segments on the dorsal and ventral faces.
The archegonia begin to form in acropetal succession, from the segments cut-off on the dorsal face. Soon eight groups of archegonia develop on the upper surface of the disc, corresponding to eight growing point of disc.
After fertilizing the stalk of the archegoniophore begin to elongated and side by side and there is a conspicuous over growth in the sterile central part of the disc, with the result that marginal apical region of the disc along with group of archegonia and pushed over the lower surface. Finally the growing apices are the turned strongly downwards and incurved close to stalk of the archegoniophore.
After curvature there is a development of one layered plate of tissue on either side of each group of archegonia. Thus, each group of archegonia (about 12 t0_15) is enclosed by a two lipped pendent, fringed involved sheath known as perichaetium or involucre. (BSc Botany Marchantia Notes Study Material)
The mature archegonium is a flask shaped structure. The bulbous venter is separated by the slender stalk. The neck consists of 6 vertical rows of Jacket cells, surrounding 4 to 6 neck canal cells. The venter has an egg and ventral canal cell.
Fertilization: Water medium is essential for fertilization. At the time of fertilization in the usual manner, the neck canal cells and ventral canal cells of archegonium disorganise forming a mass of chemical substance which help in the attraction of antherozoid. The free swimming antherozoids enter the archegonium neck. One of the antherozoid fuses with the egg and forms zygote.