MARCHANTIA - CLASSIFICATION, STRUCTURE OF THE GAMETOPHYTE, REPRODUCTION, SPOROPHYTE

A. CLASSIFICATION

Division – Bryophyta
                  Class – Hepaticopsida
                        Order – Marchantiales
                                Family – Marchantiaceae
                                      Genus – Marchantia

The genus Marchantia comprises about 65 species and is widely distributed all over the world. The genus is mainly confined to temperate regions, and is found to grow in moist, cool and shady places.

About 11 species of Marchantia have been reported from India mainly growing in Himalayas and very few species occur in plains and hills. Of these the common species are – M. polymorpha, M. nepalensis, M. palmata, etc.

B. STRUCTURE OF THE GAMETOPHYTE

1. External structure:- The plant body of Marchantia is thallus-like, prostrate, flat, dorsiventrally differentiated and richly dichotomously branched. The apex of each branched is notched. The thallus is deep-green in colour and is provided with midrib. On the dorsal surface along the midrib, there appears a cup-like structure known as gemma cup. It appears when the thallus attains maturity. Mature thallus bears also bears certain specialized sex organs i.e., antheridiophore and archegoniophore which bears in them antheridia (male sex organs) and archegonia (female sex organs) respectively.

The ventral (lower) surface of the thallus bears numerous multicellular hairs or scales, and unicellular rhizoids of two types – smooth walled rhizoids and tuberculate rhizoids. Rhizoids attach the thallus to the substratum and absorb water and solutes from the soil.

2. Internal structure:- Internally the thallus of Marchantia shows tissue differentiation of various types –

(a) Epidermal Region:- It consists of well defined upper and lower epidermis. The upper epidermis forms a surface layer over the photosynthetic region and consists of thin walled cells with slightly thickened outer walls. The epidermal cells contain few chloroplasts. Embedded in the epidermis are special chimney-like or barrel-shaped air pores.

(b) Photosynthetic Region:- Beneath the upper epidermis are the air chambers in a single horizontal layers. The chambers are bounded by a one cell layer thick partition. The partitions are three or four cells in height. Each chamber communicates with the exterior through a barrel-shaped or chimney-like pore. From the floor of each chamber arise short, simple or branched filaments of green cells, known as assimilatory or photosynthetic filaments. The cells of photosynthetic filaments contain numerous ovoid chloroplasts.

(c) Storage Region:- Just below the photosynthetic region lies the storage region. It is thickest in the centre and tappers towards the margin. It consists of a uniform tissue made up of relatively large, colourless, thin-walled polygonal, parenchymatous cells. The cells of this region lack chloroplasts but contain starch and protein grains. Isolated cells may contain a single large oil body or may be filled with mucilage. The former are called body oil cells and later mucilage cells.

The lowermost region of the storage region is composed of cells similar to those of the upper epidermis. It is the lower epidermis. From it projects project rhizoids and the scales.

C. REPRODUCTION

The thallus of Marchantia reproduces both by vegetative and sexual process of reproduction –

1. Vegetative Reproduction:- Vegetative reproduction takes place by the following process –

(a) Fragmentation:- It depends on the ageing of the vegetative cells. The cells in the older portions die of old age and eventually disorganize. When the dead and decay of the older portions of the thallus reaches the dichotomy, the young lobes become separated. Each of these by apical growth grows into a new thallus.

(b) Adventitious Branches:- This may develop from any part of the thallus particularly the ventral surface. In Marchantia palmata the development of adventitious branches from the stalk and disc of the female gametophore was reported.  They become detached from the parent thallus by the decay of the connecting tissue and form a new plant.

(c) Gemmae Formation:- Marchantia also reproduces by formation of multicellular bodies called the gemma. Gemmae are produced in large numbers within a small cup-like structure called gemma cups borne on the dorsal surface of the gametophyte along the midrib. Each gemma after detachment from the gemma cup germinates into a new plant under favourable condition.

2. Sexual Reproduction:- Marchantia is heterothallic i.e., dioecious. In female plant, the archegonia are borne on special erect, stalked branch called archegoniophore; similarly in male plants the antheridia are borne on erect, stalked branch called antheridiophore.

(a) Archegoniophore:- Each archegoniophore arises from a region just behind the apex of the thallus. The mature archegoniophore consists of a long stalk bearing at its apex a distinct, generally 8-rayed or lobed stellate disc. Between the rays, group of archegonia are arranged in radial rows, each row being borne in an inverted position.

Each archegonium is a flask shaped structure and consists of a narrow neck with several neck canal cells and a swollen venter containing a ventral canal cell and an egg. Each archegonium is attached by a short stalk to the lobe of the disc.

(b) Antheridiophore:- Each antheridiophore is similar in position to the archegoniophore. It consists of a stalk (1-3 cm long) bearing at its apex a disc with generally 8-lobed margin. Towards the upper part of the disc antheridia occur in flask shaped cavities.

Each mature antheridium is a pear-shaped or globular body with a short stalk, and a jacket layer of single layer of thin walled cells. Within the antheridial jacket lie numerous androcyte mother cells. Each androcyte mother cell divides diagonally to form two triangular androcytes (antherozoid mother cells). Ultimately, each androcyte is metamorphosed into biflagellate sperm i.e., antherozoid.

D. FERTILIZATION

            It takes place in presence of water. Generally antherozoids from the top of the archegoniophore discs are splashed by rain water on to the slightly elevated archegoniophores above the thallus. The antherozoids then swim in the moisture and ultimately reach the egg. On reaching the egg, one sperm fuses with the egg to complete the fertilization.

            As a result of fertilization, a diploid zygote (2n) is formed. With the formation of zygote, diploid sporophytic generation begins.

E. SPOROPHYTIC PHASE

Zygote is the first cell of the sporophyte. Immediately after the formation, the zygote enlarges in size until it nearly fills the cavity of the venter. The cell of the venter also divides and redivides forming a several layered calyptra which surrounds the developing sporophyte. Some of the cells of the venter also divide and redivide forming a collar like outhgrowth, one celled thickness, known as pseudoperianth or perigynium.

The zygote nucleus first divides transversely into two cells, an upper epibasal cell and the lower hypobasal cell. Then another vertical division takes place so that 8-celled structure ie., octant is formed. Derivatives of epibasal cell contribute to the formation of capsule and the upper part of the seta, while derivatives of hypobasal cell gives rise to the lower part of seta and the foot of the sporophyte.

  Next periclinal divisions of the epibasal cells take place, as a result outer amphithecium and inner endothecium. Amphithecium forms the single layered jacket of the capsule, while endothecium gives rise to sporogenous cells. About half of the cells produced by the endothecium contribute the formation of spore mother cells, while remaining cells become sterile and form long tapering, slender cells with bi-spiral wall thickenings known as elaters. Each spore mother cell by meiotic division forms four haploid spores (spore-tetrad).

The mature sporophyte is more or less elongated structure and differentiated into three distinct regions – foot, seta and capsule. The foot is bulbous or anchor shaped spreading structure directed towards the archegonium base. The seta is short and thick, and lies in between foot and capsule. The capsule is nearly spherical or oval in structure and provided with a jacket of single layered cells. The capsule contains inside spores and elaters.

After liberation, the spore germinates under favourable condition by producing a germ tube to give rise to new individual Marchantia plant.

Prema Iswary,               

Assistant Professor,     

Department of Botany.

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