Marchantia 

Marchantia is a member of the Marchantiaceae family, which belongs to the division Bryophyta. It is commonly found in moist, cool, shady areas like wet open woodlands, banks of streams, wood rocks, or shaded stub rocks.

1.0Classification of Marchantia

Marchantia is a genus of liverworts, a group of non-vascular land plants. It belongs to the kingdom Plantae and the division bryophyta. Within this division, Marchantia is categorised under the class hepaticopsida, order Marchantiales, and family Marchantiaceae.

2.0General characteristics of Marchantia 

• The main plant body is gametophytic, thalloid, flat, prostrate, generally 2-10 cm long, one or more dichotomously branched. Each branch has a thick, central midrib. The midrib ends in a depression at the apical region forming an apical notch in which the growing point is situated.

Figure : Thallus of Marchantia

• The dorsal surface of the thallus contains polygonal areas known as areolae. Each polygonal area represents the underlying air chamber, which has tiny air pores that facilitate the gaseous exchange necessary for respiration and photosynthesis. 
• The sex organs occur on the dorsal surface and are always united into groups, the dorsal surface also bears a little cup-like structure known as gemma cups; these are the vegetative reproductive structures. Sexual reproductive structures are borne on special stalked structures called gametophores.   
• The gametophores bearing archegonia are called archegoniophores and those bearing antheridia are called antheridiophores. Marchantia is dioecious; male and female sex organs are produced on separate plants.  
• The ventral surface of the thallus bears scales and rhizoids along the midrib. Rhizoids anchor the plant and absorb water and minerals. There are two types of rhizoids: smooth-walled and tuberculate. Scales are violet-colored, multicellular, one cell thick, and arranged in 2-4 rows. The vertical cross-section of the thallus can be differentiated into three regions: the Epidermal region, the photosynthetic zone region, and the storage region.

Epidermal region: this region is further divided into upper and lower epidermis.

Upper epidermis: It is a thick wall protective layer that checks transpiration. 

Lower epidermis: some lower epidermis cells extend to form scales and rhizoids.

Photosynthetic region: Just below the upper epidermis, photosynthetic chambers are in a horizontal layer. Cells of this region contain chloroplast. It is the principal centre  of photosynthesis. Many simple or branched photosynthetic filaments arise from the base of the air chambers. 

Storage region: This region has parenchymatous cells which usually lack chloroplasts and arrange compactly. Most of the cells contain starch and protein grains while some cells contain a single large oil body. 

3.0Reproduction in Marchantia

Marchantia reproduces by asexually as well as sexually: 

1. Vegetative / Asexual reproduction in Marchantia

• By Gemma: the gemmae are produced in the gemma cups, which are found on the dorsal surface of the thallus. Mature gemmae are found to be attached at the base of the gemma cup by a single stalk. After falling on a suitable substratum, gemma germinates to form a new plant. 

• By fragmentation: The basal part of the thallus rots and disintegrates due to over a period of time . When this process reaches up to the place of dichotomy, the lobes of the thallus get separated. The detached lobes or fragments develop into independent thalli by apical growth.

2. Sexual reproduction in Marchantia 

Sexual reproduction in Marchantia is oogamous. All species are dioecious. Male reproductive bodies are known as antheridia, and females as archegonia. Antheridia are bi-flagellated, and archegonia are flask-shaped; antheridia and archegonia produce particular, erect, modified lateral branches of thallus called antheridiophore and archegoniophore, respectively. 

Archegoniophore and Antheridiophore

The gametangiophores (umbrella-like branches) on the female and male plants are termed as 'archegoniophores' and 'antheridiophores', respectively. These structures arise from the meristematic region in the notch of the thallus as the result of repeated dichotomous branching within a short period and raise the receptacle of the sexual organ several centimeters off the ground on a stalk .

The female sexual organ or 'archegonium' (plural: archegonia) is formed at the underside of the 'archegonial receptacle' ('archegonial disc' or 'carpocephalum') at the top of the archegoniophore . 

The archegoniophore develops at the apical notch of female thalli  and the archegonial receptacle shows a very lobed structure bearing 9–11 (rarely more) finger-like rays (digitate rays) .

The form of digitate rays seems suitable for retaining the water drops on the lower side . The groups of archegonia are between the bases of the separate digitate rays, except between the two first-formed ones, and are covered by the 'involucre' . In basic anatomy, the mature archegonial receptacle is quite similar to the thallus in that it contains air chambers, ventral scales and rhizoids. 

The transverse section of a digitate ray shows that the digitate form is derived from inward rolling of the highly branched thallus. The stalk of the archegoniophore also has a dorsiventral differentiation similar to that of the thallus. In transverse section, two grooves of rhizoids, ventral scales and air chambers are recognised. Two grooves of rhizoids make an efficient conducting system to transport water up to the archegonial receptacle .

Figure : Marchantia (A) Male gametophyte (B) Female gametophyte 

The male sexual organ, the 'antheridium' (plural: antheridia), is produced in the cavity of the upper side of the 'antheridial receptacle' (or 'antheridial disc'). The antheridiophore originates from the apical notch of the male thallus . 

A mature antheridial receptacle is shallowly divided into eight lobes and also anatomically the same as the thallus with air chambers, ventral scales, and rhizoids . 

On the upper side of the antheridiophore, there are two types of chambers. One is the air chamber, with a barrel-shaped pore like in the vegetative thallus. The other is the antheridial chamber with a simpler pore . 

In each of these antheridial chambers, there is one antheridium. The antheridial receptacle has a warped wing on the margin of its side, so water pools easily form on the receptacle. The stalk of the antheridiophore also has two grooves with rhizoids , like that of the archegoniophore, but in general, no air chamber is developed on the stalk.

4.0Fertilisation in Marchantia

• Fertilisation depends upon the presence of water and occurs when male and female thalli grow near each other. The ventral and neck canal cells disintegrate in the mature archegonium and form a mucilaginous mass. It absorbs water, swells up and comes out of the archegonial mouth. This mucilaginous mass consists of chemical substances.
• The antherozoids are splashed by rain drops. The male gamete of Marchantia is called an antherozoid, which is a long, curved, and biflagellate sperm cell.They may fall on the nearby archegoniophore or swim the whole way.
• It is only possible if both the male and female sex organs are surrounded by water. Many antherozoids enter the archegonial neck by chemotactic response and reach up to the egg. This mechanism of fertilisation is called the splash cup mechanism.

Sporophyte stage 

• With the formation of the zygote, the sporophyte life cycle of Marchantia starts; it consists of zygote, embryo and the sporogonium. A zygote does not immediately divide meiotically; after fertilisation, it divides mitotically, forming a multicellular sporophyte it is differentiated into foot, seta, and capsule.

Figure: Sporophyte of Marchantia

• Inside the capsule, sporogenous tissue is formed. Some cells of sporogenous tissue act like spore mother cells and divide meiotically to form four haploid spores. Each spore germinates and develops into free-living male and female gametophytes. 

5.0Some common species of Marchantia

Marchantia is represented by about 65 species, 11 of which are found in India. Some common Indian species are : 

Marchantia palmata, Marchantia polymorpha, Marchantia simlana

6.0The Life Cycle of Marchantia

Figure :The Life cycle of Marchantia

7.0Evolutionary significance of Marchantia

Liverworts, including Marchantia, represent one of the earliest lineages of land plants. Their transition from an aquatic to a terrestrial environment marks a critical step in the evolution of plants from water to land. Understanding the adaptations that allowed early plants to thrive on land is crucial for tracing the evolutionary history of terrestrial life.