Marchantiales: Structure, Reproduction, Classification

Marchantiales is an order of Division Bryophyta (Class Hepaticopsida).

• This order consists of thalloid species and plants are not divisible into leaf, stem, and rhizoids.
• The gametophytes (plant bodies) of Marchantiales are prostrate, ribbon-shaped, dichotomously branched, and dorsiventrally differentiated.

Structure of Gametophyte

The gametophyte of Marchantiales is the plant body.

External Morphology

• In Monocleaceae, the simplest thallus is observed. Which are dichotomously branched mid-rib also branched. Rhizoids are two types which are smooth-walled rhizoids and tuberculated or pegged rhizoids. Monoclea foresteri is the largest thallus of Bryophyta. At dorsal region involucre present in which sex organs occur.
• In Ricciaceae, small thallus unbranched mid-rib present. Plants occur in colonies. Mature rhizoids lack protoplasm.
• In Corsiniaceae thallus are long 2-3 times dichotomously branched and ribbon-shaped. Mid-rib is also branched.
• In Targionaceae, the shape of the thallus is uneven adventitious branched tubers also occur. Archegonia are present at the apex. Mid-rib is absent.
• In Marchantiaceae, the thallus is large and 2-3 times dichotomously branched. Branched mid-rib is also present. Gemma cup is present in mid-rib at dorsal portion.

Internal Morphology

• In Monocliaceae, the thallus is not differentiated into photosynthetic and storage regions. Some cells are chlorophyllous, surrounded by the upper and lower epidermis. These plants bear smooth and tuberculated rhizoids. Scales are multicellular and violet-colored present at Ventral margins.
• In Ricciaceae, the thallus is divisible into photosynthetic and storage regions. The photosynthetic region is filamentous, while the storage region is parenchymatous.
• In Corsiniaceae, photosynthetic filaments open outside by a single air pore.
• In Targionaceae, also photosynthetic chambers are present in which 3 or more times filaments occur. Each filament is made up of 3 cells. The uppermost cell is large. The pore is a complex type and is surrounded by 4-6 cells.
• In Marchantiaceae, photosynthetic regions have two photosynthetic filaments, one is simple and the other is branched. The pore is a complex type and is surrounded by 6 cells.

Sexual Reproductive Structures of Marchantiales

• In Ricciaceae, sexual reproductive structures (sex organs) are inserted in the thallus, while in Corsiniaceae, sex organs are present on a receptacle at the dorsal surface. Receptacle present near the median axis.
• In Targioniaceae sex, organs are present at lateral branches, which arise from the ventral surface.
• In Monocleaceae, sex organs are present in the sessile receptacle, which are present at the dorsal surface, while in Marchantiaceae, specific organs are formed for sex organs, named as atheridiophore and archegoniophore.

Antheridium

• Antheridium is a male sex organ and is found on the male thallus, which is developed from an antheridial initial cell.
• This cell enlarges and divides by transverse division, which forms an upper antheridial cell and a lower stalk cell.
• The Antheridial cell is now divided by transverse division and longitudinal division and forms 4 celled structure, while the stalk cell is divided by periclinal division to form jacket initial and central cell.
• The jacket initial continues divided to form a unicellular thick jacket and the central cell is divided to form antherozoid mother cells.
• Now antherozoid mother cell forms the antherozoid (sperm) which is biflagellate and helical. When the antherozoid matured the antherozoid mother cell rupture to release the antherozoid.
• In such a way antheridium is made up of multicellular stalk and club-shaped structure.

Archegonium

• Archegonium is a female sex organ developed at the female thallus.
• Inside involucre archegonial initial is present which is divided by transverse division to form an upper cell and a basal cell.
• The basal cell divides to form a stalk, while the upper cell is divided by two longitudinal divisions to form a peripheral and a central cell.
• Peripheral cell divided by transverse division to form primary cover cell and primary jacket cell. The primary cell divides to form the jacket.
• Now the central cell is divided by transverse division which forms the central canal cell and primary neck canal cell.
• The neck canal cell divide to form 4 neck canal cell, while the central canal cell is also divided and form a ventral canal cell and an egg (oosphere).
• In such a way in the mature archegonium 4 cover cells, 4 neck canal cells are present.

Fertilization

• When archegonium matures and the antherozoid is released towards the archegonium, neck canal cells and ventral canal cells dissolved and water enters in neck portion.
• From the neck of the archegonium, some salts or proteins are excreted which stimulates the antherozoids to move in that direction. Then the antherozoids enter the venter but only one of them fertilizes the egg or oosphere and forms a diploid zygote (oospore).

Development of the Sporophyte

• The zygote is the first cell of the sporophyte, immediately after formation, the zygote increases in size.
• The zygote is divided by transverse division to form epibasal and hypobasal cells.
• Hypobasal cells divided further to form either only foot or foot and seta both. Epibasal cell contributes to the formation of the capsule but in Ricciaceae zygote directly forms the capsule.
• So, it shows the least sterilization in which directly through transverse division and periclinal division takes place and amphithecium and endothecium is formed.
• Amphithecium produces a one-celled thick jacket and endothecium forms archesporium tissue. Some cells form spore-tetrad (four haploid spores) by meiosis and some form nurse cells. But in Corsiniaceae only the foot and capsule are formed and in the capsule spore-tetrad is present, it shows more sterilization than Ricciaceae.
• In Targioniaceae, the sporophyte is divided into a foot, seta, and capsule. In the capsule, the spore-tetrad, and elaters are formed.
• In monocleaceae and Marchantiaceae well-developed foot, seta and capsule are present. In the capsule, there are elaters and spore-tetrad is present.

Structure of Sporophyte and Dispersal of Spores

• In Ricciaceae, a sporophyte is inserted in the thallus made up of a one-celled thick jacket, tetraspores, and nurse cells. Spore dispersal takes place owing to the break up of the surrounding thallus tissue.
• In Corsiniaceae, seta absent and sporophyte contains only foot and capsule. Capsule consists of a one-celled thick jacket, tetraspores, and elaters. While in Targioniaceae, the sporophyte is divided into a foot, seta, and capsule. The capsule contains elaters and tetraspore.
• In Monocleaceae and Marchantiaceae highly developed foot, seta and capsule are present. Which is surrounded by calyptra. Spores are dispersed due to the breaking of a dried jacket.

Classification of Marchantiales

Smith (1955) has divided order Marchantiales into five families:

• Ricciaceae (e.g., Riccia)
• Corsiniaceae
• Targioniaceae
• Monocleaceae
• Marchantiaceae (e.g., Marchantia)