Bryophytes

Gametophyte Of Marchantia

The real plant of Marchantia is its gametophyte. The gametophyte of Marchantia has a dorsiventral thalloid structure.

External Structure of Gametophyte of Marchantia

The thallus has dichotomous branches with an apical branch or lobes. A prominent midrib of the dorsal groove along the length of the thallus is a characteristic feature of Marchantia.

Dorsal Surface

The dorsal surface of the thallus appears dark green due to chlorophyll in the cells.
The thallus has a midrib that appears as a shallow groove on the dorsal surface. this will be ridge-shaped on the ventral surface.
The midrib ends in a depression at its apex where the growing tip is located.
On the conspicuous midrib, there are polygonal areas with a central pore, known as aereolae. Their openings are called air pores.
Cup-shaped structures with dentate rims that contain asexual reproductive structures called gemmae cups are present on the dorsal surface.
The structures for sexual reproduction are borne on specialized stalked structures known as gametophores or gametangiophores.
When they carry archegonia they are called archegoniophores and when carrying antheridium they are antheridiophores.

Ventral Surface

The ventral surface has a ridged midrib that bears rhizoids.
In addition to the rhizoids there also are two types of rhizoids on the ventral side.

Smooth Walled And Tuberculate Rhizoids

The rhizoids are appressed to the thallus. The interspaces between them develop into a conducting tissue system to help contain moisture in the ventral surface.

The ventral surface of the thallus has a median longitudinal groove from which rhizoids arise. Rhizoids can be smooth-walled or tuberculate. The smooth-walled rhizoids are mere elongations of the epidermal cells. They are fixative and absorption in function. They are live bodies with a colorless cytoplasm.
The tuberculate rhizoids are outgrowths of the epidermis and are unicellular. Their inner surface has outgrowths called tubercles and thus the name tuberculated rhizoids. These are living structures in the initial stage but later lose their cytoplasm to become dead.

Scales in Marchantia

Scales are also present on the ventral surface of Marchantia. There are three rows of scales in M.polymorpha– median, middle, and marginal.

Appendiculate Scales: Median scales have oval appendages arranged in the median longitudinal groove. They are large, broad, and wedge-shaped.
Simple Scales: Middle and marginal scales are arranged along the middle and marginal portion of the thallus. They are ligule-like, slender, narrow, and without appendages. Both these are thick, large, multicellular, membraneous, and purple. They are oriented obliquely and can retain moisture in their capillary spaces. The peripheral scales are secretory and produce mucilage, showing an adaptation to a moist environment.

Internal Structure of Marchantia Thallus

The vertical section of Marchantia thallus reveals an upper assimilatory and a lower storage region that appears green and colorless respectively.

Upper Epidermis

The upper epidermis is made of single-layered parenchymatous cells compactly arranged. They have a thicker upper side that contains chloroplast. The presence of air pores throughout the surface is one of the unique structures of Marchantia thallus.

These air pores are barrel-shaped with vertical lining of 4-8 superimposed tiers of cells. Each tier is oval shaped and made of cells arranged in oval shape, giving a barrel-shaped appearance.

Such barrel-shaped air pores have half their cells along the vertical length as projected outgrowths. This gives a star-shaped appearance in M.palacea and circular in M.polymorpha. These air pores facilitate ventilation.

Assimilatory Region

The region below the epidermis has cells containing chloroplasts and is the photosynthetic zone. This area has several air chambers separated by septa. The septa is single-layered with just 2-3 cells. The air chamber appears rectangular and the air pore is located in the middle. Cells in the base of the chamber have chloroplasts.

From the base of the air chamber, numerous assimilatory filaments arise. These filaments are short, uniseriate, and branched. The height of the chambers reduces progressively from the middle towards the periphery of the thallus.

These air pores help with gaseous exchange while the chloroplasts perform photosynthesis.

Storage Region

The lower part of the Marchantia thallus has thicker cells. The thickening of cells also reduces from the middle to the periphery. This region has thicker cells that are compactly arranged. These parenchymatous cells store starch and other protein grains. These cells also contain oils and mucilage.

The conductive strand in the midrib region has thicker cells. Moreover, the storage region is limited by an epidermis layer that contains both scales and rhizoids.