In this article we will discuss about the structure of Marchantia with the help of diagrams.
External Structure of Marchantia:
The gametophyte of Marchantia is a dichotomously branched, prostrate, dorsiventral thallus (Fig. 6.8). The dorsal surface of the thallus shows many regular rhomboidal or polygonal areas (Fig. 6.9A). Each area has a pore at the center.
A distinct median groove (midrib) is present on the upper (dorsal) surface in each branch of the thallus with a corresponding ridge on the ventral surface. The branches grow indefinitely by means of a growing point situated in the terminal groove (apical notch).
The ventral surface of the thallus bears three to four rows of scales and rhizoids on both sides of the midrib (Fig. 6.9B). The scales are membranous, one-layered thick, usually violet in colour due to the presence of anthocyanin pigments. MorphologicaUy, the scales are of two types — appendiculate and ligulate (Fig. 6.10D & E).
The appendiculate scales situated near the midrib are larger and more elaborate by the presence of an apical sub-rounded appendage (Fig. 6.10D). The ligulate scales, on the other hand, are relatively small, situated towards the margin, which do not have any appendage (Fig. 6.10E). These scales protect the growing point of the thallus from desiccation.
Besides the scales, the ventral surface of the thallus bears rhizoids between the scales. They are usually unicellular, colourless and are of two types viz., smooth walled and tuberculate as in Riccia (Fig. 6.10A-C). The rhizoids perform the functions of anchorage to the substratum as well as absorption of water and nutrients from soil.
The sexually mature thalli bear specialised erect branches called gametophores or gametan- giophores which bear sex organs. These branches are umbrella shaped and arises from the apical notch. They are of two types viz., antheridiophore and archaegoniophore (Fig. 6.8).
The antheridiophore bears antheridia and the archegoniophore, the archegonia. Marchantia is dioecious or heterothallic, therefore, a thallus bears either antheridiophores or archegoniophores.
Internal Features of Marchantia:
A section (V.T.S.) of the thallus shows three distinct regions viz., the epidermal region, the photosynthetic region, and the storage region.
1. The Epidermal Region:
It consists of a well- defined upper (dorsal) and lower (ventral) epidermis. The epidermis is formed of quadrate cells containing a few chloroplastids. An air chamber of schizogenous origin (Fig. 6.9A) is present just below the polygonal area.
The air chambers are connected with the outside atmosphere by a barrel-shaped air pore situated at the center of the polygon (Fig. 6.9A). The air pore is formed by 4 to 8 superimposed tiers of cells, and each tier composed of 4 or 5 cells (Fig. 6.9C).
The cells of the lowermost tier project inwards giving a star- shaped appearance to the pore when view from the above (Fig. 6.9D).
The lowermost ventral layer is the lower epidermis, which bears scales and rhizoids (Fig. 6.9A)
2. The Photosynthetic Region:
The upper dorsal epidermis contains a few chloroplastids. The air chamber is demarcated from others by single layered partitions of cells containing chloroplastids. Simple or branched filaments formed of cells in horizontal row full of chloroplastids arise from the floor of the air chamber.
These horizontal row of cells form the main photosynthetic tissue of the Marchantia (Fig. 6.9A, C).
3. The Storage Region:
The ventral tissue lies immediately below the air chambers forms the storage region. It is a compact zone comprised of several layers of thin-walled, polygonal parenchymatous cells devoid of chloroplasts. This region is thick in the center and gradually tapers towards the margin (Fig. 6.9A).
However, most of the cells of the storage region contain starch grains or protein granules and some isolated cells contain large oily bodies or mucilage (Fig. 6.9C). The midrib of the thallus is made up of cells elongated tangentially showing reticulate thickening.