Division Bryophyta Class Hepaticae
Order Marchantiales Family Ricciaceae Genus Riccia
The genus Riccia has aboutI30 species. Some botanists have reported its 200 species. Many of its species are found in Punjab. Kashmir and Western Himalayas. The species are found scattered from hilly and plains areas. All the species are terrestrial. They grim on damp soil or moist rocks.
I. External appearance: The adult terrestrial gametophyte is prostrate and rosette-like. It is dichotomously branched. It is deep green dorsoventrally. However, the aquatic species possess light green thallus. It is thin membranous and dichotomously branched. The dichotomous branching of the thallus is quite close to each other. It gives rosette-like appearance.
- Notches: Each branch of thallus has a conspicuous median longitudinal groove. It is present on the dorsal side of each branch of the thallus. A notch is present at the terminal end of each branch. Notch is also present on the dorsal side of each branch of the thallus. The growing pain is present in this notch.
- Scales: The ventral surface of the thallus bears a row of the one-celled scales. The scales are violet coloured and multicellular. They are arranged close to each other towards the apex of the branch. One the other hand, the scales are quite apart from each other assay from the notch. The scales are found in one row towards the apex of the branch. But they are found in two rows in the portion away from the apex. Thus the older parts of the thallus bear two rows of the scales. In Riccia crystallina the scales are either absent or rudimentary.
- Rhizoids: The rhizoids are also found on the ventral surface of the thallus. They are unicellular. Rhizoids absorb the nutrients and water from substratum. The rhizoids are of two types. Some are smooth walled and some are with tuberculate walls. The tuberculate rhizoids possess the peg-like infolding peeping. The simple or smooth walled rhizoids have no such infoldings.
Internal structure (anatomy)
Anatomy of Riccia is studied from the vertical sections of the thallus. Internally the thallus is differentiated into two regions.
a) Ventral region: The ventral region of the thallus consists of
simple colourless parenchyma. Intercellular spaces are not found in the ventral region. The cells of this region make the storage tissue. They are filled up with starch grains. The ventral region has a single layered epidermis. Epidermis gives rises to several unicellular rhizoids and multicellular scales.
b) Dorsal region: The dorsal region of the thallus consists of the chlorenchymatous cells. These cells have discoidal chloroplasts. These cells are arranged in vertical rows. There are regular air canals in between each two vertical rows. This region is photosynthetic. It prepares carbohydrates. The epidermis of tile dorsal surface of the thallus is discontinuous. It is opened outside at several places by the opening of the air canals. The epidermis is single layered. However, the epidermis is continuous in aquatic species. The exchange of gases takes place through the air canals.
A row of 3 to 5 cells is present in the notch of each branch. These cells are involved in apical growth of the thallus. Each such cell cuts
off derivatives alternately on its dorsal and ventral faces. Sometimes the segments are cut off from lateral face of the apical cell. These cells form following parts of the plant:
I . The segments of the dorsal surface give rises to dorsal
- [he segments of the ventral face give rise to the region of the ventral surface, rhizoids and scales.
- The derivatives from the outer daughter cell give rise to the epidermis and the air canals.
Air chamber formation
There are two views about the formation of air chambers:
a) Growth stops in of the tissue of the thallus in surface area at several points. The parts around these points grow vigorously upward. Thus depressions are produced in these points. These depressions become quite deep and narrow. Now these depressions become air chambers.
b) According to the second view, the air chambers develop schizogenously. These develop like intercellular spaces of the higher plants.
Formation of dichotomous branches
I. in the young thallus, divisions stop in one or more cells in the centre of the row of apical cells. Therefore, the original row of apical cells is separated into two sets of initial cells.
- Each new set of initial cells continuously grows. They develop tissue of the thallus between the two groups of initial cells. Thus the two apical growing regions are further separated from one another.
- Each of these growing points forms a separate branch. Now each branch possesses its own row of apical cells. This process continues in each of the branches. Thus it new rosette is formed.
Vegetative and sexual reproductions take places in Riccia.
It takes place by follow ing ways:
a) Death and decay of the older parts of the thallus: Sometimes, a part of the dichotomously branched thallus decays from the posterior end. But the terminal ends of the branches remain unaffected. These branches may grow Separately into new thallus.
b) By cell division or gemma formation: In this case, cell divisions occur in the young rhizoids. It develops into gemmalike structure of the cells. These structures give rise to ilex% plants.
c) By adventitious branches: It occurs in several species of Riccia. In this case, adventitious branches are produced on the ventral surface of the thallus. These branches detach from the thallus and develop into new gametophytes.
d) By thick apices: It occurs in some species like R. hirnalayensis. In this case, the apex of the thallus grows downward into the soil at the end of the growing season. This apex becomes thick. This thick apex survives in the soil. It develops into a new plant in favourable conditions.
e) By tubers: Some species of Riccia develops vegetative structure called tubers. Some branches of the Manus face adverse conditions. The tubers are developed at the apices of these branches. These tubers develop into new plants in favourable conditions.
Majority of the species of Riccia are homothallic (monoecious) like R. glauca. In this case, antheridia and archegonia are borne on the same thallus. Heterothallic (dioecious) species like R. himalayensth are also common. In such species, the antheridia and archegonia develop separately on different thalli.
The sex organs are produced in the groove. These grooves are situated on the dorsal surface of the mature gametophyte. These are arranged in acropetal succession. In a homothallic species the alternate groups of antheridia and archegonia are present on the same branches. In heterothallic species, antheridia and archegonia are present in groups. They are present separately in the medial, longitudinal grooves of different thalli. Both the antheridia an(- archegonia develop singly on the dorsal surface of the thallus.
Development of antheridium
I. The antheridium develops from a superficial dorsal ceil call the antheridial initials. These initials are present at two or th cells back of the special cell.
2. The antheridial initial soon divides. It gives rise to two cells. upper one is called outer cell and the lower one the basal The outer cell projects outside the thallus. The basal cell ref embedded in the thallus. It develops into the embedded portion of the stalk of the antheridium. The outer cell develops into the main antheridium.
- The outer cell further divides by transverse septa. It forms a vertical file of three cells:
• The two upper cells of this vertical file are called primary
antheridial cells. They produce proper antheridium.
The lowermost cell of the file is called the primary stalk cell. It produces the projecting portion of the stalk.
- Now, the two primary antheridial cells divide by two successive vertical divisions. These divisions are at right angles. As a result, eight cells are formed. These are arranged in two tiers. Each tier consists of 4 cells.
- Now each tier of four cells divides periclinally. Thus each tier has 4 jacket initials. These jacket initial encircle the 4 androgonial cells of each tier. In this way, in both tiers, ‘eight primary androgonial cells are formed. These cells are encircled by 8 jacket sterile initials.
- The sterile jacket initials repeatedly divide anticlinally. They give rise to a single layered jacket around the antheridium. The eight primary androgonial cells divide repeatedly. They give rise to a large number of cubical androgonial cells.
- %rther divisions occur in these androgonial cells. As a result, they become smaller and smaller. The last cell generation of the androgonial mother cells becomes androcyte mother cells.
- Each androcyte mother cell divides diagonally. Thus each gives rise to two androcytes.
Formation of antherozoid from androcyte
Androcytes are soon metamorphosed into antherozoid The process of metamorphosis is as follows
- The androcytes are triangular in shape. Each androcyte possesses
a big prominent nucleus. A blepharoplast appears in the cell. Blepharoplast is an extra nuclear granule. The granule becomes large in size. It elongates and develops into a cord like body. This body adheres to the plama membrane. Very soon the two flagella arise from the anterior end of the blepharoplast.
- The androcyte becomes rounded. The nucleus becomes crescent shaped. It is shifted towards the blepharoplast and unites with it firmly. The elongated cord-like blepharoplast extends about three fourth of the cell. Thus androcytes are changed into antherozoids.3. During this process, the internal cell walls of the jacket layer of the antheridium disintegrate. It produces a liquid substance. Antherozoids lie in non-motile condition in this liquid.Structure of mature antherozoidThe main body of a mature antherozoid mainly consists of nucleus. [he elongated blepharoplast adhered to the nucleus. It bears two flagella at its anterior end. These flagella are used for movement in the water. One flagellum of the antherozoid helps in propulsion. The other helps in rotating the body. Sometimes, small unused portion of cytoplasm form a vesicle. This vesicle remains attached to the posterior end of the antherozoid.
Structure of mature antheridium
The mature antheridium remains embedded in the antheridial chamber. This chamber opens by an ostiole on the dorsal side of the thalius. The mature antheridium consists of a few celled stalk and antheridium proper. The antheridium proper may be rounded or pointed at its apical end. A sterile single layered jacket-layer encircles the antheridium and protects it. The mature antheridium contains androcytes within the jacket layer. Each androcyte metamorphoses into an antherozoid
Liberation of antherozoids
The jacket layer of the fully developed antheridium is gelatinous. It imbibes water. Thus the antherozoids become gelatinized mass within the antheridium. The antherozoids lie free in the antheridium. They are embedded in the viscous substance. Liberation of antherozoids from the antheridium takes place by explosive mechanism.
Development of archegonium
I. Archegonium develops from a superficial cell. This cell lies two or three cells away from the apex of the thallus. This cell is called the archegonial initial.
- The archegonial initial divides by a transverse wall into two cells. The lower cell is the basal cell and the upper cell is the outer cell. The lowermost portion of the archegonium is produced from the basal cell. The outer cell produces the main body of the archegonium.
- The outer cell divides vertically thrice. Thus it gives rise to three peripheral initials. It itself becomes primary axial cell. The three peripheral initials are situated upon a primary axial cell.
- The three peripheral initials divide vertically. They form six jacket initials. All the six cells surround the central primary axial cell.
Soon after each jacket initial divides by a transverse division producing six neck initials and six venter initials. They are arranged in two tiers. The tier of six neck cells is situated above the tier of six venter initials.
- Several transverse divisions occur in the neck initials. It
gives rise to a neck of archegonium 6 to 9 cells in length and six-celled in perimeter. The venter initials divides to form jacket of the venter. The jacket of the venter is :2 to 20 cells in perimeter.
- The primary axial cell divides by an unequal transverse division. It produces a primary cover cell and a central cell.
- The primary cover cell divides twice vertically. It produces four cover cells at the apex ofjacket layer. The central cell divides by a transverse wall. It produces an upper canal cell and a lower canal cell.
- The upper canal cell divides twice. It gives rise to four neck canal cells. They are situated within the neck of the archegonium. The lower canal cell divides by an unequal transverse division. It gives rise to a venter canal cell and a large egg. The venter canal cell is situated above the large egg. The neck canal cells, venter canal cell and the cover cells are separated from each other in mature archegonium.
Structure of a mature archegonium
The archegonium is a flask-shaped body. It is attached to the thallus by a short stalk. It consists of an elongated neck and a bulbous venter.
a) Neck: The neck is 6 to 9 cells in height. It consists of six vertical
rows of the cells. The six vertical rows of the cells enclose a neck canal. There are four cover cells at the top of neck canal. Four neck canal cells are found %. ithin a neck canal before maturity. They disintegrate into mucilaginous mass on the maturation of the archegonium.
b) Venter: The lower bulbous structure of the archegonium is venter. This has a single layered wall around it. This wall is of 12-20 cells in perimeter. The venter encloses a venter canal cell and the large egg. The venter canal cell disintegrates on the maturity of the archegonium. Thus only the large egg remains
The mucilaginous mass absorbs water by imbibitions on the maturity. It creates pressure. Thus the cover cells become separated from each other and an opening is formed. The mucilaginous mass extrudes through this opening. It attracts the antherozoids.
Fertilization or syngamy
The water is necessary for fertilization. The antherozoids reach to the mouth of the neck of archegonium through water. The water is also essential for the separation of the cover cells. A film of water is found in the dorsal furrow of the thallus. This film acts as capillary tube. Mucilaginous substances along with proteins and certain inorganic salts ooze out through the mouth of the archegonium. The antherozoids are attracted chemotactically by this substance. The antherozoids enter the mouth of the archegonium. They travel through the neck and reach near the egg. One of the antherozoids penetrates egg cell and the fertilization takes place.
Development of embryo
I. Formation of calyptra: The zygote secretes a wall around it after the fertilization. It enlarges in size. It nearly fills the cavity of the venter. The venter cells divide periclinally and the wall of the venter becomes two-celled in thickness. Then venter cells divide anticlinally. They form two layered calyptra. The developing embryo is situated inside the calyptra.
- Formation of octant: The first division of the zygote is transverse. Thus two equal cells are formed. In most of the species these two cells are called epibasal and hypobasal. They divide vertically and a four celled embryo of quadrant type is formed. In some species, four celled filamentous type embryo is formed from the two celled embryo the. Sometimes both the types of embryo are formed in the same genus. This four celled embryo divides by a vertical wall. Thus the eight celled embryo (octant stage) is f)rmed. It is called octant stage.
- Sever-I irregular divisions occur in octant stage. Thus a 20 to 30 celiee embryo is formed. At this stage periclinal divisions occur. Now the embryo is differentiated into two regions. The outer layer is called amphithecium. The inner mass of cells is called endothecium.
- The amphithecium is protective in nature. The endothecium divides repeatedly. It gives rise to mass of sporogenous cells.
- The sporogenous tissue differentiates into sporocytes is spore mother cells and nurse cells. The sporocytes consist of dense granular cytoplasm. The nurse cells have thin vacuolated cytoplasm.
5. The nurse cells and the inner jacket cells disintegrate. They give rise to a mucilaginous mass. The sporocytes remain suspended in this mucilaginous mass. The sporocytes receive the nourishment from this mucilaginous fluid during meiosis. Finally, the walls of the spore mother cells disintegrate. Now they lie free in the sporogonium.
- Fig: Germination of spores6. The functional spore mother cells become rounded. Meiotic division occurs in the nucleus of spore mother cell in two steps. Thus a tetrad of four spores is formed. Initially, all the four spores of a tetrad remain attached to each other. They are surrounded by a common sheath. The spores separate from each other on maturation. The spores are haploid 00. The thallus decays and died. Thus the thallus the spores get free from the sporogonium. This is a long process. The spores are not liberated Unmediately after their maturation. Sometimes, they remain within the sporogonium even for a year or more.Structure of spore
Spore is surrounded by irregularly thickened spore wall. The mature spore is three layered.
a) The outermost layer is cutinized. It is called exosporium.
b) The middle layer is mesosporium. It is thick. It consists of three
c) The innermost layer is endosporium. Endosporium is composed of callose and pectose.
Germination of spore and formation of gametophyte
The spore is the beginning of the gametophytic generation. It has following process of germination.
I. The two outer layers of spore, exosporium and mesosporium
rupture. The endosporium protrudes out through the opening. It forms a germinal tube. Much of the cytoplasm is shifted in the terminal end. Now transverse walls are laid down at the apex of this tube. A rhizoid comes out near the germinal tube, of the spore.
- The distal end of the germinal tube again divides by a parallel wall. Thus two cells are formed at the distal end of the germinal tube. Now both the cells divide vertically twice. They give rise to two tiers of four cells each.
- The proximal tier of four cells divides transversely. The daughter cells elongate in longitudinal axis. They give rise to the cylindrical elongated posterior portion of a young gametophyte.
- One cell of the four celled distal tier acts as an apical cell. It possesses two cutting faces. It cuts cells repeatedly from its left and right faces. Thus it gives rise to a multicellular young thallus.