Morphology, Life Cycle, Reproduction, Economic Importance microbiologystudy

Systematic Position (Classification) of Riccia

Kingdom- Plantae

Division- Hepaticophyta

Subdivision- Hepaticae

Class- Hepaticopsida

Subclass- Marchantiae

Order- Marchantiales

Family- Ricciaceae

Genus- Riccia

Distribution and Habitat of Riccia

It is the most widely distributed genus comprising of 140 species. They are found all over the world. Riccia fluitans is the only aquatic species that occurs floating or partly submerged in still water. All the other species are terrestrial that are found in damp, shady areas with abundant moisture.

Morphology (External Structure) of Riccia

Gametophyte Phase

• The plant body is small, green, flat thallus which grows prostrate on the ground and branches freely by dichotomy.
• The branches of the thallus are called thallus lobes which are linear to wedge-shaped or obcordate in shape.
• Each lobe is thickest in the middle that constitutes the midrib region and generally thins towards the margins
• On the upper surface of each lobe there is generally a median groove known as dorsal groove. This varies from species to species. 
• The thallus is attached to the substratum by slender unicellular structures called rhizoids. These rhizoids are either smooth-walled or tuberculate. Tuberculate rhizoids contain peg-like outgrowths inside the lumen that helps in increasing the surface area for absorption. 
• The main function of rhizoid is to absorb water and solutes as well as serves organ of attachment to the substratum.
• In addition to rhizoids, there are violet membranous structures called the scales. These scales are multicellular and one cell in thickness.

Anatomy (Internal Structure) of Riccia

The vertical section of the thallus of Riccia shows two distinct regions:

Photosynthetic region or assimilatory region

This region consists of loose green tissue rich in chloroplasts and thus functions in photosynthesis. These are the type of parenchyamtic cells and known as chlorenchyma. The cells are arranged in vertical column and function as pseudo-mesophyll. Between the columns of these cells, there exists a narrow, deep vertical slits which are known as air canals or air chambers. The uppermost cells of chlorenchyma are somewhat colorless, distended or pear shaped that forms a discontinuous layer which are known as pseudo-epidermis. This layer is equivalent to epidermis in higher plants. There are small gaps in the upper pseudo-epidermis that are known as air pores. The air canals communicate to the exterior environment through these air pores.

Storage region

The lower portion of the thallus consists of closely packed undifferentiated parenchymatous cells without intercellular spaces. This layer is colorless in nature and contains starch but no chloroplasts. They serve for water and food storage. The lowermost cells of this region are small in size and regularly arranged to form the lower epidermis. 

Reproduction in Riccia

Riccia reproduces both vegetatively and sexually.

Vegetative reproduction in Riccia

1. Fragmentation- The cells in the older portions die of old age and eventually disorganize. When the death and decay reaches dichotomy, the young lobes becomes separated. Each of these grows into a new thallus.

2. Adventitious branches– In some species like R. fluitans, special adventitious branches arise from the ventral surface of the thallus in the midrib region. They get detached from the parent thallus by the decay of the connecting tissue and form new plants.

3. Persistent apices– In many species, during the dry season, the whole plant except the growing apices of the thallus lobes is dead. As soon as favourable season occurs, the apical regions resumes growth and give rise to a new plant.

4. Tuber formation- In some species, the apical region of thallus lobe is thickened to form tubers at the end of the growing season. With the advent of unfavourable conditions of the plant perishes. The tubers remain dormant and give rise to new plants with the onset of suitable conditions.

Sexual Reproduction in Riccia

Distribution of sex organs

The sex organs are developed in the dorsal furrow or groove of the thallus. They are developed in acropetal order i.e. the younger ones are near the tip and older ones are away from it. Some species are monoecious and some are dioecious.

Structure and development of sex organs

Antheridium

• The mature antheridium is elongated structure. It consists of ovoid or pear shaped body seated on a short few-celled stalk. It is present inside the antheridial chamber and the opening of each antheridial chamber is called ostiole.
• The body of antheridium has outer layer of cells called jacket cells which are sterile and forms the antheridial wall. 
• They are protective in function. It encloses a mass of fertile cubical cells known as androcyte mother cells. 
• Each androcyte mother cell has a denser cytoplasm and larger nucleus and divides diagonally to form two sperm cells called as androcytes. 
• The androcytes develops into numerous biflagellate sperms.
• Each sperm is slender, minute and curved structure with a pair of whiplash flagella. 

Dehiscence: Presence of moisture is necessary for the dehiscence of sperms. Water enters from the ostiole into the antheridial chamber. 

The cells at the apex of this chamber absorb water and disintegrate to form a pore. The sperms are then extruded out through the pore and swim freely in thin film of water in the dorsal furrow to reach archegonia.

Antheridium development

• Each antheridium develops from a cell called antheridium initial that lies on the dorsal surface of the thallus.
• It matures and divides transversely to form upper cell and lower cell. The upper cell develops into upper primary antheridial cell and the lower one develops into primary stalk cell. Both undergoes transverse cleavage and form four cells. 
• Two of them situated in the lower side functions as stalk cells and undergoes few mitotic divisions to form stalk. 
• The two upper cells on the other hand functions as antheridial cells and divides twice to form eight cells (two tiers of four cells each). 
• Periclinal division occurs in all these cells. The outer eight cells differentiate to form sterile jacket cells. 
• The inner eight cells are called primary androgonial cells and divides further to form androgonial cells. 
• These further undergo division to form sperm mother cells or androcyte mother cells. They divide diagonally to form sperm cells which later give rise to sperm. 

Spermatogenesis

Androcyte mother cells divide diagonally to form androcytes. Each androcyte has a large nucleus and dense chloroplast. 

There exists a small granular extra-nuclear body, near the periphery of the androcyte which is known as blepharoplast. 

The androcyte along with the blepharoplast and nucleus becomes elongated and the blepharoplast forms a cord like structure. At this stage, androcyte become comma shaped and the blepharoplast coils up about ¾ of the periphery.  

The nucleus becomes sickle-shaped and moves to the periphery and from the end of the blepharoplast two flagella arise.

Archegonium

• The archegonium is a flask- shaped structure that consists of a swollen portion called venter and a slender neck.
• The venter is directly attached to the tissue of thallus. There is no usually visible stalk. 
• Next to this, there exists a vertical row of four cells, the neck canal cells surrounded by a layer of sterile cells forming a protective jacket. 
• The tip of the neck consists of four specialized large cells, known as the cover cells.  
• The venter also has a jacket of sterile cells which makes up the venter wall.
•  The venter cavity consists of two cells, the larger one is the egg cell and is situated at the lower side and the upper one is ventral canal cell which is smaller in size. 
• Each archegonium lies within the archegonial cavity.

Dehiscence: When the archegonium reaches maturity, the ventral canal cells and the neck canal cells degenerate to form mucilage. This mucilage imbibes water and swells which leads to the opening up of four cover cells thus making a passage for sperms. 

Archegonium development

• The archegonium develops from archegonial initial that lies on the dorsal surface of the thallus.
• It divides transversely to form upper cell and lower cell. The lower cell takes no part in archegonium formation. The upper cell acts as archegonial mother cell. It enlarges and divides by three eccentric vertical walls and forms three peripheral initials surrounding a middle fertile cell (primary axial cell). 
• Each of these three peripheral initials divides longitudinally and gives rise to jacket initials. 
• These jacket cells undergoes transverse division and undergoes differentiation to form two tiers of six cells each. The upper tier functions as neck initials and the lower tier functions as venter initials. Neck initial forms neck and venter initial forms the venter.
• Meanwhile, the primary axial cell divides transversely to form one large lower central cell and upper primary cover cell. The central cell forms upper primary neck canal cell and a lower ventral cell. Primary neck canal cell divides to form neck canal cells. The ventral cell forms ventral canal cell and egg cell.

Fertilization in Riccia

It takes place in presence of water. 

The sperms are transferred with the help of water and in the meantime, the neck canal cells disintegrate to form mucilage and absorb water to swell which leads to opening of cover cells. 

Thus a passage is formed. 

The mucilage in archegonium contains chemical substances that attract the sperms and due to chemical interaction, they enter the venter and the most compatible one penetrates the egg and unites to form zygote.

Sporophytic Phase

It comprises of zygote, embryo and the sporogonium.

Zygote

• It is a unicellular structure formed by the fusion of male and female gametes and is the pioneer structure of the sporophytic phase. 
• It secretes a wall around it and enlarges in size. 
• The zygote is retained within the venter and has a diploid nucleus with cellulose cell wall around it. 

Embryo

• The zygote undergoes repeated division and cell enlargement.
• A spherical mass of undifferentiated cell is formed which is known as embryo.
• The venter expands as a close envelope, two cell layers thick and forms the calyptra over the developing embryo.

Sporogonium

• It is developed by further cell divisions of embryo. 
• The outer layer of cells is called ampithecium and the inner one is called endothecium. 
• The cells of ampithecium are large and fat and undergo anticlinal division to form sterile jacket layer. 
• The cells of endothecium give rise to sporogenous tissue called archesporium. 
• This archesporium matures and divides to form spore mother cells. 
• Each spore mother cell divides meiotically to form four spores that are arranged in tetrahedral manner and also known as spore tetrad. 
• However, few cells disintegrate to form nutritive fluid for the nourishment of spores, these are called nurse cells.
• The tetrad matures and secrets thick callose layer and inside this layer, there are three spore coat, the outer one is exosporium, the middle one is mesosporium and the inner one is endosporium. 
• At maturity, this layer separates to release spores.

Sporophyte

• The sporophyte of Riccia is the simplest amongst liverworts. It lacks foot and seta and only contains capsule. 
• The capsule is spherical in outline and has one layer of capsule wall that encloses the spore mother cell. There are no elaters.
•  The meiospores that are the pioneer structure of gametophytic phase are produced from spore mother cells. 
• Each mature spore is pyramidal in shape containing protective wall differentiated into two layers, the outer one is exine and the inner one is intine. 
• These spores germinate to form a new gametophytic individual.

Life cycle of Riccia

The life cycle of Riccia includes two different generations, the sporophyte and the gametophyte. 

Sporophytic generation is diploid and depend completey upon gametophytic generation. 

Gametophytic generation is haploid and is the dominant phase in life cycle of Riccia. Both the generations are morphologically dissimilar to one another so this type of alternation of generation is called heterologous or heteromorphic alternation of generation. 

These two individuals occur one after the other generation after generation.

Economic Importance of Riccia

• Riccia plays an important role in ecological system by contributing in soil formation. Its ability to retain water in its tissue helps in retention of moisture in the soil.
• Riccia is used to monitor environmental conditions, particularly air and water quality, due to its sensitivity to pollutants. Species like Riccia gangetica, Riccia frostii are used as bio indicators.
• Riccia, like other liverworts, has the ability to absorb heavy metals and other pollutants from the environment, which can be used in bioremediation to clean contaminated sites.
• Riccia also plays an important role as research tools in various fields of Botany and Genetics.
• In some cultures, liverworts, including Riccia, have been used in traditional medicine. They are believed to have various medicinal properties, although scientific validation of these uses is limited.

References

1. Bryophyte by B.R. Vashishta, A.K. Sinha, Adarsh Kumar (S.Chand & Company ltd)
2. Farmer, J.B. 1895. On spore formation and nuclear division in the Hepaticae. Ann. Bot. 9 : 469-523.
3. Hait, G., Bhattacharya, K., & Ghosh, A. K. (2012). A textbook of Botany, Volume I.
4. Hirsch, P.E. 1910. The development of air chambers in Ricciaceae. Bull. Torrey Bot. Club, 37 : 73-77.
5. Kachroo, P. 1950. A note on the morphology of some species of Riccia. Bryologist. 58 : 134-136.
6. Mitra, J. N., Mitra, D., & Choudhuri, S. K. (2010). Studies in Botany, Volume I.
7. Smith, G.M. 1955. Cryptogamic Botany. Vol. II. Bryophytes and Pteridophytes. Edi. 2, New York.
8. Srivastava, K.P. 1964. Bryophytes of India. I. Ricciaceae. Bull. Nat. Bot. Gardens. Lucknow.
9. Stange, L. 1964. “Regeneration in Lower Plants”. In Advances in Morphogenesis V. IV, New York Academic Press. Pp. 111-153.
10. T, N. (2016, August 24). Reproduction in riccia (With diagram). Biology Discussion. https://www.biologydiscussion.com/botany/bryophytes/reproduction-in-riccia-with-diagram/46221
11. Udar. 1976. Bryology in India. New Delhi.
12. Life cycle of Riccia. (n.d.). BrainKart. https://www.brainkart.com/article/Life-cycle-of-Riccia_867/
13. Riccia ppt. (2017, January 23). [Slide show]. SlideShare. https://www.slideshare.net/slideshow/riccia-ppt/71287662
14. The Editors of Encyclopaedia Britannica. (1998, July 20). Riccia | Moss, Liverwort, Bryophyte. Encyclopedia Britannica. https://www.britannica.com/plant/Riccia