Riccia

About Riccia

Systematic position

1. Kingdom – Plantae
2. Sub kingdom – Embryophyta 
3. Phylum – Bryophyta
4. Class – Hepaticopsida
5. Order – Marchantiales
6. Family – Ricciaceae
7. Genus – Riccia

Riccia Habitat

It is a cosmopolitan genus comprising 200 species and about 30 species have been reported from India. The plants occur on moist soil or rocks. Most of the species are terrestrial (on damp soils) except Riccia fluitans, which is aquatic in nature. Riccia himalayensis (R. discolor) is the most common Indian species, which is present in plains and also upto height of 9,000 ft above sea level. It occurs during july to october. The plants show rossette habit on damp soil.

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Riccia Structure

1. External structure (Gametophytic phase) : The main plant body of Riccia is gametophytic (n), which is thallose, i.e., having no true roots, stem and leaves. It is small, green, flat and fleshy. The thallus is dorsiventral and dichotomously branched. The thalli are present in the form of patches called rossettes. The dorsal surface of thallus is smooth and shows mid-dorsal groove, whereas the ventral surface is having unicellular rhizoids and multicellular scales. Scales are found on the margins, while rhizoids are present in the mid-rib region of thallus. Rhizoids are unicellular and unbranched and are of two types- smooth  and tuberculate. Rhizoids attach the plant to the substratum and absorb soil water. Scales are minute, violet coloured (due to anthocyanins in their cell sap) multicellular structures which protect the growing point. In submerged species, (e.g. R. Fluitans) scales and rhizoids are not present.  
2. Internal structure : The thallus is internally differentiated into 

• An upper or dorsal photosynthetic region.
• A lower or ventral storage region.

1. Photosynthetic region : It consists of vertical rows of chlorenchymatous cells. In between these vertical rows are present very narrow air canals or air chambers. The canals communicate with the outside through air pores. The uppermost cell of each row is enlarged and non-green. These non green cells of vertical rows form a discontinuous and poorly-defined upper epidemis. 
2. Storage region : The lower portion consists of  closely packed parenchymatous cells without intercellular spaces. The cells do not contain chloroplasts. They store water and food. The lowermost row of cells form the lower epidermis. Rhizoids and scales develop from the lower epidermis.

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In R. fluitans and R. crystallina, the photosynthetic region is made up of chlorophyllose photosynthetic lamellae running in various directions. They enclose tiny air chambers in between them.

Reproduction of Riccia 

Riccia reproduces by both vegetative and sexual method.

1. Vegetative reproduction : The plants of Riccia reproduce vegetatively by the following methods and new thalli may arise.

• By gradual death and decay of the older part of the plant body.
• By the formation of adventitious branches e.g. , R. fluitans.
• By the perennation of the apical region. e.g., R. discolor.
• By the formation of tubers which tide over unfavourable conditions e.g., R. billardieri, R. discolor, R. perennis.
• By segmentation of a rhizoid e.g. R. glauca.

1. Sexual reproduction : Sexual reproduction is oogamous type in Riccia. Antheridia and archegonia are the male and female sex organs respectively. Most of the species are monoecious or homothallic, i.e.,male and female sex organs are present on the same thallus. A few species are dioecious or heterothallic, i.e., antheridia and archegonia are present on different thalli. Common dioecious species of Riccia are R.himalayensis and R. frostii.

Also Check: Asexual Reproduction

The sex organs are produced singly on mid-dorsal groove of Riccia thallus in acropetal succession, i.e., oldest at the base and youngest at the apex. Antheridia and archegonia are present singly in antheridial cavities and archegonial cavities respectively. In homothallic species, antheridia are produced earlier than archegonia.

• Antheridium (Male sex organ) : It consists of a small stalk and an oval body. It is situated in a deep pit, called antheridial chamber. The antheridium is surrounded by an outer jacket of sterile cells. The sterile jacket encloses androcyte mother cells. Each androcyte mother cell divides to form two androcytes. The protoplast of each androcyte is metamorphosed into a single antherozoid. Each antherozoid is a narrow, curved structure with two anterior flagella. Moisture is essential for dehiscence of antheridium. Antherozoids come out through the narrow canal of antheridial chamber and swim freely in a thin film of water in the dorsal furrow.
• Archegonium (Female sex organ) : Archegonium is a flask-shaped structure. It consists of two parts the basal swollen venter and the long narrow neck. The archegonium is surrounded by a layer of sterile cells forming a protective jacket. The neck consists of a vertical row of four cells, the neck canal cells. The tip of the neck is made up of four large cap or cover cells. The venter has two cells, the lower large egg cell and the upper small ventral canal cell. The upper portion of the neck usually projects above the surface of the thallus. On maturity the neck canal and ventral canal cells of the archegonium degenerate forming mucilage. It imbibes water and swells forcing the cover cells to separate from one another.
• Fertilization : Before fertization the neck canal cells and the ventral canal cell disorganise to form a mucilagenous mass. The cover cells now split apart to give a free passage to the incoming antherozoid. The antherozoid are attracted towards the egg by chemotactic mechanism. The chemical stimulus is provided in the form of sugars. The fertilization is affected by water medium (zooidogamous). Many antherozoids may enter into the archegonium, but only one of them is able to fuse with single egg to form zygote (2n), which is beginning of sporophytic phase.
• Sporophytic phase : Soon after formation of zygote, it enlarges in size and fills up the cavity of venter. The zygote also secrets a wall around it.  Without any resting period zygote undergoes many  divisions and forms a spherical mass of undifferentiated cells called embryo. The venter becomes two cell layers thick and is known as calyptra. The neck of the archegonium disorganizes. 

Read More: Sexual Reproduction

By cell divisions in embryo two layers are formed, the outer amphithecium and the inner endothecium. The endothecuim forms the spore mother cells, each of which undergoes meiosis to form tetrahedral tetrad of four spores (n). 

A few spore mother cells, although remain sterile and act as nurse cells, which are having vacuolated cytoplasm and disorganize to provide nutrition to developing spores. Fully developed sporophyte or sporogonium is simple structure, having only capsule or spore sac. There is no foot and seta.

Sporophyte or sporogonium of Riccia is simplest among bryophytes. Sporophyte is photosynthetically independent. The capsule is having a single layered wall, i.e., outer layer of calyptra only (Inner layer of calyptra and sterile jacket formed by amphithecium also disorganize). 

• Germination of spores : Spores are the first cells of the next gametophytic generation. Spores are dispersed by the decay of the surrounding thallus tissue. The wall of the spore is thick and sculptured, and is differentiated into three layers- the outer exosporium the middle mesosporium and the inner endosporium which is made of pectose and callose. The surface is having clear triradiate mark. In the mass of cytoplasm, stored food is present in the form of oil – globules. After liberation, the spores germinate in about 6-10 days in presence of light, low temperature and sufficient moisture contents. After absorbing water, the spore swells up. The endosporium grows out in the form of a germ tube which, after further divisions, develops into a new thallus (gametophyte).

Thus there are 2 generations in life cycle of Riccia. The main plant body is gametophytic (n). The gametophytic phase starts with formation of spores and ends with fertilization. The second phase is sporophytic phase (2n), which starts with zygote and ends with reduction division of spore mother cell. The sporophytic phase is dependent upon gametophyte. Thus there is heteromorphic or heterologous alternation of generations in Riccia. So life cycle in Riccia is diplohaplontic.