Riccia:
(Family: Ricciaceae)
Taxonomic Position:
Division: Bryophyta
Class: hipaticopsida
Order: Marchantials
Family: Ricciaceae
Genus: Riccia
Occurrence: Riccia is found over all the parts of the world and it is widely distributed. The genus includes about 138 species. From India about 29 species have been recorded. Mostly these amphibious plants are terrestrial and grow as green carpet on the wet ground, on damp walls, old tree trunks and moist rocks. Riccia fluitans is aquatic.
There are two phases in the life cycle of Riccia.
(i) Gametophytic phase and (ii) Sporophytic phase
Out of these two phases gametophyte phase is the dominant phase in the life cycle.
(i) Gametophyte Phase:
This begins with the germination of spores. Each spore germinates to produce a new plant of Riccia.
External Characters:
• The plant body of Riccia is thalloid in form.
• The thallus is green, flat and fleshy.
• It grows prostrate on the ground.
• It is dichotomously branched.
• It is dorsiventrally flattened having two surfaces, i.e. dorsal and ventral.
• The branches of thallus are known as thallus lobes.
• Each thallus lobe is thickest in the middle and thin towards the margin.
• There is a presence of mid-rib in the thick middles portion of thallus.
• There is a presence of dorsal groove on the upper surface of the thallus.
• Each thallus lobe ends in a terminal notch in which lies the growing point which represents a group of meristematic cells.
• The ventral surface or lower surface consists of unicellular rhizoids and multicellular scales.
Rhizoids: - They arise from the lower surface of the thallus. They are unicellular. They are of two kinds (i) Smooth-walled (ii) Tuberculate. The tuberculate rhizoids have of peg-like outgrowths on their inner wall.
Function: They are meant for attachment and absorption. The plant is attached with the substratum with the help of rhizoids. Rhizoids also absorb water and soil solutes from the soil. The function resembles that of the root.
Scales: These are multicellular possess arise from the ventral surface of the thallus in addition to rhizoids. They are arranged in one transverse median row near the apex. They grow forward, situated closely to protect the growing point. In older portion of the thallus they are situated in lateral rows near the margin. These scales are meant for retaining water in the thallus.
Internal structure of the thallus:
The thallus is many layered in thickness. It consists of two main regions.
(i) Photosynthetic region, (ii) Storage region.
(i) Photosynthetic region:
• The dorsal or upper surface of the thallus is known as photosynthetic region.
• It consists of loose, green chlorenchymatous tissue.
• The green cells are arranged in vertical rows or columns.
• There are narrow, deep, vertical slits called the air canals or air channels between the columns.
• The uppermost cell of each row is larger and colourless.
• All the uppermost cells form ill-defined upper epidermis.
• The air canals communicate with the exterior through gaps in the upper epidermis. These gaps are called as air pores.
• This region is meant for the function of photosynthesis because. Of green, chlorophyll containing cells.
(ii) Storage Region:
• The lower or ventral region of the thallus is known as storage region.
• This region is madder up of simple, thin walled closely packed parenchymatous cells without intercellular spaces.
• These are colourless and may contain starch in the form of food material.
• The lower most cells of the region are small in size and regularly arranged to form lower epidermis.
• Scales and rhizoids are arises from the some of the cells of lower epidermis.
• Reproduction:
At maturity Riccia reproduces by two methods:
(i) Vegetative reproduction and (ii) Sexual reproduction.
(i) Vegetative reproduction:
Riccia thallus reproduces vegetatively during favourable conditions. It reproduces vegetatively by following means;
1. Fragmentation
2. Adventitious branches
3. Persistent apices
4. Tuber formation
1. Fragmentation: Aging process is responsible factor. Some of the cells of the older portion disintegrate, when decay of the older portion reaches dichotomy, the two lobes become separated and each lobe by repeated cell division and growth develops into a new individual.
2. Adventitious branches: In some species of Riccia special branches arise from the lower surface of the thallus. They arise in midrib region. They become separated from the parent thallus and grow into a new individual by repeated cell division and growth.
3. Persistent apices: In some of the species of Riccia, during unfavourable conditions, except the apical portion, whole thallus dies. These persistent apices grow into new individuals in rainy season.
4. Tuber formation: In some cases, the apical portion of the thallus becomes thickened to form tubers at the arrival of unfavourable condition. These tubers remain dormant and resume growth under favourable conditions.
(ii) Sexual reproduction: Riccia reproduces sexually at the end of growing season.
• This is a type of reproduction in which two sexes are involved.
• Two types of sex organs are developed, male sex organs are known as antheridia and female sex organs are archegonia.
• Male gametes are produced in antheridia and female gametes are formed in archegonia.
• During this phase, gametes are produced so that this phase of the life cycle is called as gametophyte phase.
• Both the sex organs are borne on the dorsal surface of the thallus in the midrib region.
• Riccia may be monoecious may be dioecious.
• In monoecious species both sex organs are developed on the same thallus. In dioecious species both of them are developed on two different thalli.
• Sexual reproduction is of oogamous type i.e. male and female gametes are different in size and shape. Male gametes are called as spermatozoids and female gametes are called as an egg cells.
Structure of sex organs:
• Antheridia:
• The mature antheridium is an elongated, ovoid or pear shaped structure situated on a short multicellular stalk.
• The antheridium is found in deep antheridial chamber.
• Each antheridial chamber opens at the dorsal surface of the thallus by a narrow pore called an ostiole.
• The wall of antheridium is made up of sterile cells of jacket layer which provides protection to developing male gametes.
• The antheridial wall encloses a mass of small cubical cells known as androcyte mother cells.
• Each androcyte mother cell divides to produce two androcytes.
• The protoplasm of these androcytes gets metamorphosed in spermatozoids.
• Each spermatozoid is a minute, delicate, curved structure with a pair of flagella at the anterior end. It has elongated nucleus and cytoplasm.
Dehiscence of Antheridia:
• Moisture is necessary factor for this process. Water enters through ostiole in the cavity. The wall of androcytes disintegrates due to absorption of water and all the spermatozoids become free to swim in the fluid in the antheridial cavity.
• All the spermatozoids escape through ostiole and swim freely in the dorsal groove on the upper surface of the thallus.
• Archegonim:
• The mature archegonium lies in the archegonial cavity.
• The archegonium is a flask-shaped, sessile (without stalk) structure directly attached to the bottom of the chamber. It consists of two parts.
• The basal swollen portion which is called venter and upper narrow elongated portion which is known as neck.
• The cavity of neck is filled with four neck canal cells.
• The wall of neck is protective. It is made up of 6 longitudinal rows of sterile cells.
• The tip of the neck is made up of four larger lid cells or cover cells.
• The wall of venter is also made up of jacket layer of sterile cells. It is single layered.
• The venter cavity consists of two cells, the lower larger an egg cell (haploid) and the smaller upper ventral canal cell.
• Fertilization:
• When male gamete unites with the female gamete the process is called as fertilization. At the end of fertilization (as a result of union of both the gametes) single celled zygote (diploid) is formed.
• Water is essential for fertilization. After bursting of antheridium, when sperms swims in a thin film of water in the dorsal groove, at the same time neck canal cells and ventral canal cells in the archegonium disintegrate due to entrance of water in archegonial cavity and forming a mass of mucilage. Pressure developed in side the archegonium.
• It opens the cover cells and continuous passage is formed. Mucilage comes out. Certain chemicals in the mucilage attract spermatozoids. This movement of spermatozoids towards archegonium is called chemotactic movement. One spermatozoid enters in the neck and reaches to the female gamete (egg) and fertilizes it. In this way a result of union of male (spermatozoid - n) and female gamete (egg- n) zygote (2n) is formed. This process called fertilization.
(ii) Sporophytic Phase:
• This phase begins with the formation of diploid zygote.
• The zygote is the pioneer structure of sporophyte phase.
• It secrets a thick wall and becomes enlarged in size to become sporogonium.
• Structure of sporogonium:
• The sporogonium of Riccia is just a spore sac or capsule.
• It is spherical in out line.
• It has a single layered wall termed as calyptra.
• The wall of sporogonium encloses a mass of diploid sporogenous tissue formed by mitotic division of zygote.
• The generation of sporophyte is spore mother cell which is diploid. All the spore mother cells divide meiotically to form haploid spore tetrads.
• The mature capsule consists of only haploid meispores. No elaters-formation in Riccia.
• It has no chlorophyll and is therefore, a total parasite getting all nutritive material from the gametophyte.
• Structure of Spores:
• They are pyramidal in shape having little cytoplasm, haploid nucleus.
• A mature spore of Riccia measures from 0.05 to 0.12 mm. in diameter.
• It has a wall composed of three layers i. e. the outer exosporium, and the middle mesosporium cutinised and the inner endosporium with cellulose and pectin.
• Inside the spore coats is the protoplasm containing a haploid nucleus and oily globules and other soluble substances.
• Dehiscence of Sporogonium:
• It never dehisces. The spores are liberated due to decay of surrounding thallus tissue. After liberation each spore in favourable condition germinates to give rise to a new thallus of Riccia to continue the life cycle.
• Life cycle of Riccia:• Riccia completes its life cycle in two phases; gametophyte and sporophyte phase.
• Gametophyte phase begins with the germination of haploid meiospores.
• Meiospores germinate to grow into mature green thallus of Riccia, which is purely vegetative.
• At maturity this haploid vegetative thallus develops sex organs during sexual reproduction. Male sex organs are known as antheridia and female sex organs are known as archegonia. Both the sex organs are developed from the haploid vegetative thallus.
• Antheridia consists of male gametes which are known as spermatozoids.
• Archegonia consists of female gametes known as an egg cell. Both the gametes are haploid.
• The union of both the haploid gametes (sexual reproduction) leads to the formation of zygote which is diploid. This process is called as fertilization.
• It is end of gametophyte phase and zygote is the pioneer structure of sporophyte phase.
• Diploid zygote undergoes many divisions and differentiation to form young sporogonium. Second individual in life cycle.
• Diploid nucleus of zygote undergoes so many mitotic divisions to form sporogonium.
• Sporogonium consists of diploid spore mother cells which undergoes meiosis to form haploid spore tetrads.
• Meiospores are the pioneer structure of new gametophyte phase. With formation meiospores sporophyte phase is over and new gametophyte phase starts.
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