Q.1 Give the systematic position and describe the structure of gametophyte of Riccia with the help of suitable diagrams.
Ans. Systematic Position
Division - Bryophyta
Class - Hepaticopsida
Order - Marchantiales
Family - Ricciaceae
Genus - Riccia
The Gametophyte of Riccia (The Adult gametophyte): -
The main plant body or thallus of Riccia is a well developed gametophyte (haploid).
Vegetative Structures: -
The thallus of Riccia is small, flat, dorsiventral and dark green in colour. It grows prostrate over, the substratum generally in the form of small patches called Riccia rosettes. The Thalli shows dichotomously branching. The dorsal surface of thallus is smooth and shows prominent mid-rib in the form of dorsal groove. The median longitudinal furrows deepens at the apex forming an apical notch in which growing point is situated.
The ventral surface bears numerous rhizoids and scales. The rhizoids are unicellular, elongated, tubular and hair like structures which attach the thallus to substratum and absorbs water and nutrient solution. Riccia has two types of rhizoids, the simple rhizoids with smooth inner walls and tuberculate rhizoids with prominent peg-like.
Internal structure or Anatomy of Terrestrial Species of Riccia: -
Internally the thallus shows differentiation in two regions, which can be seen in its vertical section.
(i) An upper assimilatory region and (ii) A lower storage region
Assimilatory region of the thallus consists of unbranched green photosynthetic filaments. These filments are arranged in vertical rows consisting of a single row of cells. Upper epidermis is discontinuous having large usually non green cells. All other cells of the filaments are a like and contain nemerous discoid chloroplast capable of manufacturing food by photosynthesis process.
Vertical rows of assimilatory filaments enclose vertical space which may be called air chambers, air canals or air channels. The air chambers may be narrow or wide due to complactly or loosely organized assimilatory filaments.
The lower zone (storage zone) consists of compact, colourless, parenchymatous tissue without intercellular spaces. The photosynthesis in photosynthetic zone and the product is stored in the storage zone. The scales are violet coloured, one celled thick and attached to the lower epidermis.
Q.2. With the help of suitable diagrams only illustrate the life cycle of Riccia.
Related Questions -
Q. Describe the vegetative reproduction in Riccia. (2004, 08)
Q. Describe the sporophyte of Ricçia. (2004)
Ans. Reproduction in Riccia: -
Riccia starts reproducing after reaching a certain stage of maturity. The reproduction occurs both vegetatively and sexually.
Vegetative Reproduction: -
The plants reproduce vegetatively by different methods. Some of them are discussed below: -
1. Fragmentation: The older or posterior portions of thalli die of old age. This progressive death and decay, on reaching to dichotomy, separate the two lobes, each survived lobe, by apical growth, develops into a new plant.
2. Formation of Adventitious Branch: In R. fluitants adventitious branches, similar to parent thallus arise from the mid-ventral surface of the thallus. These adventitious branches get detached and develop into new plants.
3. Persistent Apices: The apices grow down into the soil under unfavourable conditions. These apices become thickened and perennate the unfavourable conditions. They resume growth during next growing seasons and produce new thalli.
4. Formation of Tubers: A few species develop perennating tubers at the end of growing seasons. These tubers become thick walled and prennate the unfavourable conditions.
Sexual Reproduction: -
The sexual reproduction is oogamous type. Male sex organs are antheridia and female sex organs are archegonia. Some of the species are monoecious whereas some are dioecious.
The sex organs develop in acropetal succession, i.e. younger one produced on the apex is shifted behind at maturity. Each sex organs develops into a separate chamber and lies deeply sunken in mid-dorsal furrow of the thallus. Antheridium shows a small projection of thallus.
Antheridium: -
· A mature antheridium of Riccia is a pearshaped body within an open antheridial chamber which is formed by the overarching tissues. The antheridial chamber communicates with the dorsal surface by a pore.
· The antheridium is attached to the base of the antheridial chamber by means of a few-celled stalk.
· The pearshaped antheridial body has got a flat broad base and a conical apex.
· The antheridial body is surrounded by a single-layered wall or jacket made of thin-walled cell.
· A central mass of cuboidal cells enclosed by the jacket layer are the androgonial cells or androcyte mother cell.
· Each androcyte mother cell, on maturity, divides diagonally to produce two traingular androcytes.
· Each androcyte ultimately metamorphoses into a single biflagellate antherozoid or spermatozoid.
· During metamorphosis cell walls of the androgonial get disorganised to form a semifluid mucilaginous content in which the mature antherozoids float freely.
· Next gelatinization of jacket cells towards the apex marks it more breakable.
· When water enters into the antheridial chamber the gelatinized jacket cells absorb it and swell and finally break open.
· Then the semifluid mucilaginous content of the antheridium containing the antherozoids, oozes out of the antheridial chamber to the dorsal surface of the thallus.
Archegonium: -
· A mature archegonium is a flask-shaped body embedded within a chamber called archegonial chamber formed similarly like antheridial chamber, which communicates with the dorsal surface by a pore.
· The archegonium is attached to the base of the archegonial chamber by means of a short few-celled stalk.
· The flask-shaped archegonium is differentiated into a basal swollen part the venter, and an elongated protruding tubular portion, the neck.
· The venter consists of a single layered wall having more than six cells in perimeter and encloses a lower large egg or female gamete with an upper small ventral canal cell.
· The wall of upper tubular neck consists of 6-9 tires of elongated cells arranged in 6 vertical rows which encloses a narrow central canal consisting of 4-6 neck canal cells in a single row.
· The tip of the neck is covered by four specialized cells called cover cells.
· When the archegonium is matured, the canal cells (neck and ventral canal cells) degenerate, leaving a mucilaginous mass.
· Shortly before fertilization, when water enters into the archegonial chamber, the mucilaginous mass imbibes water, swells and sets up a force which pushes the cover cells apart. Thus a free neck canal is formed from the apex of the archegonium to the egg.
Fertilization: -
The process of fertilization occurs in presence of water provided mainly by rain or dew which forms a film over the dorsal surface of thallus. The antherozoids, liberated from antheridia, swim in the film of water in the dorsal furrow and spread in all directions. Some of the antherozoids reach near the archegonia. At maturity the neck canal cell and venter canal cells degenerated and become mucilagenous. At the same time water enters the archegonial chamber. The mucilagenous substance, when hydrated, exerts a pressure so that the cover cells spread apart. A passage is created at the opening of archegonial neck through which some of the mucilagenous substances oozes out. The antherozoids are attracted towards the mucilage. Some of the anthrozoids also enter into the neck canal. Usually a single antherozoid fuses with the egg. The fusion of anthrozoid nucleus with the egg nucleus results formation of diploid zygote. The gametophytic phase of the life cycle ends with the formation of zygote.
The Sporophyte: -
The Zygote : The oospore or zygote is the first cell of sporophytic generation, which is formed by the fusion of haploid antherozoid and haploid egg. The zygote secrete its own cell wall and enlarges in size to almost fill the cavity of venter. The zygote is retained inside the venter and starts germinating. It produces diploid sporophytic plant body which is dependent on gametophytic plant body.
The sporophyte forms the sporogonium of Riccia is considered as most primitive type. It has no diploid or sporophytic cells at maturity. The spores are haploid and they are the first cell of gametophytic generation. The spores of Riccia germinate in about 6-10 days after they are released from the sporogonium by rain drop of air.