RICCIA - CLASSIFICATION, STRUCTURE OF THE GAMETOPHYTE, REPRODUCTION, SPOROPHYTE

A. CLASSIFICATION

 Division – Bryophyta

       Class – Hepaticopsida

  Order – Marchantiales

         Family – Ricciaceae

               Genus – Riccia

The genus Riccia comprises about 138 species which are cosmopolitan and widely distributed both in tropical and temperate regions of the earth. With the exception of a single aquatic species, R. fluitans, all the other species are terrestrial.

About 28 species of Riccia have been reported from different parts of India. Of these the common species are – R. discolor, R. crystalline, R. pathankotensis, R. melanospora, R. forstii, R. gangetica, etc.

B. STRUCTURE OF THE GAMETOPHYTE

1. External structure:- The plant body of Riccia is flat, dorsiventral thallus, which is prostrate, ribbon like and more or less fleshy. The branching of the thallus is dichotomous. Due to repeated dichotomy and also due to the occurrence of a number of such dichotomously branched plants together at a place, the plants often take a rosette form.  The thallus is usually green in colour and each thallus is provided with a midrib in the middle portion, which forms the apical notch at the tip.

The ventral (lower) surface of the thallus bears numerous multicellular hairs or scales, and unicellular rhizoids of two types – smooth walled rhizoids and tuberculate rhizoids. Rhizoids attach the thallus to the substratum and absorb water and solutes from the soil.

2. Internal structure:-  Internally the thallus of Riccia shows tissue differentiation of various types –

(a) Storage Region:- The lower or ventral region in cross section shows colourless, compactly arranged parenchymatous tissue without intercellular spaces. The cells of parenchyma tissue generally lack chlorophyll and contain starch. Therefore, the ventral region primarily constitutes the storage tissue. The lower portion of storage region is bounded by a single layered epidermis, from which rhizoids of two types – smooth walled and tuberculate arises.

(b) Photosynthetic Region:- The upper or dorsal region is composed of chloroplast bearing cells arranged in vertical tiers. These tiers of cells are separated by narrow vertical air chambers. Each air chamber is generally enclosed by four rows of cells. Each air chamber communicates to the exterior by means of an air pore, which is a simple intercellular space bounded by 4-6 epidermal cells. The terminal cell of each chloroplast containing cells is colourless without pyrenoids and somewhat enlarged and forms the upper epidermis of one celled thickness.

C. REPRODUCTION

The thallus of Riccia reproduces both by vegetative and sexual process of reproduction –

1. Vegetative Reproduction:- Vegetative reproduction takes place by the following process –

(a) Fragmentation:- It depends on the ageing of the vegetative cells. The cells in the older portions die of old age and eventually disorganize. When the dead and decay of the older portions of the thallus reaches the dichotomy, the young lobes become separated. Each of these by apical growth grows into a new thallus.

(b) Adventitious Branches:- In R. fluitans special adventitious branches arise from the ventral surface of the thallus in the midrib region. They become detached from the parent thallus by the decay of the connecting tissue and form a new plant.

(c) Persistent Apices:- In species growing in prolonged dry season, the whole plant die except the growing apices. The persistent apices resume growth in the succeeding rainy season.

(d) Tuber Formation:- It has been reported in R. perennis, R. discolor, R. vescata, and R. billardieri. The apices of the thallus lobes become thickened to form tubers at the end of the growing season. With the advent of unfavourable conditions the plant dies. The tuber remains dormant and resumes growth under suitable conditions.

2. Sexual Reproduction:- Riccia is mainly homothallic (monoecious) i.e., same plant body bears both male (antheridia) and female (archegonia) sex organs. Some species like R. discolor, R. frostii, etc. are heterothallic. In monoecious species both archegonia and antheridia arise singly and each lies in the floor of a separate cavity.

(a) Antheredium:- Each antheridium stands in a deep pit called antheridial chamber, and is attached to the thallus tissue my means of multicellular stalk. The antheridial chamber opens at the tip by a narrow pore called ostiole.

The antheridium is an ovoid or pear shaped body enclosed by a single layer of sterile jacket layer known as antheridial wall. Inside the jacket layer lies the numerous androcyte mother cells, metamorphose into two sperms or spermatids or androcytes.

(b) Archegonium:- Each archegonium also stands in a pit called archegonial chamber. The archegoniun is without stalk and is directly attached with the thallus tissue.

The mature archegonium is ahort stalked, flask-shaped and consist of venter and neck. The venter consists of a ventral canal cell and an egg cell. The neck is long and consists of a row of four neck canal cells. Each archegonium is bounded on the outer portion by a single layer of jacket cells called archegonial wall.

            D. FERTILIZATION

            It takes place in presence of water. The sperms after liberation from the antheridium swim in all directions on the film of water, and some of them reach the archegonium. On reaching the archegonium, the sperms make its way through the passage led by the dissolution of the neck canal cell. On reaching the egg, one sperm fuses with the egg to complete the fertilization.

            As a result of fertilization, a diploid zygote (2n) is formed. With the formation of zygote, diploid sporophytic generation begins.

E. SPOROPHYTIC PHASE

Zygote is the first cell of the sporophyte. Immediately after the formation, the zygote enlarges in size until it nearly fills the cavity of the venter.

The zygote nucleus first divide s transversely into two cells, each of which again divide vertically to form 4-celled structure i.e., quadrant. Then another vertical division takes place so that 8-celled structure i.e., octant is formed. The cells of the octant divide and re-divide irregularly forming a spherical mass of 20 – 410 cells. Then periclinal divisions take place which cut off an outer layer of cells, called the amphithecium and inner mass of cells, called the endothecium.

The amphithecium first constitutes the jacket of capsule, but later it serves as the nutrition of the spores. The outermost layer constitutes the jacket of the mature sporophyte. The endothecium develops into sporogenous tissue. The sporogenous tissue later develops into spore mother cells. From each spore mother cell, by two successive divisions, one of which is reduction division, four haploid spores are produced. Some of the few cells of the sporogenous tissue remains sterile, and are known as nurse cells.

The mature sporophyte is a globose capsule like structure without any foot and seta. It is simply a spore case. The spore case i.e., capsule is provided with sterile jacket layer of one-celled in thickness. Inside the jacket layer, lie the spore-tetrad and nurse cells. The spores are liberated after breaking the surrounding tissue of the thallus.

After liberation, the spore germinates under favourable condition by producing a germ tube to give rise to new individual Riccia plant.

Prema Iswary,               

Assistant Professor,     

Department of Botany.

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