Lifecycle and alternation of generation in Riccia

The lifecycle of Riccia has two distinct phases called haploid i.e. gametophytic and diploid i.e. saprophytic. These phases alternate with each other.

Reproduction: The vegetative plant is a gametophyte that reproduces by vegetative and sexual methods. Asexual reproduction occurs only in saprophytic plants. 

(1) Vegetative Reproduction: It takes place by (i) By Progressive growth of the apical region and death of the older region or by decay method. When the plant becomes mature the older parts of the thallus die and decay, and the younger parts separate from older ones and lead an independent life. (ii) By adventitious branches formation: In some species, adventitious branches arise from the neutral surface of the thallus, and the separation of these branches results in the formation of new thalli. (iii) By Tubur formation: In some species there arise tubur outgrowths which are capable of producing new plants as in Riccia vesicota. (iv) Apices of plants get embedded: In some species of R. himalayensis the apex of thallus at the end of the season grows down into the soil and becomes thick. In the next season it grows up and gives rise to the new plant. (v) By rhizoids: In some species like R. glauca a young thallus may be formed at the apex of a rhizoid in manner similar to that by which the young thallus arises from the germ tube form germinating spore.

Sexual Reproduction: It is of oogamous type. Mature thallus of Riccia bears sex organs on its dorsal surface. There are male or antheridia and female or archegonia as sexual organs. Plants may be monoecious or dioecious.

Male Reproductive Organs: Male organ is autheridium cuhich develops from single superficial cell on dorsal surface of thallus called antheridial initial cell. It protrudes out and becomes papillate and then divide by transverse division forming a lower basal cell and upper outer cell. The lower basal cell undergoes further development and forms the embedded portion of the stalk of antheridium while the outer cell forms the jacket layer, antherozoids outer cells divide to form two louver cells called primary stalk cells and upper primary antheridial cells. Cells of both tiers divide to form many anderogenial cells which divide to form many androgenial cells and androcyte cells, each of which divides to form two antherozoid mother cells.

Each androcyte of antherozoid mother cells gets metamorphosed into an antherozoid. A small extra nuclear gramule the blepharoplasty appears which then elongates into a rod-shaped body around the cells. The nucleus gets the shape of a crescent and becomes homogenous. It moves to the side of blepharoplasty and becomes firmly incited with it. At the anterior end of blepharoplasty two long flagella are formed. Thus a mature antherozoid is formed which has an elongated nucleus and the blepharoplasty with two long flagella. Cilia help in movement. Antherozoids get liberated free from antheridium only when moisture is present. Mature antheridium is club-shaped with a single jacket layer and antherozoids.

Female Reproductive Organs: Archegonium develops from a single superficial cell called archegonial initial which divides by transverse division into the lower basal cell and upper outer cell. The basal cell after further divisions from the embedded stalk of archegonium while the outer cell divides by three vertical divisions in such a way that three peripheral cells surround a central primary axial cell. The three peripheral cells divide vertically from 6 jacket cells, the central axial cell divides transversely and from an upper primary cover cell and a lower central cell.

Jacket cells divide transversely and form two tiers of jacket each having six cells. The upper tier of cells is called neck initials and forms the neck of six cells in height. Primary cover cells divide into four cover cells. The central cell divides into the upper primary neck canal cell and lower primary venter cell. The primary neck canal cell further divides into four neck cells and the primary venter cell forms a small venter cell and a large egg. Thus mature archegonium looks like a flask-shaped structure with a long neck and swollen venter with an egg. Fertilization: Male and female gametes fuse together in a water medium. Free swimming antherozoid gets attached to the archegonium and reaches the mouth of the neck, enters the canal and goes into the water only antherozoid fuses with an egg and forms a zygote or oospore which is diploid in nature.

Sporophyte generation: Oospore or zygote is first cell or unit of sporophyte. The zygote secretes a cell wall of its own and enlarges in size. The wall of the venter undergoes many periclinal and anticlinal divisions to form a calyptra which is two-layered. The zygote undergoes segmentation. The first division is transverse. The second and third verticals result in the formation of & called the structure of the octant.

Eight called zygote or embryo divides by many irregular divisions to form 20 to 40 called multicellular structure. Periclinal division forms an outer layer of amphethecium and inner endothelium. Endothelium forms the spore mother cells and is diploid or sporophytic in nature and divides by reduction division and forms four spores. The sporophyte of Riccia has a capsule with a jacket and spores. There is no foot seta and no fertilization of sporogenous tissue in the capsule. Spores get liberated after maturation by death and decay of sporangous tissue in the capsule.

Gametophyte: spore is the first cell or unit of the gametophyte. Germenation of spore & formation of young gametophyte: The spores after dispersal gets a suitable substratum, and start to germinate. The emporium forms a germ tube that grows and forms a filamentous structure called protonema. The lower portion of the germ tube produces rhizoids apical portion of the germ tube divides and forms a young gametophytic thallus of Riccia.

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