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 gametophyte which reproduces by vegetative and sexual methods. Asexual reproduction occurs only in saprophytic plant.
(1) Vegetative Reproduction: It takes place by:
(i) By Progressive growth of apical region and death of older region or by decay method. When the plant becomes mature the older parts of thallus die and decay, the younger parts separate from older ones and lead independent life.
(ii) By adventitious branches formation: In some species adventitious branches arise from neutral surface of 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 season grows down into the soil and becomes thick. In next season it grows up and gives rise to 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 femal 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. Lower basal cell undergoes further development and forms the embedded portion of stalk of antheridium while outer cell forms jacket layer, antherozoids outer cells divides to form two louver cells called primary stalk cells and upper primary antheridial cells. Cells of both tiers divide to formmany anderogenial cells which divide to form many androgenial cells and androcyte cells, each of whiche divides to form two antherozoid mother cells.
Each androcyte of antherozoid mother cells get metamorphosed into an antherozoid. A small extra nuclear gramule the blepharoplast appears which then elongates into rod shaped body around the cells. The nucleus gets the shape of crescent and becomes homogenous. It moves to the side of blepharoplast and becomes firmly incited with it. At anterior end of blepharoplast two long flagella are formed. Thus a mature antherozoid is formed which has an elongated nucleus and the blephroplast with two long flagella. Cilia help in movement. Antherozoids get liberated free from antheridum only when moisture is present. Mature antheridium is club shaped with single jacket layer and antherozoids.
Female Reproductive Organs: Archegonium develops from a single superficial cell called archegonial initial which divides by transverse division into lower basal cell and upper outer cell. The basal cell after further divisions from 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 lower central cell.
Jacket cells divide transversely and form two tiers of jacket each having six cells. Upper tier of cells is called as neck initials and forms neck of six cells in height. Primary cover cells divide four cover cells. Central cell divides into upper primary neck canal cell and lower primary venter cell. Primary neck canal cell further divides into four neck cells and 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 water medium. Free swimming antherozoid gets attached with archegonium and reaches mouth of neck, enter the canal and go into water only antherozoid fuses with egg and form zygote or oospore which is diploid in nature.
Sporophyte generation: Oospore or zygote is first cell or unit of sporophyte. Zygote secretes a cell wall of its own and enlarges in size. Wall of venter undergoes many periclinal and anticlinal division to form a calyptra which is two layered. Zygote undergoes segmentation. First division is transverse. Second and third vertical result in the formation of & called structure 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 form four spores. Sporophyte of Riccia has a capsule with 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 gametophyte.
Germenation of spore & formation of young gametophyte: The spores after dispersal get suitable substratum, starts to germinate. Edoporium forms a germ tube which grown and forms filamentous structure called protonema. Lower portion of germ tube produces rhizoids apical portion of germ tube divides and forms young gametophytic thallus of Riccia.