Oedogonium Structure and Reproduction // Detailed Notes By Rohit Jirobe //
Oedogonium
Oedogonium
Oedogonium is a green alga. It is placed in the class Chlorophyceae. This genus was named by Link.
Occurrence
Oedogonium is widely distributed in freshwater habitats such as ponds, lakes, tanks, mers, etc.
The mature filaments are free-floating but the younger ones are attached.
They are found attached to stones, woods and to large algae and aquatic angiosperms. There are about 285 species in Oedogonium. Of these, 144 species are found in India.
The common Indian species are:
Oedogonium nodulosum,
Oedogonium fragile,
Oedogonium aquaticum,
Oedogonium gracilius.
The mature filaments are free-floating but the younger ones are attached.
They are found attached to stones, woods and to large algae and aquatic angiosperms. There are about 285 species in Oedogonium. Of these, 144 species are found in India.
The common Indian species are:
Oedogonium nodulosum,
Oedogonium fragile,
Oedogonium aquaticum,
Oedogonium gracilius.
Structure
Oedogonium is a unbranched filamentous green alga.
It is included in the class Chlorophyceae.
It is cosmopolitan in distribution.
The plant is a haploid gametophyte.
It is a freshwater alga.
It is an attached form.
It is a filamentous alga.
The thallus consists of a long filament and a holdfast.
The filament is unbranched.
It is included in the class Chlorophyceae.
It is cosmopolitan in distribution.
The plant is a haploid gametophyte.
It is a freshwater alga.
It is an attached form.
It is a filamentous alga.
The thallus consists of a long filament and a holdfast.
The filament is unbranched.
The filament is uniseriate and it consists of elongated and cylindrical cells ged end to end.
The basal cell of the filament is modified into the holdfast or hapteron.
The holdfast is devoid of chloroplast.
The holdfast attaches the filament with the substratum by its lobed base.
The free end of the filament is acute or round.
Some cells have ring-like thickenings called cell caps.
The cells with caps are called cap cells.
The mature filament has a few swollen oogonia and antheridia.
The cell has an outer cell wall, a middle plasma membrane and an inner protoplasm
The cell wall consists of three layers, namely an outer chitinous layer, a middle pertin layer and an inner cellulose layer.
The cell wall has ring-like thickenings called cell caps.
cell.
The cell wall has ring-like thickenings called cell caps.
The plasma membrane lies below the cell wall.
The protoplasm contains a single reticulate chloroplast with many pyrenoids, a nu-cleus and vacuole.
The nucleus is large and eukaryotic
The chloroplast contains chlorophyll-a and-b, carotenes and xanthophylls. Starch is the reserve food.
Growth
The growth of the filament takes place by the division of terminal cell or intercalary cell. The cell division is a peculiar type and it results in the formation of cap cells.
Cell Division and Formation of Cap Cells
The cell division in Oedogonium is a peculiar type. This type of cell division is not seen in other families. All cells of the filament, except the holdfast, are capable of cell division. The important events of cell division are given below:cell.
1. Just before the cell division, the nucleus moves to the centre from the periphery of the
2. The nucleus divides mitotically into two daughter nuclei. At the same time, a con-centric ring of hemicellulose develops on the inner surface of the lateral wall just below the upper transverse wall It develops by invagination of inner and middle layers of the cell wall.
3. The concentric ring becomes thickened by the addition of hemicellulose. Now a float-ing septum develops between the two daughter nuclei.
4. The floating septum moves upward and reaches near the base of the ring.
5. The septum is connected with the lateral wall of the cell.
6. As a result a small upper cell and a large basal cell are formed. In the meantime, the ring stretches slightly.
7. Due to the stretching of the ring, the outer wall layer at the ring, ruptures and appears as a ring around the cell. The ruptured outer wall layer is called cell cap.
8. The upper cell grows to the normal size, the ring becomes fully stretched and an outer wall layer develops at the point of rupture. The cell with cell cap is called cap cell.
The number of caps is corresponding to the number of previous divisions of the cap cell
Reproduction
The reproduction in Oedogonium takes place by three methods. They are:
2. Asexual reproduction
3. Sexual reproduction.
Vegetative Reproduction
The vegetative reproduction takes place by fragmentation. The filament breaks into many small fragments by mechanical actions. Each fragment grows into a new filament. This method is
called fragmentation
Asexual Reproduction
In Oedogonium, asexual reproduction takes place by three kinds of asexual spores.
They are:
1. Zoospores
2. Aplanospores
3. Akinetes
Zoospores
Asexual reproduction takes place by means of multiflagellate zoospores.
The zoospores are formed singly from any cap cells.
The cell producing the zoospore is known as zoosporangium.
The protoplast of the zoosporangium contracts from the cell wall and becomes a rounded mass. It gradually becomes oval in shape and green in colour with a hyaline anterior part.
Around this hyaline part a ring of about 120 flagella arise and now it is called zoospore
This type of flagella arrangernent is called stephanokont type
The zoospore with stephanokont arrangement of flagella is called stephankonton.
Each zoospore has a reticulate chloroplast, a haploid nucleus and plenty of reserve
food.
The cross wall at the upper end of the cell separates as a lid. The zoospore, enclosed within a vesicle, comes out of the zoosporangium. Then the vesicle disappears and the zoospore
swims freely in the water. The released zoospore becomes upside down and loses its flagella.
The cell then divides transversely into a lower cell and an upper cell.
The lower cell becomes the holdfast and the upper cell forms the uniseriate filament.
2. Aplanospores
Aplanospores are non-motile, non-flagellate, uninucleate cells.
The cell of the filament producing the aplanospore is called aplanosporangium.
The aplanospore is produced during the dry season.
When favourable season comes, the aplanospore comes out of the aplanosporangium and
germinates into a new plant.
3. Akinetes
Akinetes are thick-walled vegetative cells of Oedogonium.
Sometimes, the vegetative cell stores plenty of reserve food and develops a thick wall around it. This thick walled cell is called akinete.
During the favourable season, it germinates into a new filament.
Sexual Reproduction
The sexual reproduction in Oedogonium is oogamous type. The male sex organ is called antheridium and the female sex organ is called oogonium.
Depending upon the distribution of sex organs, all species of Oedogonium are divided into two groups. They are:
Macrandrous Species
The species of Oedogonium that produces antheridia and oogonia in normal filament, is called macrandrous species.
The macrandrous species may be homothallic or heterothallic.
The homothallic species (monoecious) produces antheridia and oogonia in the same fila-. Eg. Oedogonium nodulosum.
The heterothallic species (dioecious) produces antheridia in one normal filament and oogonia in another normal filament. Eg. Oedogonium aquaticum.
Antheridia
The male sex organs of Oedogonium are called antheridia.
They develop from any vegetative cell of the filament.
The vegetative cell functions as an antheridial mother cell.
It divides repeatedly to form a chain of 2-40 antheridia.
The protoplast of each antheridium divides mitotically into two haploid daughter protoplasts.
Each protoplast becomes pear-shaped and develops a ring of many flagella around a col-ourless portion at its one end. This becomes a sperm. Thus two sperms are produced inside each antheridium.
The wall of the mature antheridium ruptures transversely to form a gap.
The two sperms secrete a common mucilage vesicle around them.
The vesicle comes out through the gap. Later, the sperms come out of the vesicle and swim freely in the water in search of an oogonium.
The sperms are similar to the zoospores but smaller in size.
Oogonia
The female sex organs are called oogonia.
Any cell of the filament, functions as oogonial mother cell.
This cell divides transversely into an upper cell with cell cap and a lower cell.
The upper cell develops into an oogonium.
The lower cell becomes a supporting cell ot suffultory cell.
In some rare cases, the supporting cell also divides and forms one or few oogonia.
The protoplast of oogonial cell becomes round and it contracts from the oogonial wall to Bem an egg. The egg develops a small colourless part called receptive spot at its lateral side. The oogonial wall at the receptive spot forms a pore or slit to give a way for the entry of the sperms.
Fertilization
As the oogonium matures, the oogonial wall at the receptive spot gelatinises and forms a macillage mount. This mucilage attracts the sperms towards the oogonium.
One sperm enters the oogonium through the slit and fuses with the egg to form a zygote.
The zygote is diploid.
The zygote secretes a thick wall to form an oospore.
Germination of Oospore
After a period of rest, the diploid nucleus of the oospore divides meiotically into four haploid nuclei.
Each nucleus with the cytoplasm develops many flagella to form a zoospore.
The oospore wall ruptures and releases the four zoospores in water. The zoospores are kaploid.
The zoospores swim in water and germinate into haploid Oedogonium plants.
Nannandrous Species
The species of Oedogonium that produces oogonium in normal filament and antheridium in small dwarf filament is called nannandrous species.
Here, the normal filament produces a special type of zoospore called androspore.
The androspores are produced in androsporangia. The androsporangia are similar to the antheridia of macrandrous species.
The androspores settle either on the oogonium or on the supporting cell. They germinate into short filaments called dwarf males or nannandria
The nannandrous species are often known as androspore - forming species. The
nannandrous species are of two types:
1. Gynandrosporous species (monoecious)
2. Idiandrosporous species (dioecious).
In the gynandrosporous species, androsporangia and oogonia are produced in the same filament (ie, monoecious) Eg. Oedogonium concatenatum.
In idiandrosporous species androsporangia and oogonia are produced in separate fila-ments (i.e., dioecious) Eg. Oedogonium iyengarii.
The androspores are liberated from the androsporangia. Each androspore develops into a few celled filament known as dwarf male or nannandrium.
Nannandrium
The nannandrium is the male filament of Oedogonium.
It is a short filament and it produces male gametes. So it is known as dwarf male fila-ment. It develops from androspore. It is found attached to the supporting cell or the oogonium.
Each nannandrium consists of a basal stalk cell and one or two antheridia.
The protoplast of the antheridium divides mitotically into two daughter protoplasts.
Each daughter protoplast develops a ring of many flagella at one end and forms a
multiflagellate sperm or antherozoid. The sperms come out of the antheridium by the rupture of antheridial wall. They swim in water in search of an oogonium.
Oogonium
Oogonium is the female sex organ and it is produced in the oogonial filament
The vegetative cell divides transversely into an upper oogonial cell and a lower support-ing cell or suffultory cell.
The oogonial initial enlarges in size and becomes round.
The protoplast of the cell becomes an egg. A colourless area called receptive spot devel-ops at the lateral side of the oogonium.
Towards maturity, the oogonial wall at the receptive spot produces a mucilage mount
Fertilization
The sperms coming from the nannandrium move towards the oogonium and enter the oo-gonium through a pore of oogonial wall. One sperm fuses with the egg to form a diploid zygote The zygote secretes a thick wall to form an oospore.
Germination of Oospore
After a period of rest, the diploid nucleus of the oospore undergoes meiosis to form four haploid nuclei.
Each nucleus is surrounded by cytoplasm to form a haploid, multiflagellate zoospore
The zoospores germinate into new haploid filaments.
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