Give an account of the structure and life history of Oedogonium.
Q. Give an account of the structure and life history of Oedogonium.
Or, Describe the sexual reproduction in Oedogonium.
Ans. Systematic Position:
Algae
Class : Chlorophyceae
Order : Oedogoniales
Family : Oedogoniaceae
Genus : Oedogonium
Occurrence: This is common, submerged freshwater algae, which may occur attached to the solid objects in water but often it grows epiphytically on the larger green algae or upon the leaves, petioles and stems of other aquatic plants in freshwater ponds, tanks, lakes and quiet streams.
Morphology: The plant body is filamentous, the filaments are unbranched and long. The basal cell of the young filament is devoid of chlorophyll and attached to some substratum and is called the hold fast. The terminal cell of the filament is broadly rounded at its apex. The intercalary cells are cylindrical, uniform in thickness and arranged end to end. These cells possess one or more striations at their apical ends called caps or cap cells.

Fig. Oedogonium sp. Detailed structure of a cell with conspicuous reticulate chloroplast
The cells are elongated cylindrical and covered with a three layered cell wall, of which the outer chitinous layer, the middle pectic layer and the inner cellulose layer. The cell protoplast consists of the cytoplasm, a nucleus, a large central vacuole and the reticulate chloroplast. At the end of the chloroplast pyrenoids are present.
Growth or cell division or cell formation : This is mostly intercalary and quite peculiar. In this case nucleus moves towards the upper end of the cell and begins to divide mitotically, and at the same time a circular ring of wall material (hemicellulose) appears all round the inner surface of the longitudinal wall just beneath the upper septum. The nuclear division is followed by a transverse division of the protoplast. The cell wall then splits just outside the cushion or circular ring. The ruptured wall stretches and gives rise to intercalary wall. A transverse septum develops between the two nuclei. The upper of the two daughter cells thus forms a wall formed by a smaller part of the original cell membrane left above the point of rupture.
Reproduction: Oedogonium may reproduce by vegetative, asexual or sexual means.
1. Vegetative reproduction: This is a very simple method in which filament of Oedogonium breaks up into several small portions, each of which by division forms a new filament.
2. Asexual reproduction:
(a) By zoospore: Any cell of the filament except the basal and apical cell is capable of forming zoospores. The cytoplasm of the zoospore forming cell (zoosporangium) contracts and a hyaline respective spot is formed. A ring appears around the margin of the hyaline area. Each zoosporangium produces a single zoospore. The zoospore is liberated by a transverse split near the apex of the zoosporangium. On being liberated, the zoospore slips out of the gelatinous membrane and swims freely in water. Each zoospore is an oval multi-ciliate structure with a single nucleus. On germination the zoospore gives a new filament.
(b) By akinetes: Sometimes during the unfavourable period a chain of 10 to 40 thick walled akinetes may be formed. Each cell of the filament develops a thick walled akinete by rounding off its cytoplasmic contents. They germinate directly into new filaments.
3. Sexual reproduction: This is of oogamous type and is common in plants growing in standing rather than flowing water. The sex organs are antheridia and oogonia. The plants may be monoecious or dioecious. In case of Oedogonium the sexual reproduction is of two types.
(i) Macrandrous type: In this case antheridia are reproduced on the filaments of normal size either separately or on the same filaments which bears oogonia.
Antheridium: The antheridia are terminal or intercalary and are produced by the division of an antheridial mother cell. Any vegetative cell may function as an antheridial mother cell. It divides into two unequal cells at its upper end forming an antheridium. The lower sister cell divides again and a chain of 20 to 40 antheridia is formed. The protoplasmic contents of each antheridium get metamorphosed in a single antherozoid. Two antherozoids may also be produced. The structure and mode of liberation of antherozoids is just similar to zoospores. The liberated antherozoids swim freely in water.
Oogonium: The oogonia may develop by terminal or intercalary oogonial mother cell. The oogonial mother cell divides transversely and the upper daughter cell always develops into an oogonium. Each oogonium always possesses one or two caps at its upper end. The lower daughter cell is called the suffulotry cell. Each oogonium is rounded or ovoid in structure, with a hyaline spot at the upper end. It consists of a large spherical egg cell.
(ii) Nannandrous type: In this case the antheridia are not formed to the filaments of the normal size, but on special dwarf male filaments called the nannandria, which are produced by special types of motile swarm spores, called the androspores
The androspores may be produced on the same filament which bears oogonia or termed gynandrophore and the species gynandrosporous while in the later case the androspores are called indioandrospores and the species idioandrosporous. The androspores resemble in shape and structure to zoospore, but they are somewhat smaller than zoospores and larger than antherozoids. The androspores get liberated like zoospores and after swimming for sometime the androspores settle down either on an oogonium or on a supporting cell. It then germinates to form a dwarf male filament on a nannandrium consisting of an elongated attaching rhizoidal cell and one or a few small discoid antheridia on the top of it.

Fig. Oedogonium. sp. Sexual Reproduction. Nannandrous species;
A. androspore formation in androsporangia; B. ooginium with dwarf males and antheridia; C. two oogonia and unicellular dwarf males on supporting (suffultory) cell; D. fertilization
Fertilization : In both the type i.e. in Macrandrous and nannandrous types the fertilization is effected by the entrance of a swimming antherozoid into the oogonium, which unites with the eggs at the hyaline which secretes two or a three layered wall around it.
Germination of Zygote: The zygote is liberated by the decay of the oogonial wall and rests in the mud. On the approach of favourable conditions the zygote nucleus divides by a reduction division and followed by simple division so as to form four daughter nuclei. Each daughter nuclei get surrounded by the cytoplasm and get transformed into multiflagellated zoospores which come out of the vesicle and after swimming about for sometime germinate to form new plants of oedogonium.
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