Q.4. Give an account of the structure and life history of Oedogonium.
Ans.4. This is common, submerged fresh water algae, comprises about 400 species which, may occur attached to the solid objects water but often grows epiphytically on the solid object algae or upon the leaves, petioles and stems of other aquatic plants in fresh water ponds, tanks, lakes and quiet streams.
Morphology
The plant body is filamentous, the filaments are unbranched and long. The basal cell of the young filaments 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 the apex. The intercalary cell are cylindrical, uniform in thickness and arranged end to end. A few intercalary cells possess ring like structures or Caps at the apical ends. These are known as ‘caps or ‘cap cells’.
The cells are elongated cylindrical and covered with a three layered cell wall, of which the outer is 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 ends 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 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 formed has wall formed by a smaller part of the original cell membrane left above the point of rupture:
Reproduction
Oedogonium may reproduce by vegatative, asexual or sexual methods.
- Vegetative reproduction
This is a very simple method in which filament of Oedogonium breaks up into several small protions, each of which froms a new filament by divisions.
- Asexual reproduction
(a) By zoospore: Any cell of the filament except the basal and apical cell is capable of forming zoospore. The protoplasmic contents of zoospore forming cell (zoosporangium) contracts and a hyaline receptive spot is formed. A ring appears around the margin of hyaline area. Each zoosporangium produces a single zoospore. Flagellation in zoospore is stephanokont type i.e., ring of flagella (about 120) in found at the base of colour less region. The zoospore is liberated by a transverse split near the apex of the zoosporangium.
On beign liberated, the zoospore slips out of the gelationous membrane and swims freely in water. Each zoospore is a oval multicillate 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.
Sexual reproduction
This is of oogamous type and is common are 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 produced on the filament of normal size either separately or on the same filaments which bears oogonia.
(ii) Nannandrous type: In this case the antheridia are not formed to the filaments of the normal size, but on special dwarf male filament called the nannandria, which are produced by special types of motile swarm spores, called the androspores.
Antheridium: The antheridia are terminal or intercalary and are produced by the division of antheridial mother cell. Any vegetative cell may function as antheridial mother cell, it divides into unequal cells at its upper end forming an antheridium. The lower sister cell divides again and a chain of 2 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 suffultry. Each oogonium is rounded or ovoid in structure, with a hyaline spot at the upper end. It consist of a large spherical egg cell.
Nannandrous type
In this case the oogonia are formed as in macrandrous type, as described above. The antheridia are formed on special dwarf filaments or nannandria and hence the nannandrous species are dioecious. These dwarf males or nannadria originate by the germination of special type of swarmers called androspores which are produced singly in flat cell androsporangia formed by repeated transverse divisions of ordinary vegetative cells.
The androspores may be produced on the same filament which bears oogonia or termed gynadrospores and the species gynandrosporous while in the later case the androspores are called idioandrospores and the species idiondrosporous.
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 of a nannandrium consisting of an elongated attaching rhizoidal cell and one or a few small discoid antheridia on the top of it. The formation of antherozoids and their liberation is as in macrandrous type of antheridia. (BSc Botany Chlorophyta Sample Practice Question Answers)
Fertilization
In both the types i.e., in macrandrous and nannandrous type the fertilization takes place by entering of a swimming antherozoid into the oogonium, which unites with the eggs at the hyaline receptive spot. As a result of fusion zygote or oospore is formed which secretes two or a three layered wall around it.
Germination of Zygote
The zygote gets 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, followed by simply division so as to form four daughter nuclei. Each daughter nuclei get surrounded by the cytoplasm and get transformed into multiflagellate zoospores which come out of the vesicle and after swimming about for sometime, germinate to form new plants of Oedogonium.
BSc 1st Year Botany Chlorophyta Sample Model Practice Question Answer Papers