Thallus structure:
The thallus of oedogonium is multicellular filamentous, unbranched and uniseriate. All cells of the filament, except the basel and apical ones, are cylindrical and alike. The basal cell, which act as holdfast is devoid of chloroplast. Its proximal end is extended into finger like or disc like projections. The terminal or apical cell is rounded, elongated or acuminate. The cell has fairly thick rigid cell wall made up of outer chitin, middle pectin and an inner cellulose components. The chloroplast is reticulate, extending in the form of a sheet throughout the protoplast. The cells are uni nucleate, the nucleus lies in the middle region of the cell in between the chloroplast reticulum. A characteristic feature of oedogonium is the presence of distinctive transverse bands at the distal end of some cells. The bands formed at the time of cell division, is called apical cap, and the cell with apical cap is known as cap cell.
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| Oedogonium filament |
Growth in oedogonium takes place as a result of the division in the intercalary cells. All cells of filament, except holdfast and apical cell, are capable to decide. The cell division in oedogonium is a complex process. The important events during the cell division are as follows. The cell division is initiated by the formation of a ring of wall material in the upper part of the cell, which gradually increases in thickness. The nucleus migrates to the centre of the cell and divide mitotically. During telophase an unattached floating septum is laid down between the two daughter nuclei the septum is formed due to continous accumulation of vestibular material from the cytoplasm. The ring of the wall material now elongates, causing a split in the parent wall near the apical ring. In this process a part of the ruptured parent wall remains attached to the anterior end of the daughter cell in the form of cap. It is know as apical cap, and the cell having a cap is called the cap cell. The floating septum gradually migrates upward to the base of the daughter cell and develops into a mature cross wall. Thus each division of the cell results in the formation of an apical cap. The number of apical caps in a cell indicates the number of times it has decided.
Reproduction in Oedogonium:
Oedogonium reproduce by vegetative, asexual and sexual method.
1) Vegetative reproduction:
Vegetative reproduction takes place by fragmentation and akinetes.
a) Fragmentation:
Like many other algae, small fragments of oedogonium filament have the capability to grow into complete filament under favorable conditions. Fragmentation takesplace by accidental breaking of the filament, dying or dehydration of intercalary cells..
b) Akinetes:
These are thick walled reddish or brownish structures, usually formed in small chains during unfavorable conditions. These spores germinate under favourable conditions and form new filaments.
2)Asexual reproduction:
Asexual reproduction takes place by means of zoospores, usually cell with cap behaves as zoosporangium. During zoospore formation cell contents contracts slightly from the cell wall. A small lens shaped hyaline area develops on one side of the protoplast which eventually become anterior part hyaline area a ring of basal granules appears, and from each granule a single flagellum arises. The mature zoospore is ovoid, spherical or pyriform. It is uni nucleate and contains a chloroplast. At maturity of the zoospore, the wall of the zoosporangium splits near the cap region, and the adjacent cell moves apart to make passage for the liberation of the zoospore. The zoospores comes out of the zoosporangium in a delicate mucilagenous vesicle which soon gets dissolved and the zoospore is liberated.
Germination of zoospore:
Zoospore swims of about an hour and then settle on a solid substratum with its anterior and downward. It retracts it's flagella and elongates considerably. A transverse septum separates the basal hyline holdfast from the apical green cell. The apical cell devides repeatedly to form a new filament.
3) Sexual reproduction:
It takes place with the help of male and female gamets these gamets known as antherozoids and eggs, are produced in antheridium and oogonium respectively.
a) Distribution of sex organs
i) Macrandrous species:
In these species antheridia are borne on the filament of normal size. If antheridia and oogonia are found on the same filament, are called macrandrous monocious. Other where antheridia and oogonia are borne on different filaments are known as macrandrous diocious.
ii) Nannandrous species:
In this species filaments bearing antheridia and oogonia show morphological distinction. The male filament which are much smaller than the female filaments are called dwarf male or Nannandrium. These species are always diocious.
b) Antheridium:
Macrandrous species form terminal or intercalary antheridia by the division of antheridial mother cell. This cell decide transverly into an upper small antheridial cell and a lower larger sister cell. Latter undergoes several transverse division to form a row of 20-40 flat uninucleate antheridia. As mentioned earlier, in nannandrous species antheridia develop in a specialised, small, 2-4 celled filament, known as dwarf male. The dwarf males develop from androspores, formed in androsporangia. Some nannandrous, where androsporangia and oogonia develop on the same filament, are called gynandrosporous, where two are borne on different filaments, are called idioandrosporous. Androspore, when liberated from androsporangium, is also enclosed in a thin vesicle. The vesicle soon dissolves and the androspore swims freely for sometimes, it is attracted chemotactically towards oogonial mother cell and attached to it. Then it germinate to form a dwarf male. The nucleus of each antherium decides to form two antherozoids.
c) Oogonium:
Development of oogonium is however, similar in both macrandrous and nannandrous species. Any vegetative cell with apical cap can act as oogonial mother cell. It devides transverly into an upper oogonium and a lower supporting cell. Initially, oogonium is slightly wider than the vegetative cells, but at maturity it swells and becomes globose and it's protoplast forms a single uninucleate non-motile egg. At maturity, a small pore develop on the oogonial wall just opposite to this slit a hyaline area is present in the egg which is known as receptive spot.
d) Fertilization:
The process of fertilization is similar in both macrandrous and nannandrous species. Antherozoids are attracted chemotactically towards the mature oogonium through the opening present on the oogonial wall. Antherozoids entire the oogonium through pore and fuses with egg to form diploid zygote.
e) Zygote:
Soon after fertilization the zygote separates from the oogonial wall and secretes 3 layered thick wall of its own.
f) Germination of zygote:
The zygote is liberated and settles down at the bottom of the pond. It undergoes a period of rest, in favourable conditions the diploid nucleus of the zygote devides by meiosis to form four haploid nuclei followed by protoplast division to form four segments. Each segment metamorphosis into a multiflagellate zoospore, they swim for some time and then attached to substratum. Germination of zoospore result in the formation of haploid filament.
