The Algae - Chlorophycae - Oedogoniales - Oedogonium

Phylum Thallophyta - The Algae - Chlorophycae - Oedogoniales - Oedogonium 
The Oedogoniales are Chlorophyceae in which both the zoospores and the antherozoids are characterized by having a whorl of many flagella at their anterior end. Sexual reproduction is always of an advanced type; the male organs, or antheridia, giving rise to antherozoids, and the female organs, or oogonia, each producing a single oosphere.
The vegetative cells are elongated and form a filament; growth is by division of the cells, and in this process there is a characteristic annular splitting of the cell wall prior to the formation of the new cell. The filaments may be simple or branched.
Oedogonium 
Included in this genus are a large number of fresh-water species which frequently occur in ponds and tanks but are uncommon in running water. The Alga is composed of a filament of rather elongated cells, and is usually attached at one end by a rhizoid cell which is often expanded as a terminal disc or holdfast. This cell contains less chlorophyll than the other cells of the filament and is probably unable to assimilate food material.
The structure of the vegetative cell is simple; it consists of a cellulose wall, bounding the inner surface of which is a layer of protoplasm. The centre of the cell is occupied by a large vacuole with cell sap. The chloroplast is a large cylindrical network, with several included pyrenoids formed from protein granules, around which starch grains are deposited. Embedded in the peripheral protoplasm of each cell is a single large nucleus.
Growth consists in the division of the cells of the filament. The apical cell does not divide, and in some species is elongated as a fine slender terminal hair. Only certain cells of the filament divide, and these are distinguished by the formation of an internal ring on the wall, near the upper end.
The cell divides transversely and then the cell wall splits at the level of the ring, which stretches out longitudinally to form a new portion of cell wall. Another ring then forms at the upper edge of this new length of wall. Only the cell which contains the ring divides again, arid it possesses overlapping caps at its upper end, each marking the position of a ring and equal in number to the times the cell has divided. The occurrence of these cap cells in the filaments makes Oedogonium easily recognizable.
Reproduction is both sexual and asexual, the latter occurring during the summer months under conditions favourable for growth, while the former, since it results in the production of resting spores, is suitable for tiding the Alga over periods unfavourable for growth, such as, for example, drought.
Asexual Reproduction in Oedoogonium
In asexual reproduction any cell of the filament may function. The protoplast is withdrawn from the cell wall and forms a mass in the centre of the cell. One area at the side of th.is mass becomes colourless
and round this numerous fine flagella are developed. The cell wall splits across and the protoplasmic mass passes out through the opening. On being liberated the zoospore slips out of the gelatinous membrane which originally surrounded it and becomes spherical. There is no true wall around the zoospore, for the whole mass is purely protoplasmic in structure. It swims by means of the flagella, possesses an eye spot and is sensitive to light in just the same way as we have described in the case of Ulothrix. After swimming for an hour or so it settles down on its colourless end, withdraws its flagella and secretes a cell wall. The free end grows out and is divided by a transverse wall, thus beginning a new filament. The colourless portion usually develops into the branched holdfast cells. 
Sexual Reproduction in Oedogonium
The sexual reproduction of Oedogonium is of an advanced and com­plicated type. The distribution of the sexes varies in the diHerent species. In some both sexes occur in the same filament, when it is said to be monoecious, while in others the male and female organs are borne on different filaments, and the plants are spoken of as dioecious. The male cells are formed by repeated transverse divisions of a vegetative cell, which in monoecious species is close to the female organ. These divisions continue until a row of about a dozen small cells are produced, each of which is termed an antheridium. The contents of each antheridium divide into two, and each portion becomes an antherozoid. These antherozoids resemble the zoospores, but are much smaller.
The female organ, or oogonium, is formed from a single vegetative cell which divides into two, the upper cell being much richer in protoplasm than the lower one. It has a large nucleus and forms the oogonium, while the lower cell is termed the supporting cell. The oogonium swells out to about twice its original diameter and becomes rounded or oval in outline. The cell contents are withdrawn from the wall and from a free, rounded, proto­plasmic body or oosphere. A small clear patch appears on one side, which is called the receptive spot. The wall of the oogonium now opens either by a split or by a thinning of the wall above the receptive spot, and is thereupon ready for the entry of the antherozoid.
In addition to the normal monoecious and dioecious types, certain dioecious forms of Oedogonium are known in which Cell caps the male filaments are very much smaller than the female. Such plants are spoken
of as dwarf male or nannandrous forms. Cells of the parent filament divide by repeated transverse division, and from each of the cells formed a ingle zoospore escapes. I t is intermediate in size between the asexual zoospores and the antherozoids and is termed an androspore. The androspores swim till they reach the female filaments, attaching themselves either to the oogonium itself or to a cell close to it. Here they germinate and produce a filament of two or three cells. With the exception of the lowest, which forms a holdfast, each becomes an antheridium liberating two antherozoids.
Fertilization is effected by the entrance of the antherozoid through the opening in the wall of the oogonium, after which it unites with the oosphere. After fertilization the oosphere secretes a thick wall and is now known as an oospore. The contents of the oospore undergo considerable change; the green colour disappears and is replaced by a brown or red pigment, while large quantities of oil appear in the protoplasm. Such a structure is regarded as a resting spore, and it may remain for a considerable time in this con­dition before germinating. It is thus an extremely suitable structure for tiding the Alga over a period of unfavourable conditions.
When conditions are favourable for its germination the cell wall bursts and the contents slip out surrounded by a thin membrane. It now normally divides by meiosis into four monoploid cells, and from each of these a flagellated zoospore is produced exactly resembling the asexual zoospores. These zoospores free themselves from the membrane and swim away, giving rise eventually to separate plants.
In Oedogonium, then, we have an example of a green Alga, with an advanced type of sexual reproduction as compared with Ulothrix. Its method of sexual reproduction is in some respects comparable with that of the higher plants, but it is peculiar in the production of nannandrous male filaments.
Despite the fact that it is itself highly evolved, it is doubtful if Oedogonium is related to any higher type of plant. It would appear rather that it is an end line, specialized along certain directions, but not along those which produced the higher forms of plant life.

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