Oedogonium: Vegetation and Reproduction | Algae | Botany

In this article we will discuss about the vegetative body and reproduction of oedogonium.

Vegetative Body of Oedogonium:

The thallus is is an unbranched filament consisting of a single row of cylindrical cells joined end on end. The filament shows distinction into base and apex. The basal cell becomes modified and forms a holdfast by means of which the thallus fixes itself to the substratum. The apical cell usually convex, but may be pointed.

The cell wall though ap­pears to be homogeneous, con­sists of three concentric por­tions:

(1) An innermost cellulose layer lying in contact with the protoplast,

(2) A me­dian pectose layer, and

(3) An outermost layer chiefly made up of chitin.

In the basal cell this outermost chitinous layer is absent. The transverse wall of each cell towards the distal end is provided with transverse sta­tions which constitute the apical cap. Thus, the intercalary cells also show distinction into base and apex.

The protoplast, in an adult cell, lies next to the cell wall con­taining a single nucleus. The nucleus is large, biscuit-shaped, with one or more prominent nucleoli and a distinct nuclear reticulum. It lies midway between the two poles of a cell and is located in the peripheral layer of cytoplasm. The chloroplast is very characteristic.

It is in the form of a hollow cylindrical net exten­ding from pole to pole and is lodged on the peripheral layer of cytoplasm, external to the nucleus. There are numerous pyrenoids at the crossings of the reticulum, each being surrounded by a plate of starch. The starch grains, as they are formed, may move away from the pyre­noids and accumulate in the connecting strands of the chloro­plast, thereby finally obscuring the nature of it. This is known as stroma starch.

The filament elon­gates by division of any cell but the basal one. During cell division, a transverse ring of thickening appears at the upper end of the cell just beneath the septum. The cell wall breaks across transversely at this point and the ring gradually extend to form a membrane across the rent. The mitotic nuclear division is followed by cytokinesis (transverse) and elongation of daughter cells to about the same length as the mother cell.

The lower one elongates until its upper end is at about the level of the ring and its lateral wall is the lateral wall of the mother cell. The upper one elongates meanwhile, and its lateral wall is formed by a stretching of the ring except for the persistent projecting portion of the ruptured mother cell wall, the apical cap. These cap cells usually divide repeatedly showing as many caps as there are divisions.

Reproduction in Oedogonium:

Oedogonium reproduces by vegetative, asexual and sexual me­thods.

When reproduction takes place vegetatively, it is chiefly by the accidental breaking of the filaments, and in submerged aquatic species it is by the process of fragmentation.

Most species of Oedogonium reproduce asexually by the pro­duction of multi-flagellate zoospores formed within zoosporangia. Any intercalary vegetative cell, rich in accumulated oil-reserve, may become a zoosporangium and form a zoospore. Du­ring its formation, the nucleus slightly recedes inwards, and a hyaline region appears in the cytoplasm between the cell wall and the nucleus.

Around the margin of this hyaline region numerous granules (blepharoplasts) appear, each of which probably gives rise to, flagellum. After the formation of the flagella, the cell wall breaks transversely at the region of the apical cap and the metamorphozed multi-fla­gellate protoplast escapes as a zoospore, being enclosed by a hyaline vesicle which gradually increases in size and finally dis­appears liberating the zoos­pore in the surrounding water.

The zoospore, with its crown of flagella around the base of the hyaline end, swims for some time, comes to rest on some substratum with the an­terior end downwards. The flagella are finally withdrawn and the protoplast slightly elongates, develops a holdfast and finally secretes a wall around it. This one-celled plant then elongates and by repeated cell divisions gives rise to a new filament.

Sometimes Oedogonium may form akinetes in chains of 10-40, within the vegeta­tive cells. The akinetes contain sufficient amount of reserve starch and red-coloured oil. Under favourable conditions an akinete germinates into a new plant.

Sexual reproduction in Oedogonium is of an oogamous type, in which a gametic union takes place by the fusion of a small motile spermatozoid with a large, non-motile egg. The sexual organs, antheridia and oogonia, may be produced on the filaments of normal size (macrandrous species), or the oogonia are borne on normal filament, the antheridia being produced on special dwarf male fi­lament or nannandrium (nannandrous species) consisting of a few cells.

Macrandrous species may be homothallic or heterothallic. Any vegetative cell excepting the basal one may act as an antheridium mother cell. The position of the antheridium may be terminal or intercalary. The antheridium mother cell divides into two daughter cells of unequal sizes, the short terminal ceil being transformed into an antheridium. The lower much longer sister cell may, in turn, act as an antheridium mother cell, and divides repeatedly in a similar manner forming a row of 8-40 antheridia.

The protoplast of each antheridium is metamorphozed into a single spermatozoid, or it divides into two daugh­ter protoplasts and forms two spermatozoids. The spermatozoids look like zoospores, but are much smaller in size and with fewer flagella. The mode of liberation of the spermatozoids is similar to that of zoospores, and there is the usual formation of a vesicle at the time of libera­tion.

Similarly, the terminal or any intercalary vegetative cell may act as an oogonium mo­ther cell. The oogonium mo­ther cell, as before, divides into two daughter cells of un­equal sizes, the shorter cell at the distal and forms the oogonium and is always provided with one or more caps at the upper end.

The lower larger sister cell is called the suffultory cell. This cell either remains as such or acts as another oogonium mother cell, which divides and re-divides forming two or more intercalary oogonia in a chain.

The oogonium swells up and becomes more or less spherical with a diameter greater than that of any vegetative cell. The protoplast of the oogonium is gradually metamorphozed into a uninucleate oosphere or ovum. As the oogonium approaches maturity, a small pore or transverse crack is formed on its wall towards the upper end.

Just before fertilization the nucleus of the ovum migrates from the centre towards the pore, where the ovum slightly contracts from the oogonial wall and forms a hyaline receptive spot just outside the nucleus.

Nannandrous species are heterothallic. The structure and mode of development of the oogonia are similar to those in macran­drous species, and these are borne on filaments of normal size. The dwarf male filaments bear­ing antheridia are produced as a result of germination of the spe­cial type of zoospores, called androspores, formed within androsporangia.

The mode of formation of androspo­rangia is similar to that of the antheridia. The protoplast of an androsporangium is meta­morphozed into only one androspore. The androspores, similar in appearance with the spermatozoids of ma­crandrous species, are liberated in the same way, each being sur­rounded by a vesicle when first liberated.

When the vesicle dis­appears, the androspore swims freely in water and comes in the neighbourhood of an oogonium or an oogonium mother cell. In the former case, the androspore attaches itself, with its hyaline end downwards, to the oogonial wall or to the wall of the suffultory cell and forms the dwarf male filament there. In the later case, when it affixes itself to the wall of the oogonium mother cell, it has been found to form the dwarf male filament always on the suffultory cell after the division and forma­tion of oogonium.

When an androspore affixes itself to the wall it develops a holdfast, surrounds itself with a delicate wall and forms a one-celled plant (germling). This cell acts as an antheri­dium mother cell and cuts off one or more antheridia at its apex. The protoplast of each antheridium divides into two parts and forms two spermatozoids.

Nannandrous species of Oedogonium may bear both oogonia and androsporangia on the same filament (gynandrosporous), or they are borne on different filaments of normal sizes (idioandrosporous).

In both macrandrous and nannandrous species, fertilization of the ovum takes place by the entry of a spermatozoid through the pore at the hyaline receptive spot. The zygote (oospore) slightly retracts from the oogonial wall and soon forms a wall of two or three concentric layers around it.

The middle layer may often show pits or reticulation. At maturity, the oospore become red-dish brown in colour due to the accumulation of oil. Finally, the oogonial wall disintegrates, the oospore falls to the bottom of the pond, where it undergoes a period of rest for a year or more. Prior to germination the diploid nucleus of the protoplast undergoes reduction division forming four haploid nuclei. Sub­sequently, the cytoplasm divides and collects around each of the four nuclei forming four zoospores.

At the time of germination the wall of the oospore bursts and zoospores escape being surrounded by the vesicle. Finally, the vesicle disappears, and the zoospores swim freely in water. Subsequent development of the zoospore and the formation of the vegetative filament take place in the usual way.

Some species may regularly form parthenospores (aboospores) developed from unfertilized eggs. The parthenospores completely fill up the entire cavity of the oogonium and possess walls like zygotes.