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CELL STRUCTURE AND REPRODUCTION IN SPIROGYRA

It is one of the commonest and large geniuses with about 300 species growing as free floating masses in stagnant or slowly moving waters (streams). Some species, e.g., Spirogyra adnata are attached to the substratum through a basal rhizohidal cell. The masses of plants are slimy to touch because the filaments are surrounded by pectic sheaths which when come in contact water forms mucilage, therefore it is known as water-silk or pond-scum. The spirogyra grows abundantly in spring.
Plant Body_________Structure
Thallus: the thallus is filamentous. The filaments are green, simple, unbranched and consist of an indefinite number of cylindrical cells placed end to end in a single row. In free floating species there is no distinction between the basal and distal ends, whereas in attached forms the lowermost cell develops a rhizoidal outgrowth called hold fast which helps in attachment.
Cell Structure: in most species the cells are cylindrical and generally more in length than in breath. The protoplast is surrounded by a two layered cell wall. The outer layer is pectic and forms slimy mucilaginous sheath around the cell after dissolving in water and the inner layer is cellulosic. The cross walls or septa are generally plane, but in some species these are in the form of a biconvex lens, while in others H-pieces are present in the cell walls near the septa. The protoplast is bounded by plasma membrane. A large central vacuole occupies the center of the cell. It is filled with cell sap rich in tannins. The cytoplasm is peripheral in position and is in the form of a thin layer closely appressed to the plasma membrane.  A single large nucleus is suspended in the center of the cell with the help of cytoplasmic strands.
The chloroplasts are characteristics and posses either a smooth or a serrate margin and are arranged I a characteristics parietal spiral band. They are arranged in counter clockwise manner. Many small rounded rounded pyrenoids are present in each chloroplast. Electron microscopic studies show that mitochondria, endoplasmic recticulum, dictyosomes and other cytoplasmic inclusions are also present in the cell. The number of chromosomes vary from species to species from 5-24.
The growth of filament can take place by cell division of any cell of the filament except the hold fast in the attached species.

Life cycle__Reproduction:  the reproduction in Spirogyra is vegetative and asexual mostly. However. Some species exhibit asexual reproduction through rarely.
Vegetative Reproduction:  the vegetative reproduction is fragmentation is fragmentation, i. e., and breakdown of filament into fragments due to mechanical stress or dissolution of septum between the cells.
Asexual Reproduction:  the asexual reproduction, where present, is by formation of thick walled, non motile aplanospores; or if a gamete fails to fertilize, it develops a thick wall to become azygospore which give rise to a new filament.
Sexual Reproduction: the sexual reproduction occurs when the conditions are favorable for growth. It is primitive type of isogamous sexual reproduction called conjugation.
Mechanism of Conjugation: during the process two filaments come near each other and become glued together by their mucilage. Paillate (finger-like) outgrowths arise from the lateral walls of the cells, the conjugation tube. The protoplast in each cell rounds off due to loss of water to form gametes. The gametes are isogametes, however one gamete is active and move from one filament to other. It is called male gamete. The other gamete is passive and remains in the cell. It is called female gamete. Therefore, Spirogyra exhibit physiological anisogamy. All the gametes in a conjugating filament may be either male or female. The male gamete moves towards the female gamete through conjugation tube and the protoplast and nuclei of both the gametes unite to form a diploid zygote which secretes a thick wall to become a zygospore. After a period of rest the zygospore germinates into a new filament by cell division. Low nitrogen supply and high light intensity usually stimulate sexual reproduction.
The conjugation may be scalariform conjugation or lateral conjugation:
        i.            Scalariform Conjugation: It is the most common type of conjugation and occurs between opposite cells of the neighbouring filaments a described above.

      ii.            Lateral Conjugation: It is rare and occurs between the adjacent cells of the same filament. The gametes formed in the adjacent cells of the same filament act as male and female gametes. Therefore, at the end of the process one cell is empty while the other has zygospore in it.
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