Spirogyra is a freshwater alga found freely floating on the surface of the stagnant pools, ponds and ditches, etc. It is commonly known as water silk or pond silk.
Structure- Spirogyra is a unbranched filament. All the cells are alike and arranged in a single row. The cells are cylindrical, longer broad. The cell wall is two layered; the outer layer is of pectic substance and the inner of cellulose. Inside the cell, cytoplasm is present which surrounds a large central vacuole. There is a large single nucleus present in the center of the cell. The nucleus is held by cytoplasmic strands. The most characteristics feature of the cell is the ribbon or starp-shaped choloroplasts spirally arranged. The number of choloropast varies from 1 to 16. There are many pyrenoids are present on the choloroplast.
Reproduction: Spirogyra reproduces both asexually and sexually. Asexual reproduction is by means of spore productions. There is also vegetative reproduction in some species.
(1) Vegitative Reproduction:
it takes place due to fragmentation. The filament may break due to mechanical injury or due to dissoloution of middle lamella or gelatinization of cells.
(2) Asexual Reproduction:
It takes place by menas of akinetes, aplanospores and azygospores. These are formed due to the contraction of protoplasm and formation of new cellulose walls around it. These spores are formed under unfavourable conditions. During favourable conditions, they germinate to form the new filament.
(3) Sexual Reproduction:
Sexual reproduction in Spirogyra takes place by conjugation. It is isogamous type. Fusion of these non-flagllate gametes is resulted in zygospore. Two methods of conjugation have been found in Spirogyra i.e. (1) Lateral conjugation and (2) Scalariform conjugation. These are discussed below.
(i) Lateral conjugation:
It takes place between two cells of the same filament. Such species are, therefore, called hmothallic. Lateral conjugation agin is of three types:
(a) Indirect Lateral conjugation (b) Terminal conjugation (c) Direct Lateral conjugation.
(a) Indirect Lateral conjugation:
In this process there is a tube-like outgrowth in-between two adjacent cells close to the common cross walls. These outgrowths extend laterally ultimately forming conjugation tube. The contracted protoplasm of one cell moves to the adjacent cell and fuses to form the zygospore. The zygospore is diploid in nature.
(b) Terminal conjugation:
In this method, the so-called male gamete enters the female gamete by perforating the spetum of conjugation tube. The conjugation tubes are produced on either side of the septum of the two conjugating cells.
(c) Direct Lateral conjugation:
In this case, the filament is attached to the substratum by its basal cell. Lateral conjugation takes place between two cells placed immediately next to the basal cell. One cell swells known as the female cell and the other cell becomes conical known as male cell. The male gamete passes through the septum piercing it. The nuclei are fused forming the diploid zygospore.
(ii) Scalariform conjugation:
The scalariform conjugation takes place between the cells takes place between the cells of two different filaments, therefore, the speceies are heterothallic. In this case, two physiologically different filaments lie parallel to each other. Then two outgrowths arise which are called conjugation tube. Further growth of the conjugation tube pushes the filaments apart.
Later the conjugation tube dissolves forming a passage. Simultaneously the protoplasm of the conjugated cells contracts and forms gametes. The male gamete moves through the conjugation tube into the female cell of another filament. Fusion of two nuclei results in formation of diploid zygospore. After the formation of zygospore the cell of male filament becomes empty while the cells of the female filament are filled with zygospore.
Zygospore - The mature zygospore has three layered walls. These zygospores are liberated by the death and decay of the female cell wall. Zygospore germinates during favourable condition. Each zygospore nucleus divides meiotically to produce four haploid of which three degenerate and only one remains functional. On germination, the zygospore gives rise to a single haploid filament. In this way, the life cycle is completed.