Describe the life cycle in Spirogyra.
Ans: The life cycle of spirogyra is completed in the following ways.
Vegetative reproduction; fragmentation, is caused is the death of some intercalary cell, or by the injury. Filaments break into two or several small pieces. Each fragment consists of one or few living cells.
Asexual reproduction: it takes place by the following methods:
1. Akinetes: In s. farlowii akinetes formation occurs during the unfavorable conditions. It is thick-walled resting spores. On return of favorable conditions akinetes germinate to produce a new filament.
2. Aplnospore: Aplanospore is thin-walled non-motile spores. They arise singly inside the cells. The protoplast of cells loses the water and contracts. It rounds off and secretes the thin wall around it form aplanospore. The parent cell when rupture it released down.
Sexual reproduction: it is taking place by conjugation. The donor cell is designed as a male gamete while the recipient cell is designated as a female gamete. In spirogyra, any vegetative cell produces the gamete and there are no morphological differences between male and female gametes. Thus morphologically the gametes are isogamous and physiologically anisogamy. Conjugation takes place by two methods:
1. Scalariform conjugation: It occurs between the two filaments of the cell. At the time of conjugation two filaments come in contact and lie parallel to each other. The filaments form tubular outgrowth or papillae from their opposite cells. The end walls of papillae are dissolved by the enzyme action to form a conjugation tube between the two cells. Due to the formation of several conjugation tubes the two filaments give a ladder-like appearance hence it is called scalariform conjugation. The protoplast of cells functions as gametes. The protoplast of one cell squeezes its way through the conjugation tube and passes into the cell of the filament, to form a zygospore.
2. Lateral conjugation:
Occur in between the cell of the same filament. It takes place in two ways:
a) Indirect lateral conjugation: in this method the two adjacent cells develop papillae on the lateral sides of the septum. The common wall is ruptured by the action of an enzyme. Both gametes fuse to form a zygospore.
b) Direct lateral conjugation:
It is a primitive type of conjugation. The lower surface behave as female and upper surface behaves as male in the same filament the male gametes produced boring organ, this conical process produced the pore in the middle of the septum. These two gametes fuse to form the zygospore.
The zygospore is made up of three layers wall around themselves and represent the diploid phase, by the decay of this layer of the female gametes and sink to the bottom of the water. During the germination, the zygospore divides mitotically, four haploid nuclei. Three of these haploid nuclei degenerate.
The zygospore form a single filament. On return of favorable conditions, the zygospore absorbs water and swells up. The outer two-layer burst and the inner layer to form the germ tube, it divides transversely to form the two-celled structure. The lower cell is colorless and functions as a rhizoid. The upper green cell divides repeatedly to form a new filament.