Algea : Reproduction
It takes place by the following methods.
1. Vegetative Reproduction
In Spirogyra the sexual reproduction takes place by special gametes called aplanogametes and the process is aplanogamy. The motile gametes are always lacking. Aplanogammy takes place by conjugation, which may be scalariform or lateral. In each cell a single aplanogamete is produced which moves into the other cell through a conjugation tube in amoeboidal fashion. The species may be homothallic or heterothallic Scalariform conjugation is the process of reproduction of heterothallic species. Lateral conjugation, takes place in homothallic species. In scalariform conjugation the aplano gametes of two filaments opposite to one another, unite where as in lateral conjugation the aplanogametes of the two adjacent cells of the same filament unite.
This type of reproduction is found in majority of the species of Spirogyra. The filaments taking part in conjugation lie side by side. Very soon the out growths are given out from the lateral walls of the opposite cells of the filaments. The out growths of opposite cells touch each other very soon the wall of contact dissolves and a tubular passage is formed between the two opposite cells of the two filaments lying side by side. This tubular passage is called conjugation tube. Simultaneously the contents of the cells retract and aplanogametes are developed. A single aplanogamete is developed in each cell. The aplanogametes formed in the cells of one filament pass into the opposite cells of the other filament through conjugation tubes in amoeboid fashion. The plasmogamy is followed by karyogamy. The transferring aplanogametes are considered to be male gametes while the receiving aplanogametes arefemale gametes. Just after the fusion the walls are formed around the zygote and they are called zygospores. A single zygospore develops in each cell of the female filament. The wall of the female decays and the zygospores are set free in the water. Each zygospore germinate into a new plant after a resting period.
This type of reproduction occurs in homothallic species. Here the aplanogametes of the adjacent cells of the same filament Male gametangium Female gametangium unite. At the septum a tube like structure develops and through this opening the content of one cell passes into the other. Then both a manosomefes fuse to form a zygote. The empty cells are considered as male gametangia and the cells with zygotes are female gametangia. Very soon a thick wall is secreted around each zygote and dark coloured zygospores develop. They undergo a period of rest and germinate under favourable conditions.
Prior to germination, the diploid nucleus divides meiotically and, four haploid nuclei are formed. Three of the four haploid nuclei disintegrate and only one remains functional. The zygospore wall breaks and the germling comes out which soon develops into a new filament.
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