In ulothrix or spirogyra, reduction division (meiosis) occurs at the time of zygospore germination. it takes place when (+)and (-) plants /filaments results in the formation of diploid zygote (2x)

Introduction:
Ulothrix and Spirogyra are filamentous green algae belonging to the division Chlorophyta. They reproduce both vegetatively and sexually. Reduction division, or meiosis, is a type of cell division that results in the formation of gametes with half the number of chromosomes found in the parent cell. In Ulothrix and Spirogyra, meiosis occurs at a specific stage of their life cycle.

Explanation:
Meiosis occurs during the life cycle of Ulothrix and Spirogyra at the time of zygospore germination.

Life cycle of Ulothrix and Spirogyra:
1. Vegetative Reproduction:
- Vegetative reproduction in Ulothrix and Spirogyra involves the fragmentation of filaments into smaller pieces, which can grow into new individuals.
- During vegetative reproduction, meiosis does not occur.

2. Gamete Formation:
- Both Ulothrix and Spirogyra reproduce sexually by the formation of gametes.
- Gametes are haploid cells that fuse during fertilization to form a diploid zygote.
- Meiosis occurs during the formation of gametes, where the diploid cells undergo two rounds of division to produce four haploid cells.
- These haploid cells serve as the gametes, which can fuse during fertilization.

3. Zygospore Germination:
- After fertilization, the zygote develops into a thick-walled structure called a zygospore.
- The zygospore undergoes a period of dormancy before germination.
- During zygospore germination, meiosis occurs.
- The zygospore undergoes meiosis to produce four haploid cells called zoospores.

4. Zoospore Formation:
- Zoospores are flagellated cells that are capable of independent movement.
- These zoospores are released from the zygospore and can swim in water.
- Eventually, the zoospores come to rest and grow into new filamentous individuals.

Conclusion:
In Ulothrix and Spirogyra, reduction division or meiosis occurs at the time of zygospore germination. Meiosis is essential for the production of haploid cells, such as gametes and zoospores, which are involved in sexual reproduction and the formation of new individuals.