Spirogyra reproduce sexually by

The correct option is conjugation.

Step-by-step Explanation:
1. Understanding Spirogyra: Spirogyra is a genus of filamentous green algae belonging to the order Zygnematales. It is named for the helical or spiral arrangement of its chloroplasts.
2. Modes of Reproduction in Spirogyra: Spirogyra can reproduce both vegetatively and sexually:
- Vegetative reproduction: Occurs primarily by fragmentation, where the filament simply breaks into smaller pieces, and each piece grows into a new filament.
- Asexual reproduction: Occurs under unfavorable conditions through the formation of specialized spores like akinetes or aplanospores.
- Sexual reproduction: Occurs through a process called conjugation.
3. What is Conjugation? Conjugation in Spirogyra involves the fusion of two non-flagellated (non-motile) morphologically similar but physiologically different gametes. There are two main types of conjugation:
- Scalariform conjugation: Two different filaments align side-by-side, forming conjugation tubes between adjacent cells. The contents of one cell (acting as the male gamete) pass through the tube into the other cell (acting as the female gamete) to form a zygospore.
- Lateral conjugation: Occurs between two adjacent cells of the same filament.
4. Evaluating Other Options:
- Fragmentation is a vegetative method of reproduction, not a sexual one.
- Somatogamy refers to the fusion of two somatic (vegetative) cells rather than specialized gametes, commonly seen in certain fungi (like Basidiomycetes), but not in Spirogyra.
- Akinetes formation is a method of surviving unfavorable conditions (asexual/perennation), not sexual reproduction.
Therefore, the sexual reproduction in Spirogyra takes place specifically by conjugation.