Introduction:
Spirogyra is a green filamentous freshwater algae that belongs to the division Chlorophyta. It is known for its spiral-shaped chloroplasts, which give it its name. Spirogyra reproduces both asexually and sexually. Sexual reproduction in Spirogyra is considered an advanced feature because it involves the presence of physiologically differentiated sex organs.

Explanation:
Sexual reproduction in Spirogyra involves the fusion of gametes, which are produced by the conjugation of two filaments. The process of sexual reproduction in Spirogyra can be explained as follows:

1. Conjugation Tubes Formation:
During sexual reproduction, two vegetative filaments of Spirogyra come close to each other and form a conjugation bridge or tube. The conjugation tube is a hollow cytoplasmic bridge that connects the two filaments.

2. Gamete Formation:
Within each filament, the cells undergo a process called gametogenesis, where certain cells differentiate into specialized structures called gametangia. The gametangia produce gametes, which are the sex cells involved in reproduction. In Spirogyra, the gametes are known as gametospores.

3. Physiologically Differentiated Sex Organs:
The gametangia in Spirogyra are physiologically differentiated into two types: the male gametangium or antheridium and the female gametangium or oogonium.
- Antheridia: The antheridia are the male reproductive structures that produce male gametes called antherozoids. They are elongated, club-shaped structures that contain numerous biflagellated antherozoids.
- Oogonia: The oogonia are the female reproductive structures that produce female gametes called oospheres. They are rounded structures that contain a single oosphere.

4. Fusion of Gametes:
Once the gametes are formed, the contents of the antheridium are released into the conjugation tube and travel towards the oogonium. The antherozoids swim through the conjugation tube using their flagella and reach the oogonium. One antherozoid fuses with the oosphere, resulting in the formation of a zygote.

Conclusion:
Sexual reproduction in Spirogyra is considered an advanced feature because it involves the presence of physiologically differentiated sex organs, namely antheridia and oogonia. These sex organs produce specialized gametes, antherozoids and oospheres, respectively. The fusion of these gametes leads to the formation of a zygote, which continues the life cycle of Spirogyra.