Explanation:
Spirogyra is a filamentous freshwater green alga that belongs to the division Chlorophyta. It is characterized by its spiral chloroplasts, which give it its name. The cell wall in Spirogyra is made up of several layers, including an innermost layer called the inner cell wall.

Composition of the Inner Cell Wall:
The inner cell wall in Spirogyra is primarily composed of cellulose. Cellulose is a complex carbohydrate made up of long chains of glucose molecules linked together. It is the main structural component of the cell wall in many plants and algae. Cellulose provides strength and rigidity to the cell wall, allowing it to maintain its shape and protect the cell from mechanical stress.

Function of the Inner Cell Wall:
The inner cell wall in Spirogyra serves several important functions:

1. Structural Support: The cellulose in the inner cell wall provides strength and rigidity to the cell, allowing it to maintain its shape and withstand external pressures.

2. Protection: The inner cell wall acts as a barrier, protecting the cell from mechanical stress, pathogens, and other harmful substances.

3. Cell-to-Cell Adhesion: The inner cell wall helps to hold adjacent cells together, forming a filamentous structure. This allows Spirogyra to form long, multicellular filaments that can float freely in water.

4. Permeability Control: The inner cell wall regulates the movement of substances into and out of the cell. It allows the cell to selectively transport molecules and ions, maintaining cellular homeostasis.

In conclusion, the inner cell wall in Spirogyra is primarily composed of cellulose, which provides strength, rigidity, and protection to the cell. It plays a crucial role in maintaining the structural integrity of the algal filament and regulating the movement of substances in and out of the cell.