Q. A disc-shaped thickening present on the pit membrane is

Options

A. a) Torus
B. b) Callus
C. c) Tylosis
D. d) Stoma

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about a specific structure found on the pit membrane that is described as a disc-shaped thickening.

2. **Identifying the Context**: The term "pit membrane" is associated with plant anatomy, particularly in gymnosperms. It is important to recognize that this membrane plays a role in water and nutrient transport between cells.

3. **Analyzing the Options**:
- **Option A: Torus** - This is a thickening found on the pit membrane.
- **Option B: Callus** - This refers to a mass of tissue that forms at the site of a wound.
- **Option C: Tilosis** - This involves the formation of a plug in the vessel elements.
- **Option D: Stoma** - This is a pore used for gas exchange in plants.

4. **Identifying the Correct Answer**: The disc-shaped thickening on the pit membrane is specifically known as the "torus." The torus is made up of primary cell wall material and is larger in diameter compared to other structures.

5. **Explaining the Structure**: The torus is surrounded by a structure called the "Margo." Together, the torus and margo function as a barrier, regulating the movement of substances between adjacent cells.

6. **Final Conclusion**: Based on the analysis, the correct answer to the question is **Option A: Torus**.