As secondary growth proceeds, in a dicot stem, the thickness of

Options

A. heart wood increases
B. sap-wood increases
C. both increase
D. both decrease

Answer

To answer the question about what happens to the thickness of a dicot stem as secondary growth proceeds, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Secondary Growth**: Secondary growth refers to the increase in thickness of stems and roots in plants, primarily occurring in dicots and gymnosperms. This growth is facilitated by the activity of the vascular cambium and cork cambium.

**Hint**: Remember that secondary growth is different from primary growth, which increases the length of the plant.

2. **Role of Vascular Cambium**: The vascular cambium is a layer of meristematic tissue that produces secondary xylem (wood) and secondary phloem. As the cambium divides, it adds new layers of these tissues, which contribute to the overall thickness of the stem.

**Hint**: Think about how the vascular cambium contributes to the structure of the plant.

3. **Formation of Heartwood and Sapwood**: As secondary growth occurs, the inner layers of secondary xylem become heartwood, while the outer layers remain functional sapwood. The heartwood is typically denser and contributes to the overall strength of the stem.

**Hint**: Distinguish between heartwood and sapwood in terms of their functions and locations in the stem.

4. **Impact on Thickness**: As secondary growth proceeds, the thickness of the stem increases due to the accumulation of new layers of xylem. The heartwood thickness increases as sapwood is converted into heartwood over time.

**Hint**: Consider how the transformation of sapwood into heartwood affects the overall structure of the stem.

5. **Evaluating the Options**:
- Option A: "Thickness increases" - This is correct because secondary growth leads to an increase in stem thickness.
- Option B: "Sapwood increases" - This is incorrect; sapwood does not increase in volume but is converted into heartwood.
- Option C: "Both increase" - This is incorrect for the same reason as option B.
- Option D: "Both decrease" - This is also incorrect.

**Hint**: Analyze each option carefully to determine which statements align with the process of secondary growth.

### Final Answer:
The correct answer is **Option A: The thickness of the dicot stem increases** as secondary growth proceeds.