When secondary growth in girth is initiated in dicot root, which one of the following happens first?

Options

A. Primary medullary ray cells become meristematic
B. The outer parenchymatous pericycle layer divides
C. Parenchymatous cells below phloem and between xylem and phloem become meristematic
D. Vascular cambium divides

Answer

To solve the question regarding the initiation of secondary growth in girth in dicot roots, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Secondary Growth**: Secondary growth refers to the increase in girth or thickness of the plant, which occurs due to the activity of certain meristematic tissues.

2. **Identifying the Location of Growth**: In dicot roots, secondary growth primarily occurs in the vascular region, specifically between the xylem and phloem.

3. **Role of Parenchymatous Cells**: The parenchymatous cells located between the xylem and phloem, as well as those beneath the phloem, play a crucial role in initiating secondary growth.

4. **Meristematic Activity**: During the initiation of secondary growth, these parenchymatous cells become meristematic, meaning they start to divide and contribute to the formation of new tissues.

5. **Evaluating the Options**:
- Option 1: Primary medullary rays become meristematic - Incorrect.
- Option 2: Outer parenchymatous pericycle layer divides - Incorrect.
- Option 3: Parenchymatous cells below phloem and between xylem and phloem become meristematic - Correct.
- Option 4: Vascular cambium divides - Incorrect.

6. **Conclusion**: The first event that occurs when secondary growth in girth is initiated in dicot roots is the transformation of parenchymatous cells below the phloem and between the xylem and phloem into meristematic cells.

### Final Answer:
The correct answer is: **Parenchymatous cells below phloem and between xylem and phloem become meristematic.**

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