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

Options

A. Portion of conjunctive parenchyma present below the phloem bundle forms cells of vascular cambium
B. Portion of pericycle above the protoxylem becomes meristematic and forms vascular cambial cells
C. Both
D. Cambial initials between the xylem and phloem divide

Answer

To determine what happens first when secondary growth in thickness is initiated in a dicot root, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Secondary Growth**: Secondary growth refers to the increase in thickness of the plant, which occurs due to the activity of the vascular cambium. In dicots, this growth is significant as it contributes to the formation of wood and bark.

2. **Identifying the Structure**: In a dicot root, the vascular bundles consist of xylem and phloem. The cambium is the tissue responsible for secondary growth and is located between these two tissues.

3. **Role of Conjunctive Parenchyma**: The conjunctive parenchyma is the tissue located below the phloem. When secondary growth begins, the cells of this parenchyma can divide and contribute to the formation of the vascular cambium.

4. **Role of Pericycle**: The pericycle is a layer of tissue located just inside the endodermis and outside the vascular bundles. The cells above the protoxylem in the pericycle also become meristematic (capable of division) and contribute to the formation of the vascular cambium.

5. **Formation of Vascular Cambium**: Both the conjunctive parenchyma and the pericycle contribute to the formation of the vascular cambium. As these cells divide, they create a cambium ring that is essential for secondary growth.

6. **Conclusion**: Since both the conjunctive parenchyma and the pericycle contribute to the formation of the vascular cambium simultaneously, the correct answer to the question is that both processes occur first.

### Final Answer:
The correct answer is option 3: Both the conjunctive parenchyma below the phloem and the pericycle above the protoxylem form cells of vascular cambium.

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