Assertion: The ground tissue system forms the main bulk of the plant It is divided into three zones - cortex pericycle and pith
Reason: In leaves, the ground tissue consists of thin walled chloroplast containing cells and is called mesophyll

Explanation

To analyze the question, we will break it down into two parts: the assertion and the reason. We will evaluate the truth of each statement and their relationship.

### Step-by-Step Solution:

1. **Evaluate the Assertion**:
- The assertion states that "The ground tissue system forms the main bulk of the plant. It is divided into three zones - cortex, pericycle, and pith."
- The ground tissue system indeed forms a significant part of the plant structure. It is composed of three main parts: the cortex, pericycle, and pith.
- Therefore, the assertion is **True**.

2. **Evaluate the Reason**:
- The reason states that "In leaves, the ground tissue consists of thin-walled chloroplast-containing cells and is called mesophyll."
- In leaves, the ground tissue is primarily represented by mesophyll, which contains chloroplasts and is involved in photosynthesis.
- Thus, the reason is also **True**.

3. **Determine the Relationship**:
- The assertion discusses the ground tissue system in general, while the reason specifically addresses the ground tissue in leaves (mesophyll).
- The reason does not explain the assertion adequately, as it does not relate to the division of ground tissue into cortex, pericycle, and pith.
- Therefore, while both statements are true, the reason is **not the correct explanation** of the assertion.

### Final Conclusion:
- Both the assertion and reason are true, but the reason does not explain the assertion.