In roots, the primary xylem is called exarch. because -

Options

A. The protoxylem lies towards the centre and the metaxylem lies towards the periphery
B. The protoxylem lies towards the periphery and the metaxylem lies towards the centre
C. Both the protoxylem and the metaxylem lies towards the centre
D. The protoxylem is surrounded by the metaxylem.

Answer

To understand why the primary xylem in roots is called exarch, we can break down the explanation into several steps:

### Step-by-Step Solution:

1. **Understanding Xylem Structure**: Xylem is a type of vascular tissue responsible for the transport of water and nutrients in plants. It consists of two main types: protoxylem and metaxylem.

2. **Defining Protoxylem and Metaxylem**:
- **Protoxylem**: This is the first-formed xylem that develops early in the growth of the plant.
- **Metaxylem**: This is the later-formed xylem that develops after the protoxylem.

3. **Exarch vs. Endarch**:
- **Exarch**: In exarch xylem, the protoxylem is located towards the periphery (outer part) of the root, while the metaxylem is located towards the center (inner part).
- **Endarch**: In endarch xylem, the arrangement is reversed; the protoxylem is located towards the center, and the metaxylem is towards the periphery.

4. **Identifying the Correct Option**: Based on the definitions:
- Option A states that protoxylem lies towards the center and metaxylem lies towards the periphery (this describes endarch).
- Option B states that protoxylem lies towards the periphery and metaxylem lies towards the center (this describes exarch).
- Option C states both protoxylem and metaxylem lie towards the center (incorrect).
- Option D states that protoxylem is surrounded by metaxylem (this is true for exarch but does not specify their positions correctly).

5. **Conclusion**: Since in exarch xylem, the protoxylem is indeed located at the periphery and the metaxylem is at the center, the correct answer is **Option B**.

### Final Answer:
The primary xylem in roots is called exarch because the protoxylem lies towards the periphery and the metaxylem lies towards the center.

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