In stems, the primary xylem is called endarch, because -

Options

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

Answer

### Step-by-Step Solution:

1. **Understanding Primary Xylem**: The primary xylem is a type of vascular tissue in plants responsible for the transport of water and nutrients. It consists of two components: protoxylem and metaxylem.

2. **Defining Endarch Arrangement**: In the endarch arrangement, the protoxylem is located towards the center of the stem, while the metaxylem is situated towards the periphery (outer side) of the stem.

3. **Identifying the Characteristics**:
- **Protoxylem**: This is the first xylem to develop and is more flexible, allowing for growth.
- **Metaxylem**: This develops later and is more rigid, providing structural support.

4. **Comparing with Exarch Arrangement**: In contrast, the exarch arrangement is found in roots, where the protoxylem is located towards the periphery and the metaxylem is towards the center.

5. **Conclusion**: Therefore, in stems, the primary xylem is termed "endarch" because the protoxylem lies towards the center (pith) and the metaxylem lies towards the periphery.

### Final Answer:
In stems, the primary xylem is called endarch because the protoxylem lies towards the center and the metaxylem lies towards the periphery.

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