Anatomically fairly old dicotyledonous root is distinguished from the dicotyledonous stem by

Options

A. Prsence of cortx
B. Position of protoxylem
C. Absence of secondary xylem
D. Absence of secondary phloem

Answer

To solve the question regarding the anatomical distinction between a fairly old dicotyledonous root and a dicotyledonous stem, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Structure of Dicotyledonous Roots and Stems**:
- Dicotyledonous plants have distinct anatomical features in their roots and stems. The root typically has a different arrangement of vascular tissues compared to the stem.

2. **Identify Key Features of Dicotyledonous Roots**:
- In the root, the vascular tissue is organized in a way that the protoxylem (the first xylem to develop) is located near the periphery of the vascular cylinder. The metaxylem (the later-developed xylem) is found towards the center.

3. **Identify Key Features of Dicotyledonous Stems**:
- In contrast, in the dicotyledonous stem, the protoxylem is located near the center of the vascular bundle, while the metaxylem is positioned away from the center.

4. **Compare the Position of Protoxylem**:
- The primary distinguishing feature between the root and stem is the position of the protoxylem. In roots, it is peripheral, while in stems, it is central.

5. **Conclusion**:
- Based on the anatomical differences, the correct answer to the question is that the dicotyledonous root is distinguished from the dicotyledonous stem by the position of the protoxylem.

### Final Answer:
The anatomically fairly old dicotyledonous root is distinguished from the dicotyledonous stem by **the position of protoxylem** (Option B).

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