Vascular bundles in monocotyledons are considered closed because :

Options

A. There are no vessels with perforations
B. Xylem is surrounded all around by phloem
C. A bundle sheath surrounds each bundle
D. Cambium is absent

Answer

To understand why vascular bundles in monocotyledons are considered closed, let's break down the key points step by step:

### Step-by-Step Solution:

1. **Definition of Vascular Bundles**:
- Vascular bundles are structures that contain xylem and phloem, which are responsible for the transport of water, nutrients, and food in plants.

2. **Closed vs. Open Vascular Bundles**:
- Closed vascular bundles do not have cambium between the xylem and phloem, while open vascular bundles have cambium present, allowing for secondary growth.

3. **Characteristics of Monocotyledons**:
- In monocots (like grasses and wheat), the vascular bundles are arranged in a way that does not allow for secondary growth. This is primarily due to the absence of cambium.

4. **Absence of Cambium**:
- The cambium is a layer of tissue that can divide to form new xylem and phloem. In monocots, this cambium is absent, meaning that the xylem and phloem are directly adjacent to each other without any growth potential for increasing thickness.

5. **Surrounding of Xylem by Phloem**:
- In monocots, the xylem is surrounded all around by phloem, which is another characteristic of closed vascular bundles. This arrangement further supports the idea that there is no cambial layer for growth.

6. **Conclusion**:
- Therefore, the main reason vascular bundles in monocotyledons are considered closed is that cambium is absent, preventing any secondary growth and leading to the structure being classified as closed.

### Final Answer:
Vascular bundles in monocotyledons are considered closed because **cambium is absent**.

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