Which elements of the phloem become dead at maturity ?

Options

A. Sieve tube elements
B. Companion cells
C. Sieve cells
D. Phloem fibres

Answer

To answer the question "Which elements of the phloem become dead at maturity?", we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Components of Phloem**:
- Phloem is composed of several types of cells, including sieve tube elements, companion cells, phloem fibers, and phloem parenchyma.

2. **Understand the Function of Each Component**:
- **Sieve Tube Elements**: These are responsible for the transport of nutrients, particularly sugars. They remain alive at maturity.
- **Companion Cells**: These cells assist sieve tube elements in their function and are also alive at maturity.
- **Phloem Fibers**: These provide structural support and are known to become dead at maturity.
- **Phloem Parenchyma**: These are living cells involved in storage and transport.

3. **Focus on Phloem Fibers**:
- Among the components listed, phloem fibers are the ones that become dead at maturity. They lose their protoplasts and have thickened secondary walls due to lignin deposition.

4. **Explain Why Phloem Fibers Are Dead**:
- The deposition of lignin in the cell walls of phloem fibers leads to the loss of protoplasts, making these cells non-functional in terms of metabolic activity. However, they serve an important role in providing mechanical support to the plant.

5. **Conclude with the Correct Answer**:
- Therefore, the correct answer to the question is that **phloem fibers** are the elements of phloem that become dead at maturity.

### Final Answer:
**Phloem fibers become dead at maturity.**

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