Fibres are elongated cell while sclereids are broad cell.

the Main difference between fibre and sclereids is the shape of the cell.

Phloem Fibres
Phloem fibres are a type of specialized plant cells found in the phloem tissue of vascular plants. They serve various functions in the plant's transportation system. Here are the key points that distinguish phloem fibres from phloem sclerenchyma:

Structure
- Phloem fibres are elongated cells that have a narrow lumen and thick cell walls. The cell walls are composed of cellulose, hemicellulose, and lignin, providing strength and rigidity.
- They are dead cells at maturity and lack protoplasts. This means they have no cytoplasm or nucleus, giving them a hollow appearance.
- The presence of pits in their cell walls allows for the exchange of materials between neighboring cells.

Function
- The primary function of phloem fibres is to provide mechanical support and strength to the plant. They aid in maintaining the stability of the vascular tissues.
- Phloem fibres also play a role in defense against herbivores and pathogens. Their tough and rigid nature acts as a physical barrier, protecting the plant from external damage.
- Additionally, they contribute to the transport of sugars, organic molecules, and other substances through the phloem tissue. Although they are not directly involved in the active transport process, they assist in maintaining the structural integrity of the phloem.

Phloem Sclerenchyma
Phloem sclerenchyma is another type of plant cell found in the phloem tissue. It differs from phloem fibres in several aspects:

Structure
- Phloem sclerenchyma cells are elongated, narrow, and have thick cell walls. These walls are heavily lignified, making them extremely rigid and durable.
- Like phloem fibres, phloem sclerenchyma cells are also dead at maturity and lack protoplasts. Their cell walls are composed of cellulose, hemicellulose, and lignin.

Function
- Phloem sclerenchyma cells primarily provide mechanical support to the plant. They strengthen the phloem tissue, preventing it from collapsing under pressure and maintaining its structural integrity.
- These cells also play a role in conducting water and nutrients through the phloem. They contribute to the movement of sap within the plant, facilitating the distribution of essential substances to various parts.

Summary
In summary, both phloem fibres and phloem sclerenchyma cells serve mechanical support functions in plants. While phloem fibres are elongated cells with a hollow appearance, phloem sclerenchyma cells are also elongated but have heavily lignified cell walls. Both cell types contribute to the transport of substances and the structural integrity of the phloem tissue.