Describe the four elements of phloem.
The phloem is responsible for the transportation of food material from leaves to the other parts of the plant.
The four elements of the phloem are as follows:
1. Sieve tube elements:
- They are long tube-like structures arranged longitudinally.
- They are found in association with companion cells.
- The end walls are perforated in a sieve manner to from sieve plates.
- Each cell lacks a nucleus. It has a large vacuole and a peripheral cytoplasm.
2. Companion Cells:
- Companion cells are specialised parenchymatous cells.
- They help in maintaining the pressure gradient in the sieve tubes.
- Nuclei of the companion cells control the function the sieve tubes.
3. Phloem Parenchyma:
- he cells are cylindrical, elongated with tapering ends and dense cytoplasm and nucleus. The cells are connected to each other by plasmodesmatal connections.
- The cell wall is composed of cellulose.
- It also stores food and substances such as mucilage, latex and resin.
- It is absent in monocotyledons.
4. Phloem Fibres:
- They are also called bast fibres.
- They are made of sclerenchymatous cells.
- Phloem fibres are absent in the primary phloem but present in the secondary phloem.
- Fibres are elongated, unbranded and bear pointed apices.
- The cells walls are thick.
- They lose their protoplasm at maturity and become dead.
- The fibres of jute, hemp and flax are of commercial use.