Phloem is composed of several specialized cell types, each playing a crucial role in its function. The primary components include:
1. Sieve Elements: These are the main conductive cells in the phloem. Sieve elements are elongated cells that form a continuous network for the transport of nutrients. They lack a nucleus and other organelles at maturity, which helps reduce resistance to flow.
2. Companion Cells: These cells are closely associated with sieve elements and play a key role in loading and unloading sugars into the sieve tubes. Companion cells contain a dense cytoplasm and numerous mitochondria, reflecting their active metabolic role.
3. Phloem Parenchyma: These cells are involved in the storage and lateral transport of nutrients. They are living cells with a nucleus and are capable of division, thus contributing to the repair and maintenance of the phloem tissue.
4. Phloem Fibers: These are elongated, lignified cells that provide structural support to the phloem. They are typically dead at maturity and have thick cell walls.
