How many types of cells constitute phloem and what are they?

Phloem consists of four main types of cells:

1. Sieve tube elements: The primary conducting cells that transport sugars and organic nutrients. They are elongated, living cells arranged end-to-end with perforated end walls called sieve plates.
2. Companion cells: Specialized parenchyma cells closely associated with sieve tube elements, providing metabolic support and controlling their function. They have dense cytoplasm and numerous mitochondria.
3. Phloem parenchyma: Living storage cells that store food materials and assist in lateral transport of nutrients.
4. Phloem fibers: Sclerenchyma cells providing mechanical support to the phloem tissue. They have thick, lignified walls.

Together, these cells work as a coordinated system to distribute photosynthetic products throughout the plant efficiently.