Why xylem and phloem are called complex tissue?

Xylem and phloem are called complex tissues because each is made up of more than one type of cell that works together to perform a specific function in the plant. Unlike simple tissues (which consist of only one type of cell), complex tissues are heterogeneous and contain various cell types, each with specialized roles.

• Xylem consists of several types of cells, including tracheids, vessels, xylem fibers, and xylem parenchyma. These cell types work in unison to transport water and minerals from the roots to the rest of the plant and also provide mechanical support.
• Phloem is made up of sieve tube elements, companion cells, phloem parenchyma, and phloem fibers. These various cell types together facilitate the transport of food (mainly sugars) from the leaves to other parts of the plant.

The cells in xylem and phloem are structurally and functionally different, yet they coordinate to accomplish the tissues' overall function—transportation of substances throughout the plant. This complexity of structure and coordinated function is why xylem and phloem are termed complex tissues.