Why xylem and phloem are called complex tissues? How are they different from one another?

Xylem and phloem are called complex tissues because they are made up of more than one type of cell working together as a unit. The xylem transports H2O and minerals upward from the roots to the leaves, and is composed of tracheid cells which are dead at maturity (no active nucleus, makes the tree stem hard). If plants don't have xylem, they will stay lower to the ground as it is responsible for making the tree grow upward. The ends of the xylem tubes are tapered (pointed), materials are passed along one cell at a time, and the tubes are narrow and continuous. The phloem transports sugars and proteins downward from the leaves to the stems and storage roots. The cells that make up the phloem are called sieve tubules and companion cells. The companion cells have the nucleus and the sieve tubules contain the cytoplasm.

Explanation:

My teacher helped us remember using the phrase "phloem down the river" because it sounds like 'flowing down the river' (to help remember that phloem transports downward and xylem transports upward.)