Why is phloem an alive tissue and xylem dead?
Xylem tissue conducts water and mineral nutrients from the soil upward in plant roots and stems. It is composed of elongate cells with pointed ends called tracheids, and shorter, wider cells called vessel elements. The walls of these cells are heavily lignified, with openings in the walls called pits. Tracheids and vessels become hollow, water-conducting pipelines after the cells are dead and their contents (protoplasm) has disintegrated. The xylem of flowering plants also contains numerous fibers, elongate cells with tapering ends and very thick walls. This is especially true of the “ironwoods” with wood that actually sinks in water. Phloem tissue conducts carbohydrates manufactured in the leaves downward in plant stems. It is composed of sieve tubes (sieve tube elements) and companion cells. The perforated end wall of a sieve tube is called a sieve plate. Thick-walled fiber cells are also associated with phloem tissue. As phloem is associated with energy transport, hence it is alive.
