Kay Allen, age 16, of Rock Island, I11., for her question:

WHAT IS XYLEM?

Xylem is the woody tissue found in higher plants that conducts water and salts throughout the plant and provides it with mechanical support. In leaves, flowers and young stems, xylem is present in conjunction with phleom (the food conducting tissue of a plant) in the form of conducting strands called vascular bundles.

Here's how you pronounce xylem: zeye lem.

In roots there is a central core of xylem. Xylem that derives from the shoot and root growing points is called primary xylem.

Xylem may contain three types of elongated cells: tracheids, vessel elements and fibers. At maturity, when functioning in conduction, all of these cells are dead.

Because sequences in the specialization of all these tissue elements can be observed quite clearly, the study of xylem provides important clues to the evolutionary pathways of higher plants.