Another diagram on the cross section of the stem.

xy·lem : A tissue in vascular plants that carries water and dissolved minerals up from the roots through the stem to the leaves and provides support for the softer tissues. Xylem consists of various elongated cells that function as tubes. In a tree trunk, the innermost part of the wood is dead but structurally strong xylem, while the outer part consists of living xylem, and beyond it, layers of cambium and phloem. See more at cambium, photosynthesis.