In woody dicot stems, a layer of vascular cambium remains alive through the plants first winter. However, this layer does not become active until the following spring. At the surface of this layer, secondary xylem are formed. On the surface of the outside faces of the stem, the secondary phloem cells are formed. The primary xylem and phloem were produced in the first year and remain there. Each year a new xylem and phloem as well as a vascular layer are produced. These are the cells which make up the rings of the stem.
During
every growing season the new ring of xylem tissue combined with the old
xylem tissue becaomes the ring of trees. Xylem is called sapwood
as long as it conducts water. After some time, the older xylem tissue
becomes clogged with tar and resin, causing it not to be able to conduct
water. It's new purpose is to give support to the tree.
When new phloem cells are formed they are added to the inner surface of the previous years phloem cells. The phloem never becomes as thick as the xylem layer. The phloem cells have thinner walls than the xylem cells, and are crushed as the stem grows thicker. Phloem tissue forms the inner half of the tree bark where it carries sugars and other products of photosynthesis from the leaves to the other plant parts.