Like all plants, this sapling has elongated through primary growth at root and shoot tips. As a woody dicot it will also increase in girth. This is accomplished by secondary growth involving the vascular cambium, which is meristematic tissue that lies between the xylem and phloem. In a young plant, vascular bundles, composed of xylem, phloem, and vascular cambium, are distinct from one another. As growth continues, the vascular cambium forms a continuous ring near the perimeter of the stem. As cambia cells divide, differentiating cells on the interior side become secondary xylem and those on the exterior become secondary phloem. Girth increases as secondary xylem accumulates. The old phloem cells are crushed as the cambia move outwards. Rings are visible in the wood. They are the result of the difference between the large spring wood cells and the small summer wood cells. Bark replaces epidermis on the exterior of the trunk as meristem just beneath the epidermis produces cork cells. These are filled with waterproof material and help protect the tree. The role of bark is quite important, since the active xylem and phloem lie close to the vascular cambium near the surface of the tree.