Have you ever seen a tree that looks perfect on the outside, but when you cut it down, it is completely hollow on the inside? A tree like that is blighted, or diseased. Without its core, it cannot survive. The inside, where the sapwood is, provides the nourishment for the tree to continue growing strong and healthy. If it is hollow inside, a tree has no way to stay strong and healthy. No matter how good it looks on the outside, if it is rotten inside, it has no stability and no lasting life.