A surplus of glucose is transformed to starch and deposited for later user, mainly in the wood rays. The wood rays have a radial extent in the trunk and serves for storage of energy and transport back and forth between bark and xylem. It is vital to a tree, that it steadily can maintain reserves adjusted fit for its mass. If sizing to small, the tree will weaken, loose strength and ability to resist pathogens and damages. H. Sapwood I. Heartwood J. Pith W hen ageing the inner xylem of many species is altered.
Life ceases, the vessels are blocked and no longer active in the water transport.
In some species furthermore the cell walls are filled with dark chemicals that resist decay. This is called heartwood. To the tree the purpose of forming heartwood is to maintain a proper balance between mass and possible production of energy. At first energy is required for the altering. Then it becomes a static mass, still part of the supporting structure of the tree, but with no further needs of energy supply.