Heartwood or durame is the inner portion of the woody stem, containing clogged secondary xylem. Heartwood formation is genetically programmed. With growth, the stem increases its diameter, forming new xylem and phloem. Therefore, the most centered and ancient xylem becomes useless. Then, that part of the xylem becomes a part of an essential function for the tree. For this, the cells in the center of the trunk die and chemical compounds accumulate in them. Some of these compounds are resins, terpenes, and phenols.