This publication is the final one in a series on tree decay developed in cooperation with Harold G. Marx, Research Application Staff Assistant, U.S. Department of Agriculture, Forest Service, Washington, D.C. The purpose of this publication is to clarify further the tree decay concept that expands the classical concept to include the orderly response of the tree to wounding and infection-compartmentalization-and the orderly infection of wounds by many microorganisms-successions. The heartrot concept must be abandoned because it deals only with decay-causing fungi and types of decayed wood. It describes disordered wood and events tht occurred in the past. The expanded decay concept emphasizes the order of a compartmented tree, the order of compartmentalization, and the order of successions. Regulation of discoloration and decay depends on understanding compartmentalization and successions.