Sapwood of trees is a living tissue in the xylem to perform certain physiological functions. Although the heartwood does not perform physiological functions, yet it has great influence on wood utilization. The mechanism of heartwood formation is one of the unsolved issues in the field of wood science. The objective of this paper was to review key research results on the physiological activities of sapwood in relation to heartwood formation. On these grounds, the formation and control of obligatorily colored heartwood were discussed. Since the importance of moisture content to ray cell physiology, understanding of how rays control moisture content in the sapwood transition zone may be able to provide theoretical bases for artificial control of heartwood formation.