AbstractThe outer bark that includes sequent periderms is referred to as rhytidome. The defense and physiological functions of rhytidome are maintained by the continuous formation of sequent periderms. To understand the mechanisms of rhytidome growth, we examined the development of sequent periderms and the corresponding changes in the cellular contents of phloem parenchyma cells in Cryptomeria japonica. New layers of rhytidome were formed in the studied trees during the two-year course of the study. Our records showed that a new layer of periderm forms annually, and therefore, rhytidome development in C. japonica can be studied by sequential sample collection in any given year. Formation of new periderm and initiation of nuclei disappearance in phloem parenchyma in the outer layers of the developing outer bark occurred simultaneously. The early disappearance of nuclei indicates that some parenchyma cells might have been in a stage of preparation for cell death before the formation of new periderm. Four developmental stages of annual rhytidome growth were identified by structural and physiological changes of the outer layers of phloem parenchyma and the growth of the new periderm.