To understand the pathway used by Phytophthora cinnamomi Rands to penetrate the bark of jarrah, the present study describes unwounded periderm and rhytidome development. Periderm formation is described from its initiation in 4-week-old seedlings to the formation of rhytidome in saplings. Periderm in young seedlings consists of a single type of phellem, namely thin-walled suberised cells. In older seedlings where multiple layers of periderm have formed, layers of thick-walled lignified phellem cells in compacted bands alternate with thin-walled suberised cells. Rhytidome formation in older lignotuberous seedlings and in sapling jarrah occurs through the isolation of secondary phloem by periderm. The rhytidome consists of expanded and partially disintegrated secondary phloem tissue sandwiched between layers of phellem cells. Localised periderm formation beneath stomata results in the formation of lenticels, which are ephemeral features. Superficial periderms occur at sites of leaf and shoot abscission, and of lateral shoot emergence. Concealed axillary shoots lack cuticle on emergence. As the trees age, the internal production of lignified and suberised periderm and rhytidome results in an impenetrable barrier to invasion by P. cinnamomi. However, external sites including lenticels and leaf and shoot abscission and emergence areas, all provide points of ingress in unwounded stems.