After the cambium of the initial periderm begins to produce cork cells and phelloderm, new phellogens arise successively deeper in the axis, always from living cells, e.g., cortex, phloem, or phelloderm. In most species, the second and subsequent phellogens do not extend completely around the axis, but instead occupy only a portion of the circumference. This results in a set of successive periderms, arc- or shell-shaped segments that entrap pockets of living or dead cells. So forms the outer bark as a complex of these overlapping periderms. In former years, the outer bark was also called the rhytidome, but this term is now hardly ever used. Outer layers of living cells, entrapped by the segments of periderms, are blocked from nutrients and therefore die. The succession of partially overlapping periderms may mature as a patchwork design separated by weak zones, where the cells have thinner walls abutting thicker walls of the cork cells.
Some species of woody plants form deeper successive periderms that are concentric, or nearly so, around the axis. These stems thereby can produce a series of cylindrical periderms, one inside another.