Walnut bark. Cross section through the trunk of a walnut tree (Juglans regia) revealing the pattern in its bark (dark brown). This pattern is known as a rhytidome. The rhytidome is composed of alternate layers of cork cells and dead cortical cells. Cork cells are produced in the stem by a layer of meristemetic cells (phellogen). Trees usually have one layer of phellogen, but in walnut there are several, which causes the pattern seen here.