The protective layer of mostly dead cells that covers the outside of woody stems and roots. It includes the living and dead tissues external to the vascular cambium, including the secondary phloem and periderm. The term can be used more specifically to describe the periderm together with other tissues isolated by the activity of the cork cambium. In some species, such as birch, there is one persistent cork cambium but in the older stems of certain other species a second cork cambium becomes active beneath the periderm and further periderm layers are formed every few years. The result is a composite tissue called rhytidome, composed of cork, dead cortex, and dead phloem cells.