All the tissues, collectively, lying outside the vascular cambium in the stems and roots of plants showing secondary growth, i.e. the primary and secondary phloem, the cortex, and the periderm. The term is also used in a more restricted sense to mean the tissue arising to the outside of the phellogen, i.e. the *phellem
, when this is exposed by the sloughing off of the epidermis. The bark of different trees can be very distinctive and its characteristics are used to aid identification. In some species the same phellogen is active each year and a thick layer consisting solely of phellem is formed (e.g. oak, beech), but in most species a new phellogen arises annually in the cortex below: the bark thus consists of both phellem and dead cortex and is termed rhytidome. As the thickness of the bark increases the outer layers may either become fissured (e.g. elm) or be shed as scales (e.g. plane) or rings (e.g. birch).