If the same phellogen functions from year to year, as in beech and oak, then the bark is smooth and consists only of tissue external to the phloem, mainly cork.

• 如在榉木或橡木中，相同的木栓形成层年复一年的活动，那么树皮是光滑的，并且仅形成韧皮部的外方组织，主要是木栓（软木）。