If the same phellogen functions from year to year, as in beech and oak, then the bark is smooth and consists only of tis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）。
The cells of the phellogen towards outside differentiate as phellem,and towards inside as phelloderm or secon- dary cort木栓形成层不断向内分裂分化成栓内层,此时可称次生皮层。