大约半个月后，在靠近表面的愈伤组织中出现木栓形成层。
And about a half month phellogen will begin to appear in the callus near the surface.
周皮发生较迟，其木栓形成层由紧靠内皮层的皮层细胞产生。
The formation of periderm is relatively late and cork cambium is come from the cortex cells which arenear endodermis.
由于木栓形成层活动的规律不同，所以出现有各种各样的树皮。
Active rule of cork cambium is different in different trees, so form the many types of bark.
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