As the stem continues to increase in girth due to the activity of vascular cambium, the outer cortical and epidermis layers get broken and need to be replaced by another meristematic tissue called cork cambium or phellogen.
The outer cells differentiate into cork or phellem while the inner cells differentiate into secondary cortex or phelloderm.
Phellogen, phellem, and phelloderm are collectively known as periderm.
Bark refers to all tissues exterior to the vascular cambium, therefore including secondary phloem. Example- periderm and secondary phloem.
Bark that is formed early in the season is called early or soft bark and towards the end of the season late or hard bark is formed.
The parenchymatous cells soon rupture the epidermis, forming a lens-shaped opening called lenticels.