Primary and secondary growth
In woody plants, primary growth is followed by secondary growth, which allows the plant stem to increase in thickness or girth. Secondary vascular tissue is added as the plant grows, as well as a cork layer. The bark of a tree extends from the vascular cambium to the epidermis.
Source
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"OpenStax College, Stems. October 17, 2013."
http://cnx.org/content/m44702/latest/Figure_30_02_07.jpg
OpenStax CNX
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