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Fothergilla gardenii Witch Alder
Fothergilla gardenii (Witch Alder). Photo © Mary Free
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Cambium

cambium [ KAM-bee-uhm ] noun:  a formative cylindrical layer of tissue found between the bark and the wood that divides to form new wood and bark 

Vascular cambium, a cylindrical layer one to several cells thick, lies between the phloem and xylem of roots and stems in most vascular plants. It gives rise to new cells and is responsible for secondary growth that increases the plant’s girth—adds phloem outward and xylem inward. Cork cambium is a cylindrical layer in the periderm of woody plants. In new growth it lies between the epidermis and phloem and as the plant matures, replaces the epidermis with outer bark. Cork cambium produces cork outward and phelloderm inward.

Primary and Secondary Growth in Woody Stems: Image © CK-12
Primary growth adds height to a plant. Secondary growth adds girth. Note: The layer identified as “Inner Bark” on this diagram is usually called the phelloderm and the term “Inner Bark” usually refers to the phloem.
Image © CK-12 Foundation CC BY-NC 3.0

Reference

How a Tree Grows. University of New Hampshire Extension. (accessed April 11, 2023).

Wilkin D, Brainard J. 2012, modified 2021. Plant Growth. CK-12 Foundation.