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How are wood and inner bark formed by the vascular cambium?

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Tall giant sequoia trees with reddish bark and thick trunks rise closely together in a dense forest at Sequoia National Park, California. — Tree A stand of giant sequoia trees (*Sequoiadendron giganteum*) in Sequoia National Park, California.

The vascular cambium is a lateral meristematic layer responsible for the secondary growth (thickening) of stems and roots in woody plants. It functions as a cylinder of actively dividing cells located between the primary xylem and primary phloem.123

The formation of wood and inner bark occurs through tangential cell divisions within the cambial zone:3

Wood (Secondary Xylem): The cambium produces new cells toward the central axis of the stem or root. These cells differentiate into secondary xylem, which constitutes the wood of the tree. Because the cambium produces more cells internally than externally, the volume of secondary xylem significantly exceeds that of the secondary phloem.123
Inner Bark (Secondary Phloem): The cambium produces new cells toward the outside of the stem. These cells differentiate into secondary phloem, which forms the inner bark. As the tree continues to grow and add more wood internally, the older, outermost layers of the secondary phloem are eventually crushed and sloughed off as part of the bark.123

These processes are driven by two cell types: fusiform initials, which create the vertical vascular system, and ray initials, which form the horizontal vascular rays.32