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Cambial Zone - The Cambial Zone is a cell generator that is between the wood or xylem (depending on the phenological stage) and inner bark. The cambial zone produces cells on its outer side, that mature to form phloem. The cambial zone produces cells on its inner side, that mature to form xylem (Not wood)! The cambial zone is sometimes called the vascular cambium. It is not green! It is rarely made up of a single layer of cells. The cambial zone is a meristem. The cambium zone is like an accordion - during the resting period it is closed and during the growing season it is open. The cambial zone is the same as the queen bee. Both must be fed by others and their job is to pump out new living cells. The symplast is the network of connected living cells; axial and radial parenchyma in wood, the cambial zone, living cells in the inner phloem, phloem rays, and the phellogen. The cambial cells may divide in a periclinal plane which is toward the bark and Pith or in an anticlinal plane which is in circumference. By dividing in these two directions, the cells keep pace with increase in girth and circumference.
The cambial cells may divide in a periclinal plane which is toward the bark and pith or in an anticlinal plane which is in circumference. By dividing in these two directions, the cells keep pace with increase in girth and circumference.

A Bur Oak Cross Section with the cambial zone marked.

See TREE ANATOMY, SHIGO, 1994 The Book