cambium, in plants, layer of actively dividing cells between xylem (wood) and phloem (bast) tissues that is responsible for the secondary growth of stems and roots (secondary growth occurs after the first season and results in increase in thickness). Theoretically, the cambium is a single layer of cells, called initial cells; practically, it is difficult to distinguish the initials from their still-undifferentiated daughter cells, and several cell layers are collectively called the cambium, or cambial zone. Cambial cells divide to produce secondary xylem cells toward the central axis of the stem and secondary phloem cells toward the outside. The cambium originates from undifferentiated cells that have retained their embryonic capacity for continued growth and differentiation. A cambium may also form within callus tissues—masses of cells that grow over the injured surface of a wound, leading to healing.