Cambium is the layer of plant tissue between the xylem and phloem tissues that is about two cells thick. Cambium gives rise to secondary tissues, which increases the diameter of the stem or root. Because this tissue is in between the inner bark and wood, (and produces new bark and wood cells), this growth in diameter will produce the annual ring in trees. There are two main types of cambium: vascular and cork. Vascular cambium forms tissue that carries water and nutrients throughout the plant. Cork cambium creates cells that become bark on the outside and cells that add to the cortex on the inside.