Cambium is a laterally expanding meristem which divides to produce xylem to the inside and phloem to the outside. These secondary tissues are secondary xylem and secondary phloem . Herbaceous plants lack secondary growth even if a cambium layer is present. Secondary xylem cells become hard and impregnated with lignin . Only the cells closet to the cambium layer are alive and growing.
Wood is an accumulation of secondary xylem.
Sapwood is xylem which is still conducting water. Heartwood is the old, no-longer functional xylem. It may be filled with resins or tanins. Old phloem external to the newest layer of phloem can not divide or stretch to accommodate the increase in the diameter of the stem, so it is torn and crushed as the stem grows. In addition to xylem and phloem, the cambium makes ray parenchyma cells in the living part of the xylem.