In truly herbaceous dicots and monocots, all or nearly all of the vascular tissue is produced by terminal meristems. In woody plants, primary vascular tissue is also produced by terminal meristems. In plants that become woody, a cambium layer forms between the primary xylem and phloem cells. 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.