Principle purpose of this lecture is to present on Secondary Growth of Plants. Secondary thickness takes place in three steps: formation of secondary xylem and secondary phloem; formation of cork and formation of lenticels. Here also briefly present on formation of secondary xylem and secondary phloem. Parenchyma cells between the vascular bundles begin to divide. This with vascular cambium forms a complete ring of cambium. Entire cambium divides to form secondary xylem on the inside and secondary phloem on the outside. Every year a new ring of secondary xylem is formed. These are called the annual rings.