Formation of annual ring:
In plant body, usually, according to seasons the activeness is seen in cambium, such as- in spring season cambium becomes more active. During this time, plenty of secondary xylem are formed. The wood which is formed with this xylem is called spring wood. In adverse season i.e. in autumn season when plants reduce their growth, cambium produce vessels with thick wall and narrow cavities The wood that is formed with this xylem vessels in winter season is known as autumn wood.
Therefore, in one year, normally two types of Root are formed and those two types of wood combinedly form it concentric ring, with vessels of large cavities and thin walk (spring wood) and vessels of narrow cavities and thick walls (late wood autumn wood) are known as annual ring. By counting the number of annual rings, the approximate age of a plant can be estimated.
Fig: T.S. of dicot stem with secondary growth