A transverse section of the trunk of a tree shows concentric rings, which are known as annual rings. How are these rings formed? What is the significance of these rings ?

• The activity of the cambium is under the control of physiological and environmental factors. In temperate regions, the climatic conditions are not uniform through the year.
• In the spring season, vascular cambium is very active and produces a large number of secondary xylem elements having vessels with wider cavities / lumens. The wood formed during this season is called spring wood or early wood. 
• In winter and autumn season, the vascular cambium is less active and forms fewer secondary xylem elements that have vessels with narrow lumen and this wood is called winter wood or autumn wood or late wood. 
• The autumn and spring wood are formed in the form of concentric rings, called growth rings. The two kinds of woods that appear as alternate concentric rings, constitute an annual ring. 

Significance :
* Annual rings help in estimating the approximate age of the plant.