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

Explanation

Annual rings : In temperature regions and cold regions the activity of the cambium is influenced by a seasonal variations . Suring the faourable season , i.e ., in spring . When more leaves and flowers are formed the plant requires large amounts of water and minerals salts . Hence the wood formed in this period shows more number of xylem vessels with wider lumens . This is known as spring wood or early wood . The colour of this wood is light .During the unfaourable season i.e ., in autumn , the plants are less active and do not require more water and mineral salts . Hencce the wood produced in this period shows less number of xylem vessely with narrow lumens . This is known as autumn wood or late . It is dark coloured .In this way two two types of secondary xylem (wood) are produced in one year. They appear in the form of dark and light coloured circles alternately in a mature tree trunk . These are called annual rings or growth rings or seasonal rings .By counting the number of annual rings , the approximate age of trees can be estimated .This branch of science is known as " dendrochronology or growth ring analysis ". For Ex: The age of sequiod dendron , presently growing in America , is estimated to be about 3500 years .In tropical countries like India , annual rings do not appear clearly , as the seasonal variations are not sharp .Hence these are called growth marks .