$\Rightarrow$ Concentric rings : Concentric rings are also called annular rings. These rings are produced due to secondary growth. In dicot tree secondary growth takes place due to meristematic tissue cambium.

$\Rightarrow$ In spring season cambium is more active. So xylem cells produce wood of large size while in wood produced in winter have small sized and narrow xylary elements. As a result two rings are produced.

$\Rightarrow$ The age of a tree can be determined by counting the number of these rings. This branch of science is called dendrochronology.