The annular rings or the alternating dark light circles seen in a cross section of a tree are produced because of differences in the rate of growth in different seasons. As it happens, the entire tissue in the trunk of a tree does not divide or grow to increase the girth of the tree. There is a layer of dividing tissue called the cambium sandwiched between the fibrous xylem (the water carrying tissue) and the phloem (the food carrying tissue). However, the cambium divides at different rates in different seasons. In winter, its growth is slower than in other seasons, say in spring, when conditions for growth are fairly favourable. Thus, the relatively small number of cell produced in winter remain compacted together producing a dark band while the cells produced during the spring season spread out into a broader light band. These growth patterns are repeated at annual intervals and hence the name. The annular rings thus indicate the age of a tree as well as the changes in climate that might have occurred in the tree’s life time.
