As a rule, trees produce growth rings each year formed by the cambium, or growth layer, laying between the wood and the bark. When the season starts, large cells are added to the wood. These cells develop thicker walls and become samller as the growing season progresses, by the end of the growth season, cell production has ceased altogether. This process occurs every growing year, and a distinct line is formed between the wood of the previous season, with its small cells, and the wood of the next, with its new, large cells. The thickness of the rings may vary according to the tree's age and annual climatic variations, thick rings are characteristic of good growth years. Weather variations within a circumscribed area tend to run in cycles. A decade of wet years may be followed by five dry decades. One season may brak a forty-year rainfall record. These cycles of climate are reflected in patterns of thicker or thinner tree rings, which are repeated from tree to tree within a limited area. Dendrochronologists have invented sophisticated methods of correlating rings from different trees so they build up long master sequences of rings from a number of trunks that may extend over many centuries.বৃক্ষ বলয়ের পুরুত্ব নির্ভর করে-