Why Do Trees have Rings?
Trees are organisms in the plant kingdom that appear to be so simple to us but are very complex in their organization. The tree trunk is the biggest stem and the trunk possesses many branches which again have stems on them. Leaves are found on almost all the branches and stems of trees. Leaves are present on the top of the tree while the roots found at the bottom take the nutrients from the ground. All plants including the trees have a central medullary region or Pith region made up of parenchyma cells. Leaves are the sites for preparation of food necessary for the tree to survive. Hence it is essential for the tree to supply the water and nutrients to the leaves from the ground through the stem.
This transfer of water is done by Xylem vessels and that of mineral nutrients by Phloem which are surrounding the central pith region. There is a meristematic tissue called cambium which exists between the xylem and the phloem which keeps on dividing and giving rise to more xylem and phloem tissues. Each vascular bundle consists of xylem towards the center, phloem towards the periphery and cambium between them.
The primary xylem and primary phloem are formed initially by the plant. Cambium generates secondary tissues as the tree ages. The new xylem and phloem are formed towards the periphery. The old tissues have the cambium united between all the vascular bundles. There is a ring formed by the cambium and vascular bundles. These old xylem vessels do not transport water anymore and hence form heart wood. The new xylem in the new vascular bundle that surrounds the old one transports water and is called sap wood. Sap wood is surrounded by cambium, then phloem and then by cortical layers towards the periphery.
As tree age progresses, the above process is repeated and rings keep on forming inside and the heart wood grows. So, the rings are nothing but the old cambial rings. The number of rings represent the age of the tree.