In the stems of perennial plants, the layers of secondary xylem are concentric. When the plant is cut transversely, it is visible in the form of rings. These are called annual rings or growth rings. There is a great difference in the climate of some places, due to which the activity of cambium in those places is not the same throughout the year.
In the spring season, the need for convection of liquid is more in them. At this time the cambium ring is more active. For this reason, large vessels are formed at this time. In autumn, the flow of liquid is less and the activity of cambium ring is also less. The vessels formed at this time are smaller than the spring vessels. The wood formed in spring is called spring or early wood and the wood formed in autumn is called autumn or late wood respectively. In this way, in a year, one spring wood and one autumn wood are formed. These are collectively called the annual ring.
Each annual ring (growth ring) represents one year's growth. For this reason, the approximate age of a tree can be obtained by counting the annual rings present in it, for example- Pinus, Rosewood (Dalbergia sissoo) etc. Due to many reasons, more than one growth ring can also be formed in a year, such as - adverse environment, drought, cutting of leaves, various diseases etc.
Annual rings are not found in plants in places where the climate (temperature and rainfall) remains the same throughout the year, such as on the coastal areas, near the equator and high mountains. In some trees, large vessels of spring wood are located in a ring. Such wood is known as ring-porous. In some other trees, the vessels are usually of the same diameter and are scattered here and there in the spring wood. This type of wood is called diffuse porous.
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