The activity of the fascicular cambium is more periodical than continuous throughout the year. As a result of this periodic activity, distinct growth layers are formed in the secondary xylem. In a transverse section of the stem, these layers appear as rings, hence called annual lings or growth rings.
An annual ring may be defined as a ring of vascular tissue produced by the fascicular cambium during the course of one year. In each annual ring the xylem consists of two types of wood elements namely spring wood or early wood and autumn wood or late wood.