The primary xylem is gradually pushed inward and is found at the centre of the axis; whereas, the primary phloem, being soft in nature, gets completely crushed.
These activities in the stelar region exert a great pressure outwardly. The cortex cells, the pericycle and the epidermis divide anticlinally to cope with the production of tissues in the stelar region.
Formation of annual ring or growth ring
The activity of the cambium ring is under the control of series of physiological and environmental factors. For example, in spring the cambium becomes more active and forms a greater number of vessels with wider cavities.
In winter, the temperature is low due to which the cambium also becomes less active and forms narrow pitted vessels, tracheids and wood fibers.
The xylem (wood) formed during the spring is known as spring wood or early wood and which is formed in winter is called autumn wood or late wood. The spring wood is lighter in color and exhibits low density where as the autumn (or winter) wood is darker and has higher density.
These two kinds of wood appear together, in a transverse section of the stem, as a concentric ring known as the annual ring or growth ring. Successive annual rings are formed year after year by the activity of the cambium.
Each annual ring corresponds to one year's growth. Thus one can estimate the age of plant to some degree of accuracy by counting the total number of annual rings. Annual rings are readily seen with naked eye in the logs of a tree trunk.
Heart-wood and sap-wood:
In old trees, the greater part of the secondary wood is filled up with tannins resins, gums, essential oils, etc., which make it hard and durable. It looks dark or brown. This region is known as heart-wood.
The heart wood gives mechanical support...