The trunk diameter growth occurs due to the development of meristem called vascular cambium.
a) The inner portion of the tree is called heartwood and it acts as the supporting pillar of the tree. It is the dead part of the tree and does not decay . It is made up of cellulose fibers glued by lignin
b) Sapwood – it is the new wood that becomes intact to the heart wood . with every new layer of the sap wood, the older sapwood layer becomes the part of heart wood
c) Cambium cell layer – This is responsible for producing the new bark and new wood as of the hormones passed down through the phloem.
d) Inner Bark – Phloem after dying turns to cork and become part of protective outer bark
e) Outer bark – it is the outer most covering of the tree trunk which is continually renewed.
