Secondary growth : It is the permanent increase in thickness due to the activity of the vascular cambium and cork cambium in stelar and extrastelar regions.
The cork cambium gives rise to cork and secondary cortex whereas the vascular cambium gives rise to secondary xylem and secondary phloem. The roots of stem increase in girth due to secondary growth. Process of secondary growth in Typical Dicot Stem : The process of secondary growth in stem is completed into the following stages :
Stage A. (i) It shows the primary structure of a young dicot stem, (ii) The vascular bundles are open and arranged in a ring, (iii) The cortex, medullary rays and pith are well marked, (iv) The cambium between the phloem and xylem is called 'fascicular cambium' (interfascicular cambium). .
Stage B. (i) The medullary rays (parenchyma cells) in line with the intrafasdcular cambium becomes meristematic and form the 'interfascicular cambium'. (ii) The intrafasdcular and interfasdcular cambium join and form a continous cambial ring.
Stage C. (i) By the meristematic activity of cambial cells, the secondary phloem is produced towards outside and the secondary xylem towards inside, (ii) Secondary xylem is produced in much amount than secondary phloem due to more divisions taking place towards the xylem. (iii) The distance between primary phloem and primary xylem is increased, (iv) The primary xylem is pushed inward whereas the primary phloem towards outward, (v) The secondary phloem consists of sieve tubes, companion cells, phloem fibres and phloem parenchyma, (vi) The secondary xylem consists of tracheids, xylem fibres and xylem parenchyma, (vii) Both the phloem and xylem components are developed from the fusiform initials of the cambium and horizontally oriented ray parenchyma cells from the ray initials, (viii) The xylem rays and phloem rays provide for horizontal support whereas the main xylem and phloem provide for vertical support.
Stage D. (i) Secondary growth continues resulting in the increasing amount of vascular tissue, (ii) The wood (older secondary xylem) is pushed towards the centre, (iii) The pith and the primary xylem get traced. This wood is called 'heart wood'. It is harder and more durable. It is mainly for mechanical support, (iv) The wood formed due to deposition of some kind of material, each year laying outside just internal to the hard of cambium is called 'sap wood'. It is softer than heart wood. It is mainly for conducting water and minerals.
Annual rings, late wood and early wood : (i) During summer and spring seasons, the size of the secondary xylem vessels produced is much larger, whereas in winter and autumn the size of secondary xylem vessels produced is much smaller.
Thus in one year alternate smaller and larger vessels are seen as rings in the wood, (ii) A single growth ring formed each year is called 'annual ring'.(iii) These rings are helpful in determining the age of a tree, (iv) The denser wood having more fibres is called 'late wood'. The larger thinner walled wood with less fibres is called 'early wood'.