Describe the entire process of secondary development in the stems of woody angiosperms

Secondary growth refers to the formation of secondary tissues that leads to an increase in girth or width of dicot stems due to the activity of the cambium and cork cambium.

• Secondary tissues are established by two kinds of lateral meristems, vascular cambium and cork cambium. The vascular cambium produces secondary vascular tissues whereas cork cambium forms periderm.

• The vascular bundles in the dicot stem are conjoint, collateral, open, and are arranged in a ring. The cambium present among the phloem and xylem in vascular bundles is known as intrafascicular or fascicular cambium.

• Besides this, some cells of medullary rays also turn into secondarily meristematic and this is known as interfascicular cambium. Both these cambia collectively constitute a complete cambial ring. This ring of vascular cambium divides the periclinal to cut off cells both on the inner side and outer side.

• The cells cut off on the external side is the secondary phloem and the internal side is the secondary xylem.

• The amount of secondary xylem cut-off is more than secondary phloem and therefore with the development of secondary tissue, a rise in diameter or girth occurs. The structure of the secondary xylem and secondary phloem is the same as that of the primary phloem and primary xylem.

• With the rise in secondary tissue, the primary phloem and primary xylem get crushed. The ray initials of the vascular cambium ring are split by tangential divisions and add new cells. These new cells formed on both the sides of ray initials remain meristematic for some time and then distinguish into parenchymatous cells of rays.

• The rays, produced by vascular cambium among the secondary phloem and secondary xylem, are known as secondary medullary rays. They are generally one to few layers in thickness and one to numerous layers in height.

• The medullary rays form the radial system accountable for the radial conduction of solutes. They sustain the link between the cortex and pith. There is a significant difference in the activity of cambium with a change in season.

• In spring, the activity of cambium is even greater, and therefore the wood elements are bigger in size with a wide lumen. The activity of the cambium is less in the autumn and the wood components are relatively small in size with the thin lumen.

• Autumn wood and springwood of a year form the annual ring. (The age of a tree can be defined by calculating the annual rings. Numerous annual rings are consistent with the age of a tree.)

• Phellogen cells (cork cambium) split on both the inner side and the outer side as well to form secondary tissues.

• The secondary tissue developed on the inner side is known as the secondary cortex whereas the tissue developed on the outer side is known as cork.

Significance of secondary growth is as follows:

i. It adds to the girth of the plant thus provides support to the increasing weight of aerial parts due to growth.

ii. It produces a corky bark around the tree trunk that protects the interior from abrasion, heat, cold, and infection.

iii. It adds new vascular tissues for replacing old non-functioning one as well as for meeting increased demand for long-distance transport of sap and organic nutrients.