Describe/ explain the process of secondary thickening/ growth in flowering plants.

-The vascular cambium divides to produce new cambium cells between the vascular bundle called intervascular cambium; the intervascular cambium and vascular cambium form a continuous cambium ring; the new cells obtained on the outer side of cambium differentiate to form secondary phloem; and those to the outer side differentiate to form secondary xylem; more secondary xylem is formed than secondary phloem; and intervascular cambium also cuts the parenchymatous cells forming medullary rays; which allow transport of water and solutes inside the stem;
-As a result of increase in the volume of secondary tissues, pressure is exerted on the outer cells of the stem; leading to stretching and rapturing of epidermal cells; in order to replace the protective outer layer of the stem, a new band of cambium cells are formed in the cortex called cork cambium/ phellogen; the cork cambium divides to produce new cells on either side; the cells on the inner side of the cork cambium differentiate into secondary cortex; and those on the outer side become cork cells;
-Cork cells are dead with thickened walls. Their walls become coated with a waterproof substance called suberin; the cork cells increase in number and become the bark of the stem; which prevents loss of water/infection from fungi/damage from insects and acts as insulatory layer; at certain points along the stem the cork cells become loosely packed forming lenticels for gaseous exchange;
-The rate of secondary growth in the stem varies with seasonal changes. During rainy season xylem vessels and tracheids are formed in large numbers; and the cells are large, have thin walls and the wood has light texture; in the dry season, the xylem and tracheids formed are few in number; which are small, thick walled ad their wood has dark texture; this leads to two distinctive layers within the secondary xylem hence called annual rings;