Secondary growth in stems of woody angiosperms occur by two types of carnbia, i.e., vascular cambium and cork cambium.
(i) Vascular Cambium Certain cells of medullary rays become meristematic to form interfascicular cambium. The fascicular cambium and the interfascicular cambium joins to form a complete ring called cambial ring. The cells of the cambial ring undergo mitotic divisions and produce secondary phloem on its outer side and secondary xylem on its inner side.
At places, vascular cambium possesses ray initials. They form vascular rays, phloem rays in secondary phloem and wood rays in secondary xylem.
As new secondary phloem becomes functional, the previous older phloem gets crushed. Secondary xylem or wood persists. As a result wood grows with age in the form of annual rings. In each annual ring, there is wide or broader spring or early wood or spring wood and narrow autumn or late wood.
In old stems, the central part of wood becomes non-functional and dark coloured due to tyloses and deposit of resins, gums, tannins. It is called duramen or heartwood. The outer functional wood is called sapwood.
(ii) Cork Cambium As the stem continues to increase in girth due to the activity of vascular cambium the outer cortical and epidermal layers get broken and needs to be replaced to provide new protective cell layers. In this way, cork cambium or phellogen develops in the cortex region. Phellogen cuts of cells on both the sides.
The outer cells differentiate into cork or phellem while, the inner cells differentiate into secondary cortex or phelloderm. Due to the activity of cork cambium, pressure builds up on the remaining layers peripheral to phellogen and ultimately these layers die and slough off. At places, aerating pores called lenticels develop, which have loosely arranged complementary cells.
Significance of Secondary Growth
(i) It replaces old non-functional tissues.
(ii) It provides fire proof, insect proof and insulating cover around the older plant parts.
(iii) Commercial cork is a product of secondary growth.
(iv) Wood is the product of secondary growth.