Secondary growth in stems of woody angiosperms
occurs by two types of cambia, 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 join 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 the
presence of tyloses and deposition of resins, gums,
and 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 sloughed 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 fireproof, insectproof 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.