(i) vascular cambium and (ii) cork cambium (or phellogen).Stelar growth by vascular cambium
• The process of stelar secondary growth in dicotyledonous
roots begins with the initiation of vascular cambium strips
which develop from the parenchymatous cells present along
the inner edges of primary phloem. The number of cambial
strips depends on the number of phloem or xylem strands.
• The cells of cambium strips (vascular cambium) divide
repeatedly to produce new cells both towards inner as
well as outer side. The cells produced towards inner side
(centripetally) differentiate into secondary xylem elements
and those produced towards outer side (centrifugally)
differentiate into secondary phloem.
• Subsequently, the cells of pericycle lying towards outer side
of protoxylem divide by tangential division.
• The inner derivatives of these cells become meristematic and
function as cambium. These join with cambia derived from
phloem strands to form a complete ring of cambium. Thus,
a continuous wavy ring is produced which is present below
the phloem but above the xylem and is secondary in origin.
Soon, the cambium becomes circular by its divisions and
secondary tissue formation.
• Some cambial cells may function as ray initials and produce
secondary medullary rays. Secondary vascular
tissue assumes the shape of continuous
cylinder interrupted at places
where wide vascular rays
connect primary xylem to
cortex through pericycle and
endodermis.
• The primary xylem remains
in its original position but
the primary phloem is pushed
towards outer side and crushed.
Extrastelar growth by cork cambium
• Addition of more and more tissues of secondary xylem and
secondary phloem in the central part of root exerts pressure
towards periphery. The tissues outside this zone of active
growth get crushed and are sloughed off. To avoid injury to
secondary phloem, the roots develop a new cambium ring in
the pericycle, called cork cambium (phellogen).
• The cells of cork cambium (phellogen) divide to form cells
of cork (phellem) towards outer side and secondary
cortex (phelloderm) towards inner side. It results
in the formation of outer protective covering consisting
of multilayered cork, cork cambium and multilayered
secondary cortex. These three layers collectively constitute
the periderm.secondary growth in dicot stems
• The dicotyledonous stems are characterised by presence
of fascicular (intrafascicular) cambium (primary meristem)
derived from procambium of shoot apical meristem between
xylem and phloem.
• Interfascicular cambium (secondary meristem) arises from
the cells of medullary rays which occur at the level of
intrafascicular strips.
• The cambium ring formed by joining of fascicular and
interfascicular cambium, is called vascular cambium. It is
composed of two types of cells– the fusiform initials and
the ray initials.
• The cells of cambium mostly divide by periclinal divisions. Out
of the two cells produced from single cell of cambium, one
differentiates into secondary tissue and the other remains
cambial cell. In this way the cambial cells, by repeated
divisions, add new cells either towards centre or towards
periphery and the cambium still remains single layered.Formation of secondary xylem and secondary
phloem
• The fusiform initials of cambium ring divide by tangential
divisions and add new cells. The new cells produced
toward inner side (i.e., towards primary xylem) may remain
meristematic for sometime and differentiate into elements of
secondary xylem. On the other hand, the new cells
produced towards outer side (i.e., towards
primary phloem) remain meristematic
for a limited period and differentiate
into secondary phloem. The primary
phloem gets crushed.
Formation of secondary
medullary rays
• Ray initials of cambium ring divide
by tangential divisions and add new cells