[Reference- 100% From NCERT XI & XII ]
➡️SECONDARY GROWTH
The growth of the roots and stems in
length with the help of apical meristem is
called the primary growth. Apart from
primary growth most dicotyledonous
plants exhibit an increase in girth. This
increase is called the secondary growth.
The tissues involved in secondary growth
are the two lateral meristems: vascular
cambium and cork cambium.
➡️*Origin
➡️Bark is a
➡️The secondary growth occurs in most of the dicotyledonous roots and stems and
The growth of the roots and stems in
length with the help of apical meristem is
called the primary growth. Apart from
primary growth most dicotyledonous
plants exhibit an increase in girth. This
increase is called the secondary growth.
The tissues involved in secondary growth
are the two lateral meristems: vascular
cambium and cork cambium.
➡️*Origin
late 16th century (denoting one of the alimentary humours once supposed to nourish the body): from medieval Latin, ‘change, exchange’
➡️Vascular Cambium
The meristematic layer that is responsible
for cutting off vascular tissues – xylem and
pholem – is called vascular cambium. In
the young stem it is present in patches as
a single layer between the xylem and
phloem. Later it forms a complete ring
➡️Formation of cambial ring
In dicot stems, the cells of cambium present
between primary xylem and primary
phloem is the intrafascicular cambium.
The cells of medullary rays, adjoining these intrafascicular cambium become
meristematic and form the interfascicular cambium. Thus, a continuous
ring of cambium is formed.
➡️Activity of the cambial ring
The cambial ring becomes active and begins to cut off new cells, both
towards the inner and the outer sides. The cells cut off towards pith,
mature into secondary xylem and the cells cut off towards periphery
mature into secondary phloem. The cambium is generally more active
on the inner side than on the outer. As a result, the amount of secondary
xylem produced is more than secondary phloem and soon forms a
compact mass. The primary and secondary phloems get gradually
crushed due to the continued formation and accumulation of secondary
xylem. The primary xylem however remains more or less intact, in or
around the centre. At some places, the cambium forms a narrow band of
parenchyma, which passes through the secondary xylem and the
secondary phloem in the radial directions. These are the secondary
medullary rays
➡️Cork Cambium
As the stem continues to increase in girth due to the activity of vascular
cambium, the outer cortical and epidermis layers get broken and need to
be replaced to provide new protective cell layers. Hence, sooner or later,
another meristematic tissue called cork cambium or phellogen develops,
usually in the cortex region. Phellogen is a couple of layers thick. It is
made of narrow, thin-walled and nearly rectangular cells. Phellogen cuts
off cells on both sides. The outer cells differentiate into cork or phellem
while the inner cells differentiate into secondary cortex or phelloderm.
The cork is impervious to water due to suberin deposition in the cell wall.
The cells of secondary cortex are parenchymatous. Phellogen, phellem,
and phelloderm are collectively known as periderm. Due to activity of
the cork cambium, pressure builds up on the remaining layers peripheral
to phellogen and ultimately these
layers die and slough off. Bark is a
non-technical term that refers to all
tissues exterior to the vascular
cambium, therefore including
secondary phloem. Bark refers to a
number of tissue types, viz.,
periderm and secondary phloem.
Bark that is formed early in the
season is called early or soft bark.
Towards the end of the season, late
or hard bark is formed. Name the
various kinds of cell layers which
constitute the bark.
At certain regions, the phellogen
cuts off closely arranged
parenchymatous cells on the outer
side instead of cork cells. These
parenchymatous cells soon rupture
the epidermis, forming a lens shaped
openings called lenticels.
Lenticels permit the exchange of
gases between the outer atmosphere
and the internal tissue of the stem.
These occur in most woody trees
non-technical term that refers to all
tissues exterior to the vascular
cambium,
➡️Secondary Growth in Roots
In the dicot root, the vascular
cambium is completely secondary in
origin. It originates from the tissue
located just below the phloem
bundles, a portion of pericycle tissue,
above the protoxylem forming a
complete and continuous wavy ring,
which later becomes circular .
Further events are similar to
those already described above for a
dicotyledon stem.
➡️The secondary growth occurs in most of the dicotyledonous roots and stems and
it increases the girth (diameter) of the organs by the activity of the vascular cambium
and the cork cambium. The wood is actually a secondary xylem. There are different
types of wood on the basis of their composition and time of production.
➡️In dicotyledonousplants and gymnosperms, the lateral meristems,
vascular cambium and cork-cambium appear
later in life. These are the meristems that cause
the increase in the girth of the organs in which
they are active.
➡️secondary or lateral meristem
They are cylindrical meristems. Fascicular vascular cambium,
interfascicular cambium and cork-cambium are examples of lateral
meristems. These are responsible for producing the secondary tissues
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*[Reference- out of NCERT XI & XII ]
