Secondary growth occurs after primary growth through the activity of lateral meristems, resulting in increased girth. It is characterized by the deposition of secondary phloem and secondary xylem, which modifies the primary structure. In dicots, secondary growth includes intrastelar growth within the stele and extrastelar growth outside. Intrastelar growth involves the formation of the vascular cambium and secondary vascular tissues, while extrastelar growth involves the formation of a protective periderm layer and lenticels for gas exchange. The vascular cambium produces secondary phloem and secondary xylem tissues through the activity of ray initials and fusiform initials.
2. Secondary growth is that growth which
occurs after primary growth as a result of
the activity of lateral meristems. It results in
an increase in girth. It is usually associated
with deposition of secondary pholem and
secondary xylem. which completely modifies
the primary structure and is a characteristic
feature of trees and shrubs.
4. INTRASTELAR SECONDARY GROWTH
Formation of vascular cambium
Formation of secondary vascular tissue
EXTRASTELAR SECONDARY GROWTH
Formation of periderm
Formation of lenticels
5. ACTIVITY OF CAMBIUM
In the process of secondary growth, the first
step is the formation of cambium ring.
Cambium ring. The intra-fascicular cambium
or cambium of vascular bundles becomes
active and meristematic. At the same time,
some cells between the two vascular bundles
(may be called cells of medullary rays) lying
in the line with fascicular cambium also
become meristematic and form
interfascicular cambium (the cambium
between the two vascular bundles).
6. The interfascicular cambium joins the strips of
intra-fascicular cambium from the sides to form a
complete circular ring of cambium . This is known
as cambium ring which gives rise to secondary
tissues in the stelar region.
7. The vascular cambium consists of two types of cells
1.Ray intials
2.Fusiform initials
1.Ray initials : which are much smaller cells than fusiform initials
and which are almost isodiametric. It produce rays of parenchyma
in secondary xylem and pholem( medullary rays)
2. Fusiform initials: which are elongated cells with tapered ends. They
are vertically oriented and produce different elements of xylem and
pholem
8. SECONDARYPHOLEM
It consists of sieve elements (sieve tubes and
sieve plates), companion cells, phloem
parenchyma and phloem fibres and there is no
fundamental difference between primary and
secondary phloems as regards the elements .
The phloem elements are arranged in two
systems, the vertical and the horizontal.
Vertical system comprises of sieve tubes,
companion cells, phloem parenchyma and
fibres.
9. Horizontal system consists of ray parenchyma.
The phloem parenchyma are elongated and have
pointed ends. The function of vertical system is
mainly conduction of food materials from leaf to
roots while that of horizontal system is to
conduct food material laterally to cortex or
pith. Phloem fibres provide mechanical strength
to the plant and are highly valued.
10. SECONDARY XYLEM
The xylem elements formed by the activity of
cambium collectively called secondary xylem. It
consists of tracheids, vessels, fibres and wood
parenchyma.
In secondary xylem has two systems of tissues, i.e.
axial system or vertical or longitudinal system and
radial system which remains absent in primary
xylem.
11. The axial system is made up of vertical rows of
cells .It consists of tracheids, vessels (trachea),
fibres and wood parenchyma. The radial system
consists of rows of parenchymatous cells and forms
the xylem rays.
12. PERIDERM
Because of continued secondary growth , the stem
gradually Keeps on increasing in width and
secondary tissues formed exert more and more
pressure on phloem, cortical and epidermal cells.
Due to pressure, the phloem cells are pushed
outwards, the cortical cells become flattened, the
hypodermal cell are disturbed and become flattened
tangentially and epidermis is put to maximum
stretching.
13. The pressure increases so much so that epidermis
ultimately gets ruptured exposing the living cells
to the external atmosphere . At this stage is
replaced by a group of secondary tissues which
are protective in function. The term periderm is
applied to these cells. Periderm is formed by the
activity of a secondary lateral meristem called
cork cambium or phellogen. Periderm serves as
secondary protective tissue against dessication
and mechanical injuries due to compact
arrangement and suberised wall.
14. The periderm consists of three types of tissues
(i) phellem or cork
(ii) Phellogen or cork cambium
(iii) phelloderm or secondary cortex
(i) Phellem or cork-Phellem arises towards the outerside of
phellogen and are produced in more quantity than
phelloderm. The cork cells are subersied and thick walled,
dead, compactly packed and arranged in radial rows without
intercellular spaces.
15. These characteristically appear rectangular in transverse
section. They lose protoplasts after differentiation and become
dead and remain filled with air and coloured organic matter
(ii) Phellogen or cork cambium-Phellogen originates as a single
layer of initiating cells either in the sub-epidermal portion, in
the epidermis itself, in the cortex and some times may even
extend to the phloem. These are arranged just like the cambial
cells.
16. (iii) Phelloderm or Secondary cortex – Secondary cortex
is formed towards the inner side of the cork cambium.
The cells of this region are thin-walled and living and
have pits on their cellulose wall. So these cells resemble
those of cortex in their nature, wall structure and
contents. These are arranged in radial rows and
isodiametric in shape. Some intercellular spaces are
found between them. The cells serve in storage of food.
Sometimes, these may contain chloroplast and may carry
on photosynthesis.
17. LENTICELS
Due to continued secondary growth, the
protective tissues or periderm formed, of
which, the cork cells are impervious to
water and gases. Thus, the gaseous
exchange between the internal living cells
and, outer atmosphere becomes difficult
and is brought about by lenticels. Lenticels
are pore-like openings which look-like lens-
shaped raised spots on the surface of stem
These take up the function of gaseous
exchange. Lenticels are found in most
woody plants where periderm is developed,
except a few climbers
