2. Secondary Growth –
The growth which responsible for increase in height of plant is called primary
growth & produces primary tissues.
It is brought about by the activity of the meristems. The meristem which
involved in increase in height of plant either be apical or intercalary meristem.
These meristematic tissue responsible for primary growth of plant.
Many monocots & some herbaceous dicots possess only primary growth.
In Gymnosperms, woody as well as herbaceous dicots, & some monocots the
stem & root continue to grow, not only in length but also in diameter. This is
called secondary growth.
Secondary growth can be defined as production of secondary tissues by the
activity of the vascular cambium & cork cambium in the stelar & extra-stelar
regions leading to an increase in the girth of the stem or root. The vascular
cambium and cork cambium are also called as lateral meristem because they
present at lateral side of plant.
Complete secondary growth occur in two region ,stelar and extrastelar region .
3. Secondary growth in Dicot stem
Stelar secondary growth
Stelar secondary growth occur in plant due to vascular cambium . In plant there are two kind of
vascular cambium present inside stele i.e. interfascicular and fascicular cambium
Cambium which is located in between two vascular bundle called as interfascicular and cambium
present between xylem and phloem of single vascular bundle. Parenchyma cells which are a part
of the medullary ray & are in line with the fascicular cambium start becoming meristematic. Thus,
interfascicular cambium is formed.
Fascicular & interfascicular cambium now join hands to form a complete cambial ring.
The vascular cambium i.e., fascicular cambium is composed of two types of cells – the fusiform
initials & the ray initials.
The fusiform initials are elongated & spindle shaped & produce vascular tissues. The fusiform
initials are elongated tapering cells that give rise to all cells of the vertical system of the secondary
phloem and xylem (secondary tracheary elements, fibres, and sieve cells and the associated
companion cells).
The ray initials are smaller & isodiametric & give rise to the xylem & phloem rays. They
produce the vascular rays, which constitute the horizontal system of secondary tissues; this
horizontal system acts in the translocation and storage of food and water.
4. The cambial cells divide tangentially & produce secondary xylem on
the inner side & secondary phloem on the outer side.
The amount of secondary xylem produced is more than the amount of
secondary phloem.
Secondary xylem is composed of xylem vessels & fibres. There is less
of wood parenchyma.
Secondary phloem consists of sieve tubes, companion cells & phloem
parenchyma.
Due to increase in the girth, the cells of the non-vascular tissue, i.e.,
medullary rays, pith, hypodermis & cortex become tangentially
compressed.
The secondary tissues put pressure on the primary phloem lying
immediately outside & therefore it gets crushed.
T.S of Dicot Stem Showing normal growth
7. Extrastelar secondary growth
When a large amount of secondary tissues are produced in the stelar region,
the epidermis gets stretched & ultimately ruptures.
To protect the inner tissues, the epidermis is replaced by a new secondary
tissue called periderm.
The periderm consists of three layers which are meristematic in nature –
i) Cork cambium /Phellogen ,
ii)First cork cell /The phellem or cork produced by the phellogen
towards the outer side &
iii) The phelloderm or secondary cortex, a tissue that resembles cortical
parenchyma & consists of the inner derivatives of the phellogen.
Phellogen originates in the primary cortex through dedifferentiation. It is
reversion of permanent parenchymatous cells into meristematic cells.
8. The phelloderm is composed of cells that remain arranged in definite
radial rows. They help in storage of food.
Phellem or cork is composed of cells which are arranged in distinct
radial rows, without intercellular spaces & are without protoplasts at
maturity.
They are filled with either air or with highly coloured organic
substances which may be resinous or tanniferous compounds.
In the big perennial trees, the phellogen originates successively towards
the inner side.
The earlier formed phellogen does not function throughout the life but
is replaced by many successive phellogen layers.
This causes an accumulation of dead tissue on the outer side of the stem
& root.
Such composite periderm is called Rhytidome (shell bark). It consists
of cork, cortical layers & dead phloem placed alternately.
Bark is a non-technical term used to refer to all the tissues lying
outside the vascular cambium, but often used to designate the
phellem.
9. Kinds of bark
1. Ring Bark
Continuous bark of equal thickening is called ring bark.
It is formed around the stem in the form of a complete ring. In
ring bark cork cambium is continuous.
A complete distinct ring bark is formed in this plant. Its bark
was used as a writing material as a paper in ancient period.
Example: Bhojpatra (Betula utilis) and it is also formed in
Eucalyptus. Ring Bark In bhojpatra Tree
11. 2. Scaly Bark
Discontinuous bark of unequal thickening is called scaly
bark.
This bark is formed around the stem in the form of
pieces or fragments. In scaly bark the ring of cork
cambium is not continuous.
This scaly bark formed in Neem (Azadirachta indica),
Mango (Mangifera indica) and Imli ( indica) etc.
Scaly Bark is also formed in Neem
(Azadirachta indica)
13. Lenticels –
These are structurally differentiated portions of the periderm that are
characterized by relatively loose arrangement of the cells without any
suberization.
During periderm formation, gaseous exchange between the internal living
cells & the outer atmosphere becomes difficult; lenticels take over this
function of gaseous exchange.
Lenticels are first formed beneath the stomata. They originate either
before, simultaneously with or after the initiation of the periderm.
Parenchyma near the sub-stomatal chamber loses chlorophyll & divides in
many planes to form a mass of colourless cells with inter cellular spaces.
When phellogen is formed, it also produces the same type of loosely
arranged cells on the outer side instead of normal cork cells.
They increase in numbers & ultimately the epidermis ruptures & these
cells called complementary cells are exposed, forming a lenticel.
14. Functions of Secondary Meristem
1. Healing of wounds
When any plant part gets injured wound is formed there.
Boundary of the wound is raised outside and composed of similar type of living cells
(parenchyma) called callus.
Living cells of wound are responsible to form a cambium.
This is called wound cambium. It is also called inducible cambium.
This newly formed cambium forms cork towards the outside.
This cork covers the wound entirely. Wound cambium is lateral meristem.
15. 2. Abscission
Falling of any plant organ is called as abscission.
Abscission takes place due to formation of abscission layer at the
base of plant organ and it is composed of parenchyma.
Middle lamella is dissolved in abscission layer during abscission
and primary walls also dissolve partially or completely.
Sign of leaf fall on stem is called leaf scar and it is a type of
wound. The living cells of leaf scar are responsible to form cork
cambium, which produce cork.
Cork covers the wound. At the site of abscission protective layer is
found which is suberized.
16. 3. Knots
Knot is formed when branches are embedded inside the main stem. In most
cases knots are caused by the natural growth of the tree.
As a tree grows and increases the circumference of its trunk, the growing trunk
begins to overtake the branches that grow out from it. Knots form around these
branches, building up trunk material as the tree continues to expand.
The wood of the knot is typically tougher than the surrounding wood and may
form a bulge around the branch emerging from its center and known as tight
knot.
If a branch becomes injured or otherwise dies while still attached to the tree, a
loose knot forms as the trunk grows larger. Loose knots are similar to tight
knots, but instead of having living wood in the center of the knot there is only a
dark plug of dead or decaying material.
