!

Navigation

Secondary Growth
in Dicot Stem
(With Diagram)
Article Shared by
ADVERTISEMENTS:

The below mentioned article

provides study notes on
Secondary Growth in Dicot Stem
of plants.

Primary growth produces growth in

length and development of lateral
appendages. Secondary growth is the
formation of secondary tissues from
lateral meristems. It increases the
diameter of the stem. In woody plants,
secondary tissues constitute the bulk of
the plant. They take part in providing
protection, support and conduction of
water and nutrients.

Secondary tissues are formed by two

types of lateral meristems, vascular
cambium and cork cambium or
phellogen. Vascular cambium produces
secondary vascular tissues while
phellogen forms periderm.

ADVERTISEMENTS:

Secondary growth occurs in perennial

gymnosperms and dicots such as trees
and shrubs. It is also found in the
woody stems of some herbs. In such
cases, the secondary growth is
equivalent to one annual ring, e.g.,
Sunflower.

A. Formation of Secondary
Vascular Tissues:

They are formed by the vascular

cambium. Vascular cambium is
produced by two types of meristems,
fascicular or intra-fascicular and inter-
fascicular cambium. Intra-fascicular
cambium is a primary meristem which
occurs as strips in vascular bundles.
Inter-fascicular cambium arises
secondarily from the cells of medullary
rays which occur at the level of intra-
fascicular strips.

These two types of meristematic tissues

get connected to form a ring of vascular
cambium. Vascular cam​bium is truly
single layered but appears to be a few
layers (2-5) in thickness due to
presence of its immediate derivatives.
Cells of vascular cambium divide
periclinally both on the outer and inner
sides (bipolar divisions) to form
secondary permanent tissues.

ADVERTISEMENTS:
 Seeking an
Ideal
Partner?

Friendly & Safe Dating Community

For Over 40s Near You. Sign Up
Free!
www.DateMyAge.com

The cells of vascular cambium are of

two types, elongated spindle-shaped
fusiform initials and shorter
isodiametric ray initials (Fig. 6.29).
Both appear rectangular in T.S. Ray
initials give rise to vascular rays.

Fusiform initials divide to form

secondary phloem on the outer side and
secondary xylem on the inner side (Fig.
6.28 B). With the formation of
secondary xylem on the inner side, the
vascular cambium moves gradually to
the outside by adding new cells.

The phenomenon is called dilation.

New ray cells are also added. They form
additional rays every year (Fig. 6.28 D).
The vascular cambium undergoes two
types of divisions— additive (periclinal
divisions for formation of secondary
tissues) and multi​plicative (anticlinal
divisions for dilation).

Ray initials produce radial system (=

horizontal or transverse system) while
fusiform initials form axial system (=
vertical system) of sec​ondary vascular
tissues.

1. Vascular Rays:

The vascular rays or secondary

medullary rays are rows of radially
arranged cells which are formed in the
secondary vascular tissues. They are a
few cells in height.

Depending upon their breadth, the

vascular rays are uniseriate (one cell in
breadth) or multiseriate (two or more
cells in breadth). Vascular rays may be
homo-cellular (having one type of cells)
or hetero-cellular (with more than one
type of cells). The cells of the vascular
rays enclose intercellular spaces.

The part of the vascular ray present in

the secondary xylem is called wood or
xylem ray while the part present in the
secondary phloem is known as phloem
ray. The vascular rays conduct water
and or​ganic food and permit diffusion
of gases in the radial direction. Besides,
their cells store food.
2. Secondary Phloem (Bast):

It forms a narrow circle on the outer

side of vascular cam​bium. Secondary
phloem does not grow in thickness
because the primary and the older sec​-
ondary phloem present on the outer
side gets crushed with the development
of new functional phloem (Fig. 6.28 D).
Therefore, rings (annual rings) are not
produced in secondary phloem. The
crushed or non-functioning phloem
may, however, have fibres and
sclereids.

Secondary phloem is made up of the

same type of cells as are found in the
primary phloem (metaphloem)— sieve
tubes, companion cells, phloem fibres
 and phloem paren​chyma.

Phloem pairenchyma is of two types—

axial phloem parenchyma made up of
longitudinally arranged cells and
phloem ray parenchyma formed of
radially arranged parenchyma cells that
constitute the part of the vascular ray
present in the phloem.
Ad
ADVERTISEMENTS:

Here`s how I coped with my gray hair without

expensive procedures. Learn more.Read Next Story
ww.naturalcoloration.com | Sponsored

Yamaha Big Bike

Protection
Big Bike Tech

Premium Big Bike Protection and Luggage

Brand from Germany

The Korper-Kappe
How To Lose Weight Theory
Quickly
of Root Apex | Essay |
Botany
FruThin | Sponsored
Read Next Story Read Next Story
OPEN

Elements of secondary phloem show a

more regular arrangement. Sieve tubes
are comparatively more numerous but
are shorter and broader. Sclerenchyma
fibres occur either in patches or bands.
Sclereids are found in many cases. In
such cases secondary phloem is
differentiated into soft bast (secondary
phloem without fibres) and hard bast
(part of phloem with abundant fibres).

3. Secondary Xylem:

It forms the bulk of the stem and is

commonly called wood. The secondary
xylem consists of vessels, tracheids
(both tracheary elements), wood fibres
and wood parenchyma.

ADVERTISEMENTS:

Wood parenchyma may contain tannins

and crystals besides storing food. It is of
two types— axial parenchyma cells
arranged longitudinally and radial ray
parenchyma cells arranged in radial or
horizontal fashion. The latter is part of
vascular ray present in secondary
xylem.

Secondary xylem does not show

distinction into protoxylem and meta-
xylem elements. Therefore, vessels and
tracheids with annular and spiral
thicken​ings are absent. The tracheary
elements of secondary xylem are similar
to those of meta-xylem of the primary
xylem with minor differences. They are
comparatively shorter and more thick-
walled. Pitted thickenings are more
common. Fibres are abundant.

Width of secondary xylem grows with

the age of the plant. The primary xylem
persists as conical projection on its
inner side. Pith may become narrow
and ultimately get crushed. The yearly
growth of secondary xylem is distinct in
the areas which expe​rience two seasons,
one favourable spring or rainy season)
and the other un-favourable (autumn,
winter or dry summer).
In favourable season the temperature is
optimum. There is a good sunshine and
humidity. At this time the newly formed
leaves produce hormones which
stimulate cambial activity. The activity
decreases and stops towards the
approach of un-favourable sea​son.
Hence the annual or yearly growth
appears in the form of distinct rings
which are called annual rings (Fig.
6.30).

ADVERTISEMENTS:

Annual rings are formed due to

sequence of rapid growth (favourable
season, e.g., spring), slow growth
(before the onset of un-favourable
period, e.g., autumn) and no growth
(un-favourable season, e.g., winter).
Annual rings are not distinct in tropical
areas which do not have long dry
periods.
Annual Rings (Growth Rings). It is the
wood formed in a single year. It consists
of two types of wood, spring wood and
autumn wood (Fig. 6.31). The spring or
early wood is much wider than the
autumn or late wood. It is lighter in
colour and of lower density. Spring
wood consists of larger and wider xylem
elements.

The autumn or late wood is dark

coloured and of higher density. It
contains compactly arranged smaller
and narrower elements which have
comparatively thicker walls. In autumn
wood, tracheids and fibres are more
abundant than those found in the
spring wood.

The transition from spring to autumn

wood in an annual ring is gradual but
the transition from autumn wood to the
spring wood of the next year is sudden.
Therefore, each year’s growth is quite
distinct. The number of annual rings
corresponds to the age of that part of
the stem. (They can be counted by
increment borer).

Besides giving the age of the plant, the

annual rings also give some clue about
the climatic conditions of the past
through which the plant has passed.
Dendrochronology is the science of
counting and analysing annual growth
rings of trees.
Softwood and Hardwood:

Softwood is the technical name of

gymnosperm wood be​cause it is devoid
of vessels. Several of the softwoods are
very easy to work with (e.g., Cedrus,
Pinus species). However, all of them are
not ‘soft’. The softness depends upon
the content of fibres and vascular rays.
90-95% of wood is made of tracheids
and fibres. Vascular rays constitute 5-
10% of the wood.

Hardwood is the name of dicot wood

which possesses abundant vessels. Due
to the presence of vessels, the
hardwoods are also called porous
woods. In Cassia fistula and Dalbergia
sisso the vessels are comparatively very
broad in the spring wood while they are
quite narrow in the autumn wood. Such
a secondary xylem or wood is called
ring porous.

In others (e.g., Syzygium cumini) larger

sized vessels are distributed throughout
spring wood and autumn wood. This
type of secondary xylem or wood is
known as diffuse porous. Ring porous
wood is more advanced than diffuse
porous wood as it provides for better
translocation when the requirement of
the plant is high.

Sapwood and Heartwood:

The wood of the older stems (dalbergia,

Acacia) gets differentiated into two
zones, the outer light coloured and
functional sapwood or alburnum and
the inner darker and nonfunctional
heartwood or duramen (Fig. 6.33). The
tracheids and vessels of the heart wood
get plugged by the in growth of the
adjacent parenchyma cells into their
cavities through the pits. These
ingrowths are called tyloses (Fig. 6.32).

Ultimately, the parenchyma cells

become lignified and dead. Various
types of plant products like oils, resins,
gums, aromatic substances, essential
oils and tannins are deposited in the
cells of the heartwood. These
substances are collectively called
extractives. They provide colour to the
heartwood. They are also antiseptic.
The heartwood is, therefore, stronger
and more durable than the sapwood.

It is resistant to attack of insects and

microbes. Heart wood is commercial
source of Cutch (Acacia catechu),
Haematoxylin (Haematoxylon
campechianum), Brasilin (Caesalpinia
sappan) and Santalin (Pterocarpus
santalinus). Heart​wood is, however,
liable to be attacked by wood rotting
fungi. Hollow tree trunks are due to
their activity.

B. Formation of Periderm:

In order to provide for increase in girth

and prevent harm on the rupturing of
the outer ground tissues due to the
formation of secondary vascular tissues,
dicot stems produce a cork cambium or
phellogen in the outer cortical cells.
Rarely it may arise from the epidermis
(e.g., Teak, Oleander), hypodermis
(e.g., Pear) or phloem parenchyma.

Phellogen cells divide on both the outer

side as well as the inner side (bipolar)
to form secondary tissues. The
secondary tissue produced on the inner
side of the phellogen is parenchymatous
or collenchymatous. It is called
secondary cortex or phelloderm. Its
cells show radial arrangement.

Phellogen produces cork or phellem on

the outer side. It consists of dead and
com​pactly arranged rectangular cells
that possess suberised cell walls. The
cork cells contain tannins. Hence, they
appear brown or dark brown in colour.
The cork cells of some plants are filled
with air e.g., Quercus suber (Cork Oak
or Bottle Cork). The phelloderm,
phellogen and phellem together
constitute the periderm (Fig. 6.34).

Cork prevents the loss of water by

evaporation. It also protects the interior
against entry of harmful micro-
organisms, mechanical injury and
extremes of temperature. Cork is light,
compressible, nonreactive and
sufficiently resistant to fire.

It is used as stopper for bottles, shock

absorption and insulation. At places
phellogen produces aerating pores
instead of cork. These pores are called
lenticels. Each lenticel is filled by a
mass of somewhat loosely arranged
suberised cells called complementary
cells.

Lenticels:

Lenticels are aerating pores in the bark

of plants. They appear on the surface of
the bark as raised scars containing oval,
rounded or oblong depressions (Fig.
6.34 A). They occur in woody trees but
not in climbers. Normally they are
formed in areas with underlying rays
for facilitating gas exchange. Lenticels
may occur scattered or form longi​-
tudinal rows.

A lenticel is commonly produced

beneath a former stomate or stoma of
the epidermis. Its margin is raised and
is formed by surrounding cork cells.
The lenticel is filled up by loosely
arranged thin walled rounded and
suberised (e.g., Prunus) or un-suberised
cells called comple​mentary cells (Fig.
6.34 B).

They enclose intercellular spaces for

gaseous exchange. The complementary
cells are formed from loosely arranged
phellogen cells and division of sub-
stomatal parenchyma cells. The
suberised nature of complementary
cells checks excessive evaporation of
water.

In temperate plants the lenticels get

closed during the winter by the
formation of com​pactly arranged
closing cells over the complementary
cells.

Bark:

In common language and economic

botany, all the dead cells lying outside
phello​gen are collectively called bark.
The outer layers of the bark are being
constantly peeled off on account of the
formation of new secondary vascular
tissues in the interior. The peeling of
the bark may occur in sheets (sheets or
ring bark, e.g., Eucalyptus) or in
irregular strips (scaly bark).

The scaly bark is formed when the

phellogen arises in strips instead of
rings, e.g., Acacia (vem. Kikar). Bark
formed in early growing season is early
or soft bark. The one formed towards
end of growing season is late or hard
bark.

ADVERTISEMENTS:
Bark is insect repellent, decay proof,
fire-proof and acts as a heat screen.
Commercially it is employed in tanning
(e.g., Acacia), drugs (e.g., Cinchona—
quinine) or as spice (e.g., Cannamon,
vem. Dalchini). The cork of Quercus
suber is employed in the manufacture
of bottle stoppers, insulators, floats,
sound proofing and linoleum.

Significance of Secondary
Growth:

1. Secondary growth adds to the girth of

the plant. It provides support to
increasing weight of the aerial growth.

2. Secondary growth produces a corky

bark around the tree trunk that protects
the interior from abrasion, heat, cold
and infection.

3. It adds new conducting tissues for

replacing old non-functioning ones as
well as for meeting increased demand
for long distance transport of sap and
organic nutrients.

Anomalous Secondary Growth:

It is abnormal type of secondary growth

that occurs in some arborescent
monocots (e.g., Dracaena, Yucca,
Agave) and storage roots (e.g., Beet,
Sweet Potato). In arborescent monocot
stems, a secondary cambium grows in
hypodermal region. The latter forms
con​junctive tissue and patches of
meristematic cells. The meristematic
patches grow into sec​ondary vascular
bundles.

Anomalous vascular bundles also occur

in cortex (cortical bundles, e.g.,
Nyctanthes) and pith (e.g., Boerhaavia).
In storage roots (e.g., Beet), accessory
cambial rings appear on the outside of
endodermis. They produce less
secondary xylem but more secondary
phloem. The secondary phloem
contains abundant storage parenchyma.

Importance of Secondary Growth:

1. It is a means of replacement of old

non-functional tissues with new active
tissues.

2. The plants showing secondary

growth can grow and live longer as
compared to other plants.

3. It provides a fire proof, insect proof

and insulating cover around the older
plant parts.

4. Commercial cork is a product of

secondary growth. It is obtained from
Quercussuber (Cork Oak).

5. Wood is a very important product of

secondary growth. It represents
secondary xylem.

Related Articles:

1. Secondary Growth of Dicot Steam

 and Dicot Roots
2. Secondary Growth in Dicot Stem |
Botany

You May Like Sponsored Links by Taboola

Here`s how I coped with my gray hair

without expensive procedures. Learn more.
ww.naturalcoloration.com

How To Lose Weight Quickly

FruThin

Get Rid Of Dry And Red Eyes Using This

Simple Solution
OptiV

At 80, Al Pacino Lives Modest Life With His

Partner
Cash Roadster

High speed Wi-Fi booster takes Philippines

by storm
iBooster

The 'Matilda' Actress Is Now 32 - Take A

Deep Breath Before You See Her
Refinance Gold

Tapusin ang pagkawala ng buhok na may

simpleng trick. Subukan ito ngayon!
Asami
Getting an Online Job Might be Easier Than
You Think
Work at Home Jobs | Search Ads

Welcome to
BiologyDiscussion!
Our mission is to
provide an online
platform to help
students to share
notes in Biology.
This website
includes study
notes, research
papers, essays,
articles and other
allied information
submitted by
visitors like YOU.

Before sharing your

knowledge on this
site, please read the
following pages:

1. Content
Guidelines
2. Privacy Policy
3. TOS
4. Disclaimer
Copyright
Share Your
Knowledge

Share Your
Word File

Share Your
PDF File

Share Your
PPT File
QUESTIONS

Name the types of

nitrogenous bases
present in the DNA. 0
Answers
Which organelle is known
as “power house” of the
cell? 42 Answer

Give the examples of

blue green algae. 0
Answers

What is open and closed

type of circulatory
system? 0 Answers

How does XXY condition

arise in humans? 1
Answer

What is the world’s most

endangered animal? 0
Answers

What is meant by a
species? 0 Answers

What measures, as an
individual you would take
to reduce environmental
pollution? 2 Answer

What are the functions of

nucleus? 0 Answers

What is an ecosystem?
1 Answer

Why do you think that

carbohydrates are not
digested in the stomach?
0 Answers
What are the three
important stages of
aerobic respiration? 0
Answers

You Sponsored Links

May
Like
Here`s how I
coped with my
gray hair witho…
ww.naturalcoloration.com

How To Lose
Weight Quickly
FruThin

Get Rid Of Dry

And Red Eyes
Using This Simp…
OptiV

At 80, Al Pacino
Lives Modest Life
With His Partner
Cash Roadster
High speed Wi-Fi
booster takes
Philippines by…
iBooster
by Taboola

LATEST

Yeast: Origin,
Reproduction,
Life Cycle and
Growth
Requirements |
Industrial
Microbiology

How is Bread
Made Step by
Step? (With
Methods)|
Industrial
Microbiology

How is Cheese
Made Step by
Step:
Principles,
Production and
Process

Enzyme
Production and
Purification:
Extraction &
Separation
Methods |
Industrial
Microbiology

Fermentation
of Olives:
Process,
Control,
Problems,
Abnormalities
and
Developments

ABOUT
US

Home

Publish
your
Articles

Privacy
Policy

Contact
Us

SUGGEST
IONS

Report
Spelling
and
Gramm
atical
Errors

Suggest
Us
NEW
QUESTIO
NS AND
ANSWER
S AND
FORUM
CATEGOR
IES

Animal
Kingdom

Biodivers
ity

Biologica
l
Classific
ation

Biology
An
Introducti
on 11

Biology
An
Introducti
on
Biology
in
Human
Welfare
175

Biomolec
ules

Biotechn
ology 4
3

Body
Fluids
and
Circulatio
n

Breathin
g and
Exchang
e of
Gases

Cell 21
6

Cell-
Structure
and
Function

Chemical
Coordina
tion

Digestion
and
Absorpti
on
Diversity
in the
Living
World 1
25

Ecology
93

Ecosyste
m

Environ
mental
Issues

Evolution

Excretor
y System

Flowerin
g Plants

Food
Producti
on

Genetics
and
Evolution
110

Human
Health
and
Diseases

Human
Physiolo
gy 242

Human
Reprodu
ction

Immune
System

Living
World

Locomoti
on and
Moveme
nt

Microbes
in
Human
Welfare

Mineral
Nutrition

Molecual
r Basis of
Inheritan
ce

Neural
Coordina
tion

Nutrition

Organis
ms and
Populatio
n

Photosyn
thesis

Plant
Growth
and
Develop
ment

Plant
Kingdom

Plant
Physiolo
gy 261

Principle
s and
Processe
s

Principle
s of
Inheritan
ce and
Variation

Reprodu
ction 2
45

Reprodu
ction in
Animals

Reprodu
ction in
Flowerin
g Plants

Reprodu
ction in
Organis
ms

Reprodu
ctive
 Health

Respirati
on

Structura
l
Organisa
tion in
Animals

Transpor
t in
Plants

Trending
14

This is a question and answer forum for

students, teachers and general visitors for
exchanging articles, answers and notes.
Answer Now and help others.

Answer Now

Here's how it works:

 Anybody can Anybody can
ask a question answer

The best answers are

voted up and rise to the top
Forum Categories
Biological Biology An Biology An

Animal Kingdom Biodiversity Classification Introduction

11 Introduction

Biology in Human Body Fluids and Breathing and

Welfare 175 Biomolecules Biotechnology 43 Circulation Exchange of Gases

Cell- Structure and Chemical Digestion and Diversity in the

Cell
216 Function Coordination Absorption Living World
125

Environmental

Ecology 93 Ecosystem Issues Evolution Excretory System

Genetics and Human Health and Human

Flowering Plants Food Production Evolution 110 Diseases Physiology 242

Human Locomotion and Microbes in Human

Reproduction Immune System Living World Movement Welfare

Molecualr Basis of Organisms and

Mineral Nutrition Inheritance Neural Coordination Nutrition Population

Plant Growth and Plant Principles and

Photosynthesis Development Plant Kingdom Physiology 261 Processes

Principles of

Inheritance and Reproduction in Reproduction in Reproduction in

Variation Reproduction 245 Animals Flowering Plants Organisms

Structural

Organisation in

Reproductive Health Respiration Animals Transport in Plants Trending

14
What is diarrhoea? Mention its causes. Answer Now