Anatomy of Flowering Plants

Study of internal structure of plant is called anatomy.

In plants cells are the basic unit.

Cells organized into tissues and tissues organized into organs.

Anatomy is studied with the help of microscope & is equivalent to histology.

Histology is study of tissues and their organization into organs and tissue systems.

Importance:

Gives information about the internal structure of plant.

Indicates simplicity or complexity of organs.

Homology & analogy can be known only through the study of internal structure

Helps in differentiating various groups of plants, like stem & leaves of dicots &

monocots.

Provides information about adaptations to diverse environments.

Anatomical studies can tell about adulteration, identification in forensic science and

quality of wood.

THE TISSUES :

A group of cells having common origin and perform one function.

Plant tissues are classified into two types:-

◦ Meristematic tissue.

◦ Permanent tissue

Meristematic Tissues:

Growth in plants is restricted to specific regions with active cell division called

meristems (Gk. meristos: divided).

Location of Meristems:

Depending on their location, the meristems are of three types, viz., apical, intercalary & lateral.

◦ Apical meristem: Occurs in the shoot and root tips.

Primary meristem

Increase the length of plant

◦ Intercalary meristem : Present in-between mature tissues.

Primary meristem

Occurs in grasses and regenerate parts removed by grazing.

◦ Lateral meristem: Occurs in the mature regions of roots and shoots.

Also known as secondary meristem.

Responsible for producing secondary tissues.

Fascicular vascular cambium, interfascicular cambium and cork cambium

are example of lateral meristem.

◦ Axillary bud :

◦ During formation of leaves and elongation of stem, some cells ‘left behind’ from

shoot apical meristem, constitute the axillary bud.

◦ Present in the axils of leaves and are capable for forming a branch or a flower.

Primary & Secondary Meristems:

Primary meristem is a meristematic tissue derived directly from embryonic meristem like

apical meristem & its left out parts in axillary buds and intercalary meristem. Fascicular

(intrafascicular) cambium is primary in origin but secondary in structure and function.

Secondary meristem is the one which has formed secondarily from permanent cells

through dedifferentiation – vascular cambium of root, cork cambium, wound cambium.

It gives rise to secondary tissues.

Characteristics of Meristematic Cells: Small, rounded/polygonal cells with no intercellular spaces

Cell walls thin and elastic

Nucleus large with dense cytoplasm

Central vacuole absent but small vacuoles may occur

Respiration & biosynthetic activities high. Food flow is high but there is no storage of

food.

Meristematic cells grow and undergo repeated divisions.

Functions of Meristems:

◦ Responsible for growth in the plant body by adding new cells followed by differentiation.

Differentiated cells form permanent tissues.

◦ Responsible for the formation of new organs like buds, flowers, leaves, etc.

◦ Help to heal the injured parts of the plant body.

Vegetative Shoot Apex: It has dome-shaped apical meristem surrounded by leaf primordial. According tunica-corpus

theory proposed by Schmidt (1924), consists of an outer layer of tunica (forms protoderm for

epidermal tissues) and inner mass of corpus (forms procambium for vascular tissues & ground

meristem for ground tissues). According to histogen theory of Hanstein (1868), apical

meristem produces three meristematic zones or histogens – outer dermatogens (forms

epidermis), middle periblem (forms tissues outside vascular strand) and central plerome (forms

vascular strand). Apical meristem of shoot broadens on the flanks at regular intervals to form

leaf primordial and their axillary buds. The interval between the formation of two successive

leaf primordial is known as plastochron.

Root Apex: It has a hemispherical apical meristem which appears subterminal due to presence of root cap

over it. Root apical meristem contains a small disc of low mititic activity towards the base which

is known as quiescent centre (Clowes, 1961). Quiescent centre is considered to be reserve

meristem. Root apical meristems gives rise to 3 – 4 types of initials or histogens:

i. Calyptrogen initials for root cap.

ii. Protoderm or dermatogens initials for epiblema.

iii. Periblem initials for cortex (and endodermis)

iv. Plerome initials for central cylinder of vascular strand, pericycle & pith, if present.

Permanent Tissues:

Following divisions of cells in both primary and as well as secondary meristems –

The newly formed cells become structurally and functionally specialised and lose the

ability to divide.

Such cells are termed permanent or mature cells and constitute the permanent tissues.

During the formation of the primary plant body, specific regions of the apical meristem

produce dermal tissues, ground tissues and vascular tissues.

Permanent tissues having similar structure and function are called simple tissues.

Permanent tissues having many different types of cells are called complex tissues.

Simple Tissues: A simple tissue is the one which is homogenous, made of only one type of cells, having a

common origin & function.

The various simple tissues in plants are parenchyma, collenchyma & sclerenchyma.

Parenchyma : Forms major component within organs.

Cells are isodiametric (having similar diagrams in all planes)

Thin cell wall made of cellulose.

Cells may be spherical, oval, round, polygonal or elongated shape.

Cells are closely packed or have small intercellular space.

Perform various functions such as photosynthesis, storage, secretion.

Based on structure & function, there are different types of parenchyma viz.

chlorenchyma, aerenchyma, storage parenchyma, xylem parenchyma, phloem

parenchyma & stellate parenchyma.

Chlorencyma – contain chloroplasts & is specialized for photosynthesis. Found

in mesophyll of leaves & are of two types viz. palisade parenchyma & spongy

parenchyma.

Aerenchyma – here the intercellular spaces have lot of air & is present in

hydrophytes, helps in buoyancy & respiration. Eg. Eichhornia

Storage parenchyma – cells store sugars, , protein granules, oil drops, etc. found

in cotyledons, endosperm of seeds, pulp of fruits, etc.

Xylem parenchyma & Phloem parenchyma – associated with xylem & phloem

respectively and help in conduction.

Stellate parenchyma – star shaped parenchymatous cells with large spaces found

in the petiole of Canna & banana.

Functions of Parenchyma:

1) Helps in storage of food, water & air.

2) Carries on vital activities like respiration, photosynthesis & conduction.

3) Capable of meristematic activity & can give rise to secondary meristems.

4) Aerenchyma gives buoyancy to aquatic plants.

5) Parenchyma cells associated with xylem & phloem help in conduction of water & food

materials respectively.

6) Parenchyma cells can dedifferentiate, acquire the power of division to form secondary

meristem which produce secondary tissues.

Collenchyma : Found either in homogeneous layer or in patches.

Cell wall thickened in the corner due to deposition of cellulose, hemicelluloses and

pectin. Cells are oval, spherical or polygonal in shape

Often contain chloroplasts.

No intercellular spaces.

Provide mechanical support to the growing part of the plant such as young stem and

petiole of a leaf.

Functions of Collenchyma:

1) Provides tensile support to the young stems & leaves during development, provides

elasticity to the plant organs, preventing tearing action of wind on dicot leaves.

2) Presence of chloroplasts helps it to carry out photosynthesis.

3) Collenchyma cells have active protoplasts capable of removing the extra wall thickenings

during dedifferentiation which happens during the formation of cork cambium in dicot

stems & in response to wound healing.

Sclerenchyma : Consists of long, narrow cells

Cell wall is thick and lignified.

Cell wall with few or numerous pits.

Cells are usually dead and without protoplast.

Provides mechanical support to the organs.

Sclerenchymas are of two types on the basis of origin, form & structure.

Fibres : Thick walled

Elongated and pointed cells

Generally occurs in group in various parts of the plant.

Develop directly from derivatives of meristamatic cells.

Occur in xylem, phloem,sheaths of vascular bundles, hypodermis, pericycle etc.

Provide mechanical strength against compression, pull, bending & shearing.

Sclereids : Spherical, oval or cylindrical in shape.

Highly thickened dead cells with very narrow cavities (lumen).

Occur singly or in small groups.

Formed from modification of parenchyma cells.

Provide stiffness & protection

Commonly found in fruit walls of nuts; pulp of guava, pear and sapota; seed coats of

legumes and leaves of tea.

Important types of sclereids are

1. Brachysclereids/Stone cells – short isodiametric sclereids with branched pits,

e.g., grit of pear, sapota

2. Macrosclereids/Rod cells – rod-like elongated sclereids with branched pits, e.g.,

testa of lrgume seeds.

3. Osteosclereids – bone-shaped sclereids (rod-like with swollen ends),

subepidermal region of legume seeds.

4. Astrosclereids – star-like branched sclereids, e.g., leaves of tea

5. Filiform sclereids – elongated sclereids with occasional branches, e.g., olea,

Brachysclereids Macrosclereids

Astrosclereids Filiform sclereids

Functions of Sclerenchyma:

1) Fibres provide strength & rigidity to the various organs of plants & enable them to

withstand various strains caused by external agencies without any damage to the inner

thin-walled cells, hence they are purely mechanical in support.

2) Sclerenchyma fibres, in the form of sheaths provide protection to internal parts of plant

orghans.

3) Sclereids provide hardness & mechanical strength to the plant organ where they are

present.

4) Sclereids provide toughness to the otherwise soft pul of fruits & provide efficient

protection to the seeds in the form of shells.

Comparison between the three types of simple tissues:

Complex tissues : Made of more than one type of cells and these work together as a unit.

Meant for conduction of water & food materials, hence are known as vascular tissues or

conducting tissues.

Xylem and phloem constitute the complex tissues in plants.

Xylem : (Gr. Xylos – wood)

Functions as conducting tissues for water and minerals from roots to stem and leaves.

Provides mechanical support to the plant.

Also called as wood.

It consists of four different kinds of elements:-

◦ Tracheids

◦ Vessels

◦ Xylem fibres and

◦ Xylem parenchyma.

Tracheids : Elongated or tube like cells.

Thick and lignified walls and tapering ends.

Cells are dead and without protoplasm.

Inner layers of cell walls have thickenings which vary in forms.

Arranged parallel to the long axis of the plant body.

Have an empty lumen through which the water passes without any obstruction.

Vessels: Is a cylindrical tube-like structure made up of many cells called vessel members,

which are short & wide without any end walls.

Each with lignified walls and a large central cavity.

Cells are devoid of protoplasm.

Vessel members are interconnected through perforations in their common walls.

Presences of vessels are the characteristics of the angiosperm

Xylem fibres :

Have highly thickened walls and obliterated central lumen.

Either septate or aseptate.

Xylem Parenchyma :

Cells are living and thin-walled.

Cell walls are made up of cellulose.

They stored food materials in the form of starch or fat.

Also store materials like tannins.

The radial conduction of water takes place by the ray parenchymatous cells.

The primary xylem is of two types-

Protoxylem Metaxylem.

The first formed xylem elements are called protoxylem, made up of

smaller & narrower tracheary elements having primitive type of

thickenings (annular, spiral & scalariform).

The later formed primary xylem is called metaxylem, which develops

after the stoppage of elongation. Tracheary elements are larger and wider

having scalariform, reticulate and pitted thickenings.

Based on the position of protoxylem in relaation to central axis of the

organ, xylem is of three types.

1. Endarch: the protoxylem lies towards pith and metaxylem towards

the periphery of the organ ( in stem)

2. Exarch: in root the protoxylem lies towards periphery and

metaxylem lies towards the centre.

3. Mesarch: Protoxylem is in the middle of xylem while metaxylem

is towards both outer & inner sides, as in leaves of ferns & cycads.

Phloem : (Gk. Phlos – bark)

Transports food materials usually from leaves to other part of plant.

Also called as bast as well as living conducting tissue.

It is composed of four elements:-

◦ Sieve tube elements.

◦ Companion cells.

◦ Phloem parenchyma.

◦ Phloem fibres. Sieve tube elements : Long tube like structure arranged longitudinally

Associated with companion cells.

End walls are perforated to form sieve plates & the perforations are called sieve pores

A mature sieve element possesses peripheral cytoplasm and a large vacuole but lacks

nucleus.

Cytoplasmic strands are continuous from one sieve element to the next through sieve

pores.

P-proteins are proteinaceous material seen in sieve tubes as tubules and act as damage

control.

The function of sieve elements controlled by nucleus of companion cells.

Companion cells : Specialized parenchymatous cells.

Closely associated with sieve tube elements.

Connected with sieve tube element by pit field.

Have dense cytoplasm & prominent nucleus.

Control the functioning of sieve tube elements.

Helps in maintaining pressure gradient in the sieve tubes.

Phloem parenchyma : Made up of elongated tapering cylindrical cells

Have dense cytoplasm and nucleus.

Cell wall made of cellulose and has pits. Plasmodesmatal connections exist between

the cells through the pits.

Store food materials and other substances like resins and latex and mucilages.

It is absent in monocotyledons.

Phloem fibres : Also known as bast fibres.

Made of sclerenchymatous cells.

Absent in primary phloem but present in secondary phloem.

Much elongated, unbranched and have pointed, needle like apices.

Cell wall is quite thick.

On maturity loose their protoplasm and become dead.

Phloem fibres of jute, flax and hemp are used commercially.

The first formed primary phloem consists of narrow sieve tubes and referred as

protophloem.

The later formed phloem has bigger sieve tubes and is referred to as metaphloem.

The Tissue System: On the basis of their development, structure and location Sachs (1975) has classified tissue

systems into three types viz.

Epidermal tissue system.

Ground or fundamental tissue system.

Vascular or conducting tissue system.

Epidermal tissue system : Forms the outermost covering of the whole plant body and comprises:

◦ Epidermal cells.

◦ Stomata

◦ Epidermal appendages like trichomes and hairs.

Epidermis (Gk. Epi – outer; dermis – skin) consists of single layer parenchymatous cells..

Cells are elongated, compactly arranged, which form continuous layer.

Epidermis is usually single layered. In some plants like leaf of Nerium & in epiphytic

roots of orchids it is multiple layered.

Outside the epidermis covered with waxy thick layer called cuticle.

Outer layer of root is known as epiblema/piliferous layer/rhizodermis.

Cuticle absent in epiblema of root.

Stomata are the structure present in the epidermis of leaf.

Stomata regulate the process of transpiration and gaseous exchange

Each stoma composed of two bean shaped cell called guard cells.

In grasses the guard cells are dumb-bell shaped.

Outer wall of guard cell is thin and inner wall is thick.

Guard cell possesses chloroplast and regulates the opening and closing of stomata.

Epidermal cells in the vicinity of guard cell called subsidiary cells.

Stomatal aperture, guard cells and subsidiary cells together called stomatal apparatus.

Based on the distribution of stomata on the leaf surface, they are of three types viz.,

o Hypostomatic – stomata only on lower surface eg. Most of the dicot plants

o Epistomatic – stomata only on the upper surface eg. Hydrophytic plants

o Amphistomatic – stomata found both on upper & lower surface eg. Grass, maize,

paddy, etc.

Epidermal Appendages:

Root hairs:

The root hairs are unicellular elongations of the epiblemal cells and help absorb water

and mineral from the soil.

Trichomes:

On stem the epidermal hairs are called trichomes.

Trichomes are usually multicellular.

May be branched or unbranched and soft or stiff.

Sometimes secretory. & are known as glandular hairs.

Glandular hairs producing sticky secretion are called colleters.

Trichomes help in preventing water loss due to transpiration.

Emergences:

Are multicellular appendages that contain some tissues from subepidermal region.

Most common emergences are prickles.

Functions: 1. Protection – Epidermal system protects the plant against mechanical injury, pathogens &

pests.

2. Water loss – Cuticular covering reduces the rate of transpiration.

3. Stomata – They regulate gaseous exchange and transpiration.

4. Insulation – Trichomes produce an insulating stationary layer of air.

5. Epiblema – Epiblema of root takes part in absorption of water and minerals.

6. Browsing – Stinging hair, prickles and silica protect the plants against browsing animals

7. Glandular hair – Produce various types of secretion.

The ground tissue system : All the tissues except epidermis and vascular bundles constitute the ground tissue.

It consists of simple tissues such as parenchyma, collenchyma, sclerenchyma, glandular

& laticifer tissues.

Different components of ground tissue system are –

Hypodermis – region found internal to epidermis & as an outer region to cortex,

consisting of one, two or few continuous or discontinuous layers of collenchymas

( in dicots) or sclerenchyma ( in monocots) can be protective or mechanical in

function.

General cortex – mainly parenchymatous, cells contain starch grains, oil, tannins

& crysrtals. Some cortical cells contain chloroplasts & are known as

chlorenchyma. In aquatic plants it consist of aerenchyma cells. Also special types

of cells like sclereids, resin ducts, oil glands, laticifers are found in this region.

Cortex helps in vital functions & storage.

Endodermis – represents the innermost layer of the cortex. Generally made up of

a single layer of livinf\g cells which are barrel shaped & arranged without

intercellular spaces. In stems, it stores starch & is known as starch sheath

(endodermoid layer). In root endodermis, the radial & tangential walls possess

special thickenings called casparian strips (discovered Caspary) which mainly

consist of suberin. The cells of the endodermis opposite the protoxylem elements

lack the deposition of suberin & are called passage cells. The function of

endodermis is to control the movement of water between cortex & xylem & also

helps to maintain the root pressure.

Pericycle – is a single layer with compactly arranged parenchyma cells present

internal to the endodermis. Represents the outermost layer of stele. In stems,

pericycle may be multilayered. In Cucurbita stem pericycle is multilayered

sclenrenchyma, whereas in Tridax it is multilayered with alternating patches of

parenchyma & sclerenchyma. Functionally, in roots pericycle gives rise to latral

roots & in dicot roots vascular cambium originates in the pericycl.

Pith & medullary rays – pith, also known as medulla is the central region of dicot

stem, dicot & monocot roots. It is generally made up of parenchyma cells. In

some cases the pith may be sclenrenchymatous or it is a hollow cavity as in some

hydrophytes. Main function of pith is storage of water & food. Parenchyma cells

between the vascular bundles in dicot stem are radially elongated & extend from

pith outwards & are known as medullary rays (pith rays), which help in radial

translocation of organic food and water.

In leaves, the ground tissue consists of thin-walled chloroplast containing cells called

mesophyll.

The vascular tissue system : Vascular system consists of complex tissues – xylem and phloem.

Xylem and phloem together constitute the vascular bundle.

In addition to xylem and phloem, sometimes there is another region known as cambium.

Based on the arrangement of xylem & phloem in a vascular bundle there are three types

viz. –

Radial vascular bundle: xylem and phloem form separate bundles & are

arranged alternately on different radii eg. Roots of seed plants.

Conjoint vascular bundle: xylem and phloem are present together in the same

bundle situated on the same radius. They are of two types viz. –

Collateral – Xylem & phloem lie together on the same radius, wherein

xylem lies towards the inner side & phloem towards the outside. In dicot

stem between xylem & phloem cambium is present and is known as open

type whereas in monocot cambium is absent between xylem & phloem

and is known as closed type.

Bicollateral – In some plants like the members of Cucurbitaceae family

xylem is sandwiched between outer & inner cambium & ploem i.e. outer

phloem, outer cambium, xylem, inner cambium & inner phloem and are

always open.

Concentric vascular bundle: Xylem is surrounded by phloem & is known as

amphicribal found in some of the ferns; or phloem is surrounded by xylem & is

known as amphivasal found in Dracaena & Yucca. Such type of vascular bundles

are known as concentric vascular bundle.

Anatomy of Dicot Root: The outermost layer is epiblema.

Presence of unicellular root hairs in epiblema.

The cortex constitutes many layer thin-walled parenchyma cells with intercellular spaces.

The innermost layer of cortex is endodermis.

Endodermis consists of single layered barrel-shaped cells without intercellular spaces.

Presence of casparian strip in the endodermis.

Next to endodermis there is few layer parenchymatous cells form pericycle.

Initiation of lateral root and vascular cambium during secondary growth takes place from

the cells of pericycle.

The parenchymatous cells present in-between xylem and phloem is called conjuctive

tissue.

The number of xylem and phloem bundle is three or four.

All the tissues on the inner side of endodermis such as pericycle, vascular bundles and

pith constitute the stele.

Anatomy of Monocot Root: Monocot root have similar tissues as in dicot except :-

◦ It contains more than six xylem bundles called polyarch.

◦ Pith is large and well developed.

◦ Do not undergo any secondary growth.

Anatomy of Dicot Stem: Outermost layer is epidermis.

Epidermis covered with thin layer of cuticle and has trichomes and few stomata.

The cells arranged in multiple layers in-between epidermis and pericycle constitute the

cortex.

Cortex has three sub-zones :

◦ Hypodermis: a few layers of collenchymatous cells below epidermis.

◦ Cortical layers: consists of rounded thin walled parenchymatous cells with

intercellular spaces.

◦ Endodermis: it is the innermost layer of cortex. Cells are rich in starch grains and

are referred to as starch sheath.

Pericycle : present on the inner side of the endodermis and above the phloem in the form

of semi-lunar patches of Sclerenchyma.

Medullary rays: a few layers of radially placed parenchymatous cells present in between

vascular bundles.

A large number of vascular bundles arranged in a ring.

Each vascular bundle is conjoint, open and endarch protoxylem.

The central portion of stem constitutes the pith.

Anatomy of Monocot Stem:

It has similar tissues with the dicot stem except in following-

◦ Sclerenchymatous hypodermis.

◦ Vascular bundles are scattered in the ground tissue.

◦ Each vascular bundle is covered by bundle sheath cells.

◦ Vascular bundles are conjoint and closed.

◦ Peripheral vascular bundles are smaller than central one.

◦ Phloem parenchyma is absent.

◦ Water containing cavities are present within the vascular bundles.

Anatomy of Dicot Leaf:

Dorsiventral (Dicotyledonous) Leaf : Vertical section of a Dorsiventral leaf shows three main parts:

Epidermis.

Mesophyll cells.

Vascular systems.

Epidermis covers both upper (adaxial) and lower (abaxial) surface of the leaf has a

conspicuous cuticle.

Upper epidermis is continuous whereas lower epidermis is discontinuous due to the

presence of stomata, such a condition is known as hypostomatic leaf.

Abaxial surface has more stomata than the adaxial epidermis.

Tissue between upper and lower epidermis called mesophyll.

Mesophyll cells are two types:

Palisade parenchyma

Spongy parenchyma Adaxially placed palisade parenchyma is made up of elongated cells arranged

vertically, parallel to each other.

Spongy parenchyma: oval or round and loosely arranged cells below the palisade

parenchyma.

Vascular bundles are seen in the midrib and veins.

Xylem is towards the upper epidermis & phloem towards lower. Cambium is absent.

The vascular bundles are surrounded by a layer of thick walled bundle sheath cells

which are parenchymatous.

Between the bundle sheath & the epidermal layers there is collenchymatous bundle

sheath extension.

Anatomy of Monocot Leaf: Isobilateral leaf :

It is similar with Dorsiventral leaf in many respect except –

Stomata are equally distributed on upper and lower epidermis, such

condition is known as amphistomatic leaf.

Mesophyll cells are not differentiated into palisade and spongy.

Vascular bundle is surrounded parenchymatous bundle sheath & there is

sclerenchymatous extension on either side.

In grasses, certain adaxial epidermal cells along the veins modified themselves into large,

empty, colourless cells called bulliform cells.

Causes rolling of leaves to reduce transpiration during water stress.

SECONDARY GROWTH :

Apart from primary growth the dicot plant exhibit an increase in girth is called secondary

growth. The tissues involved in secondary growth are two lateral meristem:

o Vascular cambium.

o Cork cambium.

Vascular cambium :

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.

Intrafascicular cambium and interfascicular cambium joined to form complete cambium

ring. The cambial ring becomes active and begins to cut off new cells both toward inner side

and outer side.

Cells produced toward pith mature into secondary xylem.

Cells produced towards periphery mature into secondary phloem.

Secondary xylem forms a compact mass, retaining primary xylem in the centre.

At some places, the cambium forms a narrow band of parenchyma, which passes through

secondary xylem and phloem is said to be secondary medullary rays.

Spring wood and autumn wood :

In temperate region during spring season, cambium becomes more active and produces a

large number of xylary elements having vessels with wider cavities. The wood formed in

spring is called spring wood or early wood.

In winter the cambium is less active and forms fewer xylary elements with narrow vessels

thus called autumn wood or late wood.

Spring wood is lighter in colour and lower density, where as the autumn wood is darker

and has higher density.

The two woods that appear as alternate concentric rings constitute an annual ring.

Annual rings seen in a cut stem give an estimate of the age of the tree. The determination

of the age of the plant by counting annual rings is called dendrochronology.

Heartwood and sapwood :

In old trees the secondary xylem is dark brown due to deposition of organic compounds

like tannins, resins, oils, gums, aromatic substances and essential oils in the central or

innermost layers of the stem.

These substances make it hard, durable and resistance to insect.

These regions comprise dead elements with high lignified walls and are called

heartwood or duramen. The heartwood never conducts water, provide mechanical support to plant.

The peripheral region of secondary xylem is lighter in colour and is known as sapwood

or alburnum. It involve in transport of water and minerals from root to leaf.

Cork cambium :

Cork cambium or phellogen developed usually in the cortex region.

Phellogen is a couple of layers in thick.

Made of narrow, thin walled and nearly rectangular cells.

Phellogen cut new cells to both inner and outer side.

The outer cells differentiated into cork or phellem.

The inner cells differentiated into secondary cortex or phelloderm.

The cork is impervious to water due to suberin deposition.

Phellogen, phellem and phelloderm are collectively known as periderm.

Bark is a non-technical term that refers to all tissues exterior to the vascular cambium,

there fore including secondary phloem and periderm.

At certain region the phellogen cuts off parenchymatous cells instead of cork.

These parenchymatous cells ruptured the epidermis and forms lens-shaped opening called

lenticels. Lenticels permit the gaseous exchange between the outer atmosphere and internal tissues

of the stem.

Lenticel Bark

Secondary growth in Roots :

In dicot root the vascular cambium is completely secondary in origin.

It originates from the tissue below the phloem, a portion of pericycle tissue, above the

protoxylem forming a complete and continuous wavy ring.

Later it becomes circular. Rather all events are similar with dicot stem.