THE PLANT TISSUESA group of cells with similar origin, structure and function is called tissue.The plant tissues are mainly of two types:1. Meristematic (dividing)2. Permanent (non-dividing)1. Meristematic tissues

Composed of immature or undifferentiated cells without intercellular spaces. The cells may be rounded, oval or polygonal; always living and thin walled. Each cell has abundant cytoplasm and prominent nuclei in it. Vacuoles may be small or absent.

Root apical meristem: occupies the tip of a root shoot apical meristem occupies the stem tip. primary meristems secondary or lateral meristemthey appear early in life of a plant appear later than primary meristemcontribute to the formation of the primary plant body

These are responsible for producing the secondary tissues.

occupies the tip of a root and shoot, at the base of nodes

occurs in the mature regions of roots and shoots

Both apical meristems and intercalary meristems Fascicular vascular cambium, interfascicular cambium and cork-cambium

2. Permanent tissues Permanent tissues are those in which growth has stopped either completely or for the time being. Cells of these tissues may be living or dead; and thin walled or thick walled. Thin walled permanent tissues are generally living whereas the thick walled tissues may be living or

dead.Types of permanent tissues(i) Simple tissues : Simple tissue is made up of only one type of cells. Common simple tissues are parenchyma, collenchyma and sclerenchyma (ii) Complex tissues : Complex tissue is made up of more than one type of cells working together as a unit. Common examples are xylem and phloem Simple Plant TissuesThere are three types of simple plant tissues 1. Parenchyma (Chlorenchyma and Aerenchyma)

2. Collenchyma3. Sclerenchyma

Complex tissues: The complex tissues are made of more than one type of cellsComplex tissues are mainly of two types :(i) Xylem (ii) Phloem

They are known as vascular tissues and form vasular bundles in roots and stems.Xylem:o conducts water and salts upward from roots to leaves. o It also provides mechanical strength to the plant parts.o Xylem is composed of

(a) Tracheids, (b) Vessels (c) Fibres (d) Xylem Parenchyma

Tracheids: o elongated or tube like cells o thick and lignified walls o tapering endso dead and without protoplasmo The inner layers of the cell walls have thickenings which vary in form. o In flowering plants, tracheids and vessels are the main water transporting elements.

Vessel: o long cylindrical tube-like structure made up of many cells called vessel members o lignified walls o a large central cavity o devoid of protoplasm o interconnected through perforations in their common walls o Gymnosperms lack vessels in their xylem. The presence of vessels is a characteristic feature of

angiosperms. Xylem fibres:

o highly thickened walls and obliterated central lumens. o These may either be septate or aseptate.

Xylem parenchyma:o cells are living and thin-walled o cell walls are made up of cellulose. o They store food materials in the form of starch or fat, and other substances like tannins. o The radial conduction of water takes place by the ray parenchymatous cells.

protoxylem metaxylemThe first formed primary xylem elements are called protoxylem

the later formed primary xylem is called metaxylem

In stems, the protoxylem lies towards the centre (pith)

In stems, metaxylem lies towards the periphery

In roots, the protoxylem lies towards periphery In roots, metaxylem lies towards the centrehaving narrow vessels and tracheids wider vessels and tracheids

Endarch Exarchwhen the protoxylem lies towards the centre and metaxylem lies towards the periphery Such arrangement of primary xylem is called endarch

when the protoxylem lies towards periphery and metaxylem lies towards the centre. Such arrangement of primary xylem is called exarch

e.g. In stems e.g. In roots

Phloem: o conducts food synthesised in the leaves to different parts of the plant.o Phloem is composed of

(a) Sieve Tubes (b) Companion Cells (c) Phloem Fibre (d) Phloem Parenchyma

Sieve tube elements: long, tube-like structures arranged longitudinally associated with the companion cells. end walls are perforated in a sieve-like manner called sieve plates. A mature sieve element possesses a peripheral cytoplasm and a large vacuole but lacks a nucleus. The functions of sieve tubes are controlled by the nucleus of companion cells.

companion cells : specialised parenchymatous cells The sieve tube elements and companion cells are connected by pit fields present in their walls. The companion cells help in maintaining the pressure gradient in the sieve tubes.

Phloem parenchyma : elongated, tapering cylindrical cells which have dense cytoplasm and nucleus cell wall is composed of cellulose plasmodesmatal connections exist between the cells.

The phloem parenchyma stores food material and other substances like resins, latex and mucilage. Phloem parenchyma is absent in most of the monocotyledons.

Phloem fibres (bast fibres) : made up of sclerenchymatous cells. elongated, unbranched and have pointed, needle like apices. The cell wall of phloem fibres is quite thick. dead At maturity These are generally absent in the primary phloem but are found in the secondary phloem. Phloem fibres of jute, flax and hemp are used commercially.

protophloem metaphloemThe first formed primary phloem The later formed phloemIt consists of narrow sieve tubes It consists of narrow sieve tubes

THE TISSUE SYSTEM:Epidermal Tissue System:

The epidermal tissue system forms the outer-most covering of the whole plant body. epidermal tissue system comprises-

o epidermal cellso stomata o epidermal appendages –

Trichomes hairs

I. epidermis: outermost layer of the primary plant body Epidermis is usually single layered It is made up of continuous and compactly arranged Epidermal cells. Epidermal cells are elongated, parenchymatous cells with a small amount of cytoplasm and a large

vacuole. II. Stomata: Stomata are present in the epidermis of leaves.

Structure: Each stoma is composed of guard cells.guard cells:

o These are two bean shaped cells or dumbbell shaped cells (In grasses) o The outer walls of guard cells (away from the stomatal pore) are thin and the inner walls

(towards the stomatal pore) are highly thickened.o The guard cells possess chloroplasts and regulate the opening and closing of stomata. o a few epidermal cells, in the vicinity of the guard cells become specialized in their shape and

size and are known as subsidiary cells. o The stomatal aperture, guard cells and the surrounding subsidiary cells are together called

stomatal apparatus Function: Stomata regulate the process of transpiration and gaseous exchange.

III. Cuticle: It is present outside the epidermis of leaf and stem but absent in roots. It is a waxy thick layer.

Function: It prevents the loss of water. IV. root hairs:

unicellular elongations of the epidermal cells Function: they absorb water and minerals from the soil. V. Trichomes: epidermal hairs On the stem are called trichomes which are-o usually multicellular o branched or unbranched o soft or stiff

function: o They may even be secretoryo The trichomes help in preventing water loss due to transpiration.

The Ground Tissue System All tissues except epidermis and vascular bundles constitute the ground tissue. It consists of simple tissues such as parenchyma, collenchymas and sclerenchyma. Parenchymatous cells are usually present in cortex, pericycle, pith and

medullary rays, in the primary stems and roots. In leaves, the ground tissue consists of thin-walled chloroplast containing

cells and is called mesophyll.

The Vascular Tissue System: The vascular system consists of complex tissues, the phloem and the xylem.vascular bundles: The xylem and phloem together constitute vascular bundles

open vascular bundles closed vascular bundlescambium is present between phloem and xylem

no cambium present

possess the ability to form secondary xylem and phloem tissues

they do not form secondary tissues

In dicotyledonous stems In monocotyledonous stems

conjoint vascular bundles radial vascular bundlesxylem and phloem are situated at the same radius of vascular bundles The conjoint vascular bundles usually have the phloem located only on the outer side of xylem.

When xylem and phloem within a vascular bundle are arranged in an alternate manner on different radii

in stems and leaves in roots

ANATOMY OF Dicotyledonous Root:(i) Epiblema :

Single, outermost layer of thin-walled cells. Some cells form unicellular root hairs. It protects and absorbs water.

(ii) Cortex : Large zone, multi layered thin-walled parenchymatous cells

intercellular spaces present stores food and water.

(iii) Endodermis : Innermost layer of cortex barrel-shaped, closely packed cells cells show band like thickenings of waterimpermeable, waxy material-suberin on their

radial walls called casparian strips. Some cells (opposite the protoxylem) which lack these strips are called passage cells. They help in the movement of water and dissolved salts from cortex directly into xylem. Stele : All tissues inner to endodermis comprise stele.

(iv) Pericycle : Inner to endodermis lies a single layer of pericycle. It is the seat of origin of lateral roots and vascular cambium during secondary growth.

(v) Vascular Bundle : radial vascular bundle Xylem is exarch root may be diarch (di-2 patches) to hexarch (hexa- 6 patches).

(vi) Pith : pith is absent or is small and parenchymatous.

(vii) Conjunctive parenchyma: Parenchyma which separates xylem and phloem.

Monocotyledonous Root:(i) Epiblema :

Outermost, single layer of thin-walled, closely packed cells.

Some cells are prolonged into unicellular root hairs.

(ii) Cortex : Large zone, multilayered, of parenchymatous cells

with intercellular spaces stores water and food material.

(iii) Endodermis : Innermost layer of cortex with characteristic

casparian strips and passage cells. Stele : All the tissue inner to endodermis is called

stele(iv) Pericycle :

Single layered, of thin walled cells. The lateral roots originate from this layer.

(v) Vascular Bundle : Closed bundles The xylem is exarch and polyarch (poly-many).

(vi) Pith : Lies in the center, large, well developed,

parenchymatous or sclerenchyamatous , stores food.

(vii) Conjunctive Parenchyma : Lies in between strands of xylem and phloem.

Differences between Dicot and Monocot root:

Characters Dicot root Monocot rootNumber of vascular Bundles 2-6 (di-hexarch) Many (polyarch)Pericycle Seat of origin of lateral roots,

vascular and cork cambiumSeat of origin of lateral roots only

Cambium Present AbsentSecondary growth Present AbsentPith Very small or absent Large

Dicotyledonous Stem(e.g. sunflower):

I. Epidermis – Outermost single layered, covered with cuticle, bears multicellular hairs, protective function.

II. Cortex – Inner to epidermis, there are three regions.

A. Hypodermis - 4-6 layers of collenchyma for mechanical support.B. Middle layers - Few layers of parenchyma.C. Endodermis - Innermost layer of cortex, has barrel shaped cells. As cells contain starch

grains, it is also called starch sheath. Stele - All the tissues lying internal to endodermis constitute the stele.

III. Pericycle – Inner to endodermis, multilayered, parenchymatous with patches of sclerenchyma.

IV. Vascular bundles – Arranged in a ring each vascular bundle is -

(a)conjoint (xylem and phloem together in one bundle),(b) collateral (xylem and phloem on the same radius with phloem towards the periphery) (c) open (cambium present in between xylem and phloem).

Xylem is endarch V. Medullary rays - Narrow regions of parenchymatous cells in between the vascular bundles.VI. Pith - The central parenchymatous zone with intercellular spaces.

Internal structure of monocot stem (e.g., maize):

1. Epidermis – Single layered, covered with cuticle, stem hairs absent.

2. Ground tissue- A mass of parenchymatous tissue. Only a few

peripheral layers below epidermis are sclerenchymatous called hypodermis.

3. Vascular bundle- Numerous, scattered in the ground tissue each

enclosed by sclerenchymatous bundle sheath. Each bundle is

(a) colateral (b) closed (no cambium strip between xylem and phloem) (c) endarch xylem.

Xylem occurs in the form of letter ‘Y ‘ innermost protoxylem disintegrates to form a

water cavity.

Dorsiventral (Dicotyledonous) Leaf:I. Epidermis –

Present on both upper(adaxial epidermis) and lower (abaxial epidermis) surface of leaf. lower epidermal cells form more stomata

II. Mesophyll – Consists of chloroplast - containing

parenchyma (chlorenchyma) and is responsible for carrying out photosynthesis.

It is differentiated into palisade and spongy cells.

III. Palisade cells – occur below upper epidermis. Cells are radially elongated, compactly arranged. Possess abundant chloroplasts

IV. Spongy cells – Occur below the palisade cells Cells irregular and loosely arranged - Contain

fewer chloroplasts Store gases in the inter cellular spaces

V. Vascular Bundles – They are collateral and closed In each bundle, xylem is located on dorsal and

phloem on ventral side Most vascular bundles are surrounded by colourless parenchyma called bundle sheath or border

parenchyma.

Isobilateral (Monocotyledonous) LeafI. Epidermis –

Present on both upper(adaxial epidermis) and lower (abaxial epidermis) surface of leaf.

stomata are present on both the surfaces of the epidermis

bulliform cells: In grasses, certain adaxial epidermal cells along the veins modify into large, empty, colourless cells called bulliform cells.

o When the bulliform cells in the leaves have absorbed water and are turgid, the leaf surface is exposed.

o When they are flaccid due to water stress, they make the leaves curl inwards to minimise water loss.

II. Mesophyll – Consists of chloroplast - containing parenchyma (chlorenchyma) and is responsible for

carrying out photosynthesis. mesophyll is not differentiated into palisade and spongy parenchyma Cells irregular and loosely arranged - Contain chloroplasts

Store gases in the inter cellular spacesIII. Vascular Bundles –

They are collateral and closed nearly similar sizes In each bundle, xylem is located on dorsal and phloem on ventral side Most vascular bundles are surrounded by colourless parenchyma called bundle sheath or

border parenchyma.

Secondary growth: The growth of the roots and stems in length with the help of apical meristem is called the primary

growth. In most dicotyledonous plants due to the activity of two lateral meristems, the increase in girth is

called the secondary growth. The tissues involved in secondary growth are the two lateral meristems: vascular cambium and

cork cambium. The tissues formed after secondary growth are known as secondary tissues

(i) Activity of vascular cambium It Forms secondary vascular tissue as follows-

The strip of cambium present in the vascular bundle is called Fascicular Cambium The cells of medullary rays adjoining the strip of vascular (Fascicular) cambium become

meristematic and form interfascicular cambium Both fascicular and inter-fascicular cambium join to form a continuous cambium ring Cambium divides and adds cells on internal side (towards pith) which mature into secondary

xylem and cells added towards external side (periphery ) mature into secondary phloem Amount of secondary xylem produced is more than secondary phloem 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

(ii) Activity of cork cambium-It Forms periderm as follows :

Cork cambium or phellogen develops in the cortex. Phellogen divides and adds cells on both the inner and the outer side. The inner cells differentiate into phelloderm or secondary cortex while outer cells into phellem

or cork Cork cells are compactly arranged and become dead and suberized (deposition of suberin) except

in regions of lenticels where cells are loosely arranged (complimentary cells) and non-suberized.

Through the lenticels woody branches and tree trunks can undergo gaseous exchange. Phellogen, phelloderm and phellem together constitute the periderm. Due to internal increase in thickness, periderm replaces the epidermis, becomes protective in

function. All the dead cells lying outside the active phellogen constitute the bark. 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. In Betula bhojpatra bark peels off like sheets of paper. Ancient manuscripts are still preserved on

them.

Cork tissue becomes very thick in Cork tree (Quercus suber) and is used commercially as, bottle-stoppers, insulators, shoe soles etc.

Wood: Wood is the secondary xylem produced by the activity of vascular cambium in dicot stem.Annual Rings (A secret to know the age of tree)

In temperate regions, the climatic conditions show seasonal variations. The activity of vascular cambium also becomes periodical as a result distinct growth layers are

formed in xylem. In spring season cambium is very active and produces a greater number of vessels with wider

cavities. The wood formed during spring is called early wood (or spring wood). In summer, cambium is less active and forms narrow vessels, this wood is called late wood (or

summer wood). These two kinds of woods in a transverse view appear as alternate concentric rings together

forming an annual ring. By counting the number of these rings we can know the age of a tree. Science dealing with

predicting the age of a tree by counting the annual rings is called as Dendrochronology.

Sap Wood and Heart Wood: Outer part of wood which is functional and consists of recently formed secondary xylem having

some living cells is called sap wood. As the plant ages in the central part of stem, the inner cells of sap wood becomes non-functional

and dark in colour it is called heart wood

Secondary Growth in Roots:Activity of vascular cambium:

Pericycle cells outside the protoxylem divide to form a strip of cambium Another strip of vascular cambium appears in the conjunctive tissue on the inner side of phloem

bundle These two vascular cambium strips join laterally to form a ring which may initially be wavy but later it becomes circular due to over production of secondary xylem tissue inner to primary

phloem Cambium cells divide and add cells on its either side Those added towards the periphery differentiate into secondary phloem and the ones formed

towards the center differentiate into secondary xylem. Secondary tissue formed outer to the protoxylem bundle differentiates into prominent primary

medullary ray thus, protoxylem does not get crushed

Activity of cork cambium: Later, cork cambium (Phellogen) also differentiates in the pericycle The cork cambium divides and gives rise to cork (Phellem) towards outside and secondary cortex

(Phelloderm) towards inside. All the three i.e. Phellogen, Phellem and Phelloderm together form the Periderm of the root and

has protective function. Finally all the primary tissue outside the developing cork (i.e. endodermis, cortex and epiblema) are

sloughed off.