From the last chapter, we recall that all livingorganisms are made of cells. In unicellularorganisms, a single cell performs all basicfunctions. For example, in Amoeba, a singlecell carries out movement, intake of food andrespiratory gases, respiration and excretion.But in multi-cellular organisms there aremillions of cells. Most of these cells arespecialised to carry out a few functions. Eachspecialised function is taken up by a differentgroup of cells. Since these cells carry out onlya particular function, they do it veryefficiently. In human beings, muscle cellscontract and relax to cause movement, nervecells carry messages, blood flows to transportoxygen, food, hormones and waste materialand so on. In plants, vascular tissues conductfood and water from one part of the plant toother parts. So, multi-cellular organismsshow division of labour. Cells specialising inone function are often grouped together inthe body. This means that a particularfunction is carried out by a cluster of cells ata definite place in the body. This cluster ofcells, called a tissue, is arranged and designedso as to give the highest possible efficiency offunction. Blood, phloem and muscle are allexamples of tissues.

A group of cells that are similar instructure and/or work together to achieve aparticular function forms a tissue.

6.1 Are Plants and Animals Madeof Same Types of Tissues?

Let us compare their structure and functions.Do plants and animals have the samestructure? Do they both perform similarfunctions?

There are noticeable differences betweenthe two. Plants are stationary or fixed – theydon’t move. Most of the tissues they have aresupportive, which provides them withstructural strength. Most of these tissues aredead, since dead cells can provide mechanicalstrength as easily as live ones, and need lessmaintenance.

Animals on the other hand move aroundin search of food, mates and shelter. Theyconsume more energy as compared to plants.Most of the tissues they contain are living.

Another difference between animals andplants is in the pattern of growth. The growthin plants is limited to certain regions, whilethis is not so in animals. There are sometissues in plants that divide throughout theirlife. These tissues are localised in certainregions. Based on the dividing capacity of thetissues, various plant tissues can be classifiedas growing or meristematic tissue andpermanent tissue. Cell growth in animals ismore uniform. So, there is no suchdemarcation of dividing and non-dividingregions in animals.

The structural organisation of organs andorgan systems is far more specialised andlocalised in complex animals than even in verycomplex plants. This fundamental differencereflects the different modes of life pursuedby these two major groups of organisms,particularly in their different feeding methods.Also, they are differently adapted for asedentary existence on one hand (plants) andactive locomotion on the other (animals),contributing to this difference in organ systemdesign.

It is with reference to these complexanimal and plant bodies that we will now talkabout the concept of tissues in some detail.

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Chapter

uestions1. What is a tissue?2. What is the utility of tissues in

multi-cellular organisms?

6.2 Plant Tissues6.2.1 MERISTEMATIC TISSUE

• From the above observations, answerthe following questions:1. Which of the two onions has longer

roots? Why?2. Do the roots continue growing even

after we have removed their tips?3. Why would the tips stop growing in

jar 2 after we cut them?

The growth of plants occurs only in certainspecific regions. This is because the dividingtissue, also known as meristematic tissue, islocated only at these points. Depending onthe region where they are present,meristematic tissues are classified as apical,lateral and intercalary (Fig. 6.2). New cellsproduced by meristem are initially like thoseof meristem itself, but as they grow andmature, their characteristics slowly changeand they become dif ferentiated ascomponents of other tissues.

Fig. 6.1: Growth of roots in onion bulbs

Activity ______________ 6.1• Take two glass jars and fill them with

water.• Now, take two onion bulbs and place

one on each jar, as shown inFig. 6.1.

• Observe the growth of roots in both thebulbs for a few days.

• Measure the length of roots on day 1,2 and 3.

• On day 4, cut the root tips of the onionbulb in jar 2 by about 1 cm. After this,observe the growth of roots in both thejars and measure their lengths eachday for five more days and record theobservations in tables, like the tablebelow:

Length Day 1 Day 2 Day 3 Day 4 Day 5

Jar 1

Jar 2

Q

Apical meristem is present at the growingtips of stems and roots and increases thelength of the stem and the root. The girth ofthe stem or root increases due to lateralmeristem (cambium). Intercalary meristem isthe meristem at the base of the leaves orinternodes (on either side of the node)on twigs.

Apical meristem

Intercalary meristem

Lateral meristem

Fig. 6.2: Location of meristematic tissue in plant body

Jar 1 Jar 2

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As the cells of this tissue are very active,they have dense cytoplasm, thin cellulosewalls and prominent nuclei. They lackvacuoles. Can we think why they would lackvacuoles? (You might want to refer to thefunctions of vacuoles in the chapter on cells.)

6.2.2 PERMANENT TISSUE

What happens to the cells formed bymeristematic tissue? They take up a specificrole and lose the ability to divide. As a result,they form a permanent tissue. This processof taking up a permanent shape, size, and afunction is called differentiation. Cells ofmeristematic tissue differentiate to formdifferent types of permanent tissue.

• Now, answer the following on the basisof your observation:1. Are all cells similar in structure?2. How many types of cells can

be seen?3. Can we think of reasons why there

would be so many types of cells?• We can also try to cut sections of plant

roots. We can even try cutting sectionsof root and stem of different plants.

6.2.2 (i) SIMPLE PERMANENT TISSUE

A few layers of cells form the basic packingtissue. This tissue is parenchyma, a type ofpermanent tissue. It consists of relativelyunspecialised cells with thin cell walls. Theyare live cells. They are usually loosely packed,

Trichome

Mucilaginous canalCuticle

Epidermis

Hypodermis

Cortex

Endodermis

Pericycle

PhloemCambium

Metaxylem

Protoxylem

Vascular bundlePith

Medullary ray

Xylem

Fig. 6.3: Section of a stem

Activity ______________ 6.2• Take a plant stem and with the help

of your teacher cut into very thin slicesor sections.

• Now, stain the slices with safranin.Place one neatly cut section on a slide,and put a drop of glycerine.

• Cover with a cover-slip and observeunder a microscope. Observe thevarious types of cells and theirarrangement. Compare it with Fig. 6.3.

so that large spaces between cells(intercellular spaces) are found in this tissue[Fig. 6.4 a(i)]. This tissue provides support toplants and also stores food. In somesituations, it contains chlorophyll andperforms photosynthesis, and then it is calledchlorenchyma. In aquatic plants, large aircavities are present in parenchyma to givebuoyancy to the plants to help them float.Such a parenchyma type is calledaerenchyma. The parenchyma of stems androots also stores nutrients and water.

TISSUES 71

Fig. 6.4: Various types of simple tissues: (a) Parenchyma (i) transverse section, (ii) longitudinal section;(b) Collenchyma (i) transverse section, (ii) longitudinal section; (c) Sclerenchyma (i) transverse section,(ii) longitudinal section.

b (i)

Wall thickenings

Nucleus

VacuoleCell wall

b (ii)

End wall

Primary cell wall(thickened at corners)

Chloroplast

Nucleus

Vacuole

Cytoplasm

Intercellular space

Cytoplasm

Nucleus

Middle lamella

Chloroplast

Vacuole

Intercellular space

Primary cell wall

a (ii)

a (i)

Intercellular spaces

Narrow lumen

Lignifiedthick wall

c (ii)

Simplepit pair

c (i)

The flexibility in plants is due to anotherpermanent tissue, collenchyma. It allowseasy bending in various parts of a plant (leaf,stem) without breaking. It also providesmechanical support to plants. We can find

this tissue in leaf stalks below the epidermis.The cells of this tissue are living, elongatedand irregularly thickened at thecorners. There is very little intercellularspace (Fig. 6.4 b).

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Yet another type of permanent tissue issclerenchyma. It is the tissue which makesthe plant hard and stiff. We have seen thehusk of a coconut. It is made ofsclerenchymatous tissue. The cells of thistissue are dead. They are long and narrow asthe walls are thickened due to lignin (achemical substance which acts as cement andhardens them). Often these walls are so thickthat there is no internal space inside the cell(Fig. 6.4 c). This tissue is present in stems,around vascular bundles, in the veins ofleaves and in the hard covering of seeds andnuts. It provides strength to the plant parts.

Activity ______________ 6.3• Take a freshly plucked leaf of Rheo.• Stretch and break it by applying

pressure.• While breaking it, keep it stretched

gently so that some peel or skinprojects out from the cut.

• Remove this peel and put it in a petridish filled with water.

• Add a few drops of safranin.• Wait for a couple of minutes and then

transfer it onto a slide. Gently place acover slip over it.

• Observe under microscope.

epidermis may be thicker since protectionagainst water loss is critical. The entiresurface of a plant has this outer covering ofepidermis. It protects all the parts of the plant.Epidermal cells on the aerial parts of the plantoften secrete a waxy, water-resistant layer ontheir outer surface. This aids in protectionagainst loss of water, mechanical injury andinvasion by parasitic fungi. Since it has aprotective role to play, cells of epidermaltissue form a continuous layer withoutintercellular spaces. Most epidermal cells arerelatively flat. Often their outer and side wallsare thicker than the inner wall.

We can observe small pores here and therein the epidermis of the leaf. These pores arecalled stomata (Fig. 6.5). Stomata areenclosed by two kidney-shaped cells calledguard cells. They are necessary forexchanging gases with the atmosphere.Transpiration (loss of water in the form ofwater vapour) also takes place throughstomata.

Think about which gas may be requiredfor photosynthesis.Find out the role of transpiration in plants.

Epidermal cells of the roots, whosefunction is water absorption, commonly bearlong hair-like parts that greatly increase thetotal absorptive surface area.

In some plants like desert plants,epidermis has a thick waxy coating of cutin(chemical substance with waterproof quality)on its outer surface. Can we think of a reasonfor this?

Is the outer layer of a branch of a treedifferent from the outer layer of a young stem?

As plants grow older, the outer protectivetissue undergoes certain changes. A strip ofsecondary meristem replaces the epidermisof the stem. Cells on the outside are cut offfrom this layer. This forms the several-layerthick cork or the bark of the tree. Cells ofcork are dead and compactly arrangedwithout intercellular spaces (Fig. 6.6). Theyalso have a chemical called suberin in theirwalls that makes them impervious to gasesand water.

Fig. 6.5: Guard cells and epidermal cells: (a) lateralview, (b) surface view

(a) (b)

Guardcell

Stomata

Epidermalcell

Guardcells

What you observe is the outermost layerof cells, called epidermis. The epidermis isusually made of a single layer of cells. In someplants living in very dry habitats, the

TISSUES 73

6.2.2 (ii) COMPLEX PERMANENT TISSUE

The different types of tissues we havediscussed until now are all made of one typeof cells, which look like each other. Suchtissues are called simple permanent tissue.Yet another type of permanent tissue iscomplex tissue. Complex tissues are made ofmore than one type of cells. All these cellscoordinate to perform a common function.Xylem and phloem are examples of suchcomplex tissues. They are both conductingtissues and constitute a vascular bundle.Vascular or conductive tissue is a distinctivefeature of the complex plants, one that hasmade possible their survival in the terrestrialenvironment. In Fig. 6.3 showing a section ofstem, can you see different types of cells inthe vascular bundle?

Xylem consists of tracheids, vessels,xylem parenchyma (Fig. 6.7 a,b,c) and xylemfibres. The cells have thick walls, and manyof them are dead cells. Tracheids and vesselsare tubular structures. This allows them totransport water and minerals vertically. Theparenchyma stores food and helps in thesideways conduction of water. Fibres aremainly supportive in function.

Phloem is made up of four types ofelements: sieve tubes, companion cells,phloem fibres and the phloem parenchyma[Fig. 6.7 (d)]. Sieve tubes are tubular cells withperforated walls. Phloem is unlike xylem inthat materials can move in both directions init. Phloem transports food from leaves to other

Cork cells Ruptured epidermis

Phloem

Fig. 6.6: Protective tissue

parts of the plant. Except for phloem fibres,phloem cells are living cells.

Xylem

Sieve plate

Sieve tube

Phloemparenchyma

Companion cell

Nucleus

CytoplasmPits

Pit

(a) Tracheid (b) Vessel (c) Xylem parenchyma

Fig. 6.7: Types of complex tissue

(d) Section of phloem

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uestions1. Name types of simple tissues.2. Where is apical meristem found?3. Which tissue makes up the husk

of coconut?4. What are the constituents of

phloem?

6.3 Animal TissuesWhen we breathe we can actually feel themovement of our chest. How do these bodyparts move? For this we have specialised cellscalled muscle cells (Fig. 6.8). The contractionand relaxation of these cells result inmovement.

During breathing we inhale oxygen. Wheredoes this oxygen go? It is absorbed in thelungs and then is transported to all the bodycells through blood. Why would cells needoxygen? The functions of mitochondria westudied earlier provide a clue to this question.Blood flows and carries various substancesfrom one part of the body to the other. Forexample, it carries oxygen and food to all cells.It also collects wastes from all parts of thebody and carries them to the liver and kidneyfor disposal.

Blood and muscles are both examples oftissues found in our body. On the basis ofthe functions they perform we can think ofdifferent types of animal tissues, such asepithelial tissue, connective tissue, musculartissue and nervous tissue. Blood is a type ofconnective tissue, and muscle formsmuscular tissue.

6.3.1 EPITHELIAL TISSUE

The covering or protective tissues in theanimal body are epithelial tissues. Epitheliumcovers most organs and cavities within thebody. It also forms a barrier to keep differentbody systems separate. The skin, the liningof the mouth, the lining of blood vessels, lungalveoli and kidney tubules are all made ofepithelial tissue. Epithelial tissue cells aretightly packed and form a continuous sheet.They have only a small amount of cementingmaterial between them and almost nointercellular spaces. Obviously, anythingentering or leaving the body must cross atleast one layer of epithelium. As a result, thepermeability of the cells of various epitheliaplay an important role in regulating theexchange of materials between the body andthe external environment and also betweendifferent parts of the body. Regardless of thetype, all epithelium is usually separated fromthe underlying tissue by an extracellularfibrous basement membrane.

Different epithelia (Fig. 6.9) show differingstructures that correlate with their uniquefunctions. For example, in cells lining bloodvessels or lung alveoli, where transportationof substances occurs through a selectively

Q

Fig. 6.8: Location of muscle fibres

Smooth muscle fibres

Smooth muscle fibre

(Cell)

Nucleus

TISSUES 75

permeable surface, there is a simple flat kindof epithelium. This is called the simple

squamous epithelium. Simple squamousepithelial cells are extremely thin and flat andform a delicate lining. The oesophagus andthe lining of the mouth are also covered withsquamous epithelium. The skin, whichprotects the body, is also made of squamousepithelium. Skin epithelial cells are arrangedin many layers to prevent wear and tear. Sincethey are arranged in a pattern of layers, theepithelium is called stratified squamousepithelium.

Where absorption and secretion occur, asin the inner lining of the intestine, tallepithelial cells are present. This columnar(meaning ‘pillar-like’) epithelium facilitatesmovement across the epithelial barrier. In therespiratory tract, the columnar epithelialtissue also has cilia, which are hair-likeprojections on the outer surfaces of epithelialcells. These cilia can move, and theirmovement pushes the mucus forward to clearit. This type of epithelium is thus ciliatedcolumnar epithelium.

Cuboidal epithelium (with cube-shapedcells) forms the lining of kidney tubules andducts of salivary glands, where it providesmechanical support. Epithelial cells oftenacquire additional specialisation as glandcells, which can secrete substances at theepithelial surface. Sometimes a portion of theepithelial tissue folds inward, and amulticellular gland is formed. This isglandular epithelium.

6.3.2 CONNECTIVE TISSUE

Blood is a type of connective tissue. Whywould it be called ‘connective’ tissue? A clueis provided in the introduction of this chapter!Now, let us look at this type of tissue in somemore detail. The cells of connective tissue areloosely spaced and embedded in anintercellular matrix (Fig. 6.10). The matrixmay be jelly like, fluid, dense or rigid. Thenature of matrix differs in concordance withthe function of the particular connectivetissue.

Take a drop of blood on a slide and observedifferent cells present in it under a microscope.

(a) Squamous

(d) Stratified squamous

(c) Columnar (Ciliated)

(b) Cuboidal

Fig. 6.9: Different types of epithelial tissues

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Blood has a fluid (liquid) matrix calledplasma, in which red blood cells (RBCs), whiteblood cells (WBCs) and platelets aresuspended. The plasma contains proteins,salts and hormones. Blood flows andtransports gases, digested food, hormonesand waste materials to different parts of thebody.

Bone is another example of a connectivetissue. It forms the framework that supportsthe body. It also anchors the muscles andsupports the main organs of the body. It is astrong and nonflexible tissue (what would bethe advantage of these properties for bonefunctions?). Bone cells are embedded in ahard matrix that is composed of calcium andphosphorus compounds.

Two bones can be connected to each otherby another type of connective tissue calledthe ligament. This tissue is very elastic. It hasconsiderable strength. Ligaments containvery little matrix. Tendons connect bones tomuscles and are another type of connectivetissue. Tendons are fibrous tissue with greatstrength but limited flexibility.

Another type of connective tissue,cartilage, has widely spaced cells. The solidmatrix is composed of proteins and sugars.Cartilage smoothens bone surfaces at jointsand is also present in the nose, ear, tracheaand larynx. We can fold the cartilage of theears, but we cannot bend the bones in ourarms. Think of how the two tissues aredifferent!

Areolar connective tissue is found betweenthe skin and muscles, around blood vesselsand nerves and in the bone marrow. It fillsthe space inside the organs, supports internalorgans and helps in repair of tissues.

Where are fats stored in our body? Fat-storing adipose tissue is found below the skinand between internal organs. The cells of thistissue are filled with fat globules. Storage offats also lets it act as an insulator.

6.3.3 MUSCULAR TISSUE

Muscular tissue consists of elongated cells,also called muscle fibres. This tissue isresponsible for movement in our body.

Reticular fibre Fibroblast

Macrophage

Collagen fibreMast cell Plasma cell

Fat droplet Nucleus

Adipocyte

(a)

(b)Haversian canal(contains blood vessels

and nerve fibres)

Canaliculus (containsslender process of bone

cell or osteocyte)

Hyaline matrix

Chondrocyte

Different whiteblood corpuscles

Neutrophil(polynuclearleucocyte)

BasophilEosinophil

Nucleus

Cytoplasm

Red bloodcorpuscle

Lymphocyte Monocyte Platelets

(c)

(d)

(e)

Fig. 6.10: Types of connective tissues: (a) areolartissue, (b) adipose tissue, (c) compactbone, (d) hyaline cartilage, (e) types ofblood cells

TISSUES 77

Muscles contain special proteins calledcontractile proteins, which contract and relaxto cause movement.

to bones and help in body movement. Underthe microscope, these muscles show alternatelight and dark bands or striations whenstained appropriately. As a result, they arealso called striated muscles. The cells of thistissue are long, cylindrical, unbranched andmultinucleate (having many nuclei).

The movement of food in the alimentarycanal or the contraction and relaxation ofblood vessels are involuntary movements. Wecannot really start them or stop them simplyby wanting to do so! Smooth muscles [Fig.6.11(b)] or involuntary muscles control suchmovements. They are also found in the iris ofthe eye, in ureters and in the bronchi of thelungs. The cells are long with pointed ends(spindle-shaped) and uninucleate (having asingle nucleus). They are also calledunstriated muscles – why would they becalled that?

The muscles of the heart show rhythmiccontraction and relaxation throughout life.These involuntary muscles are called cardiacmuscles [Fig. 6.11(c)]. Heart muscle cells arecylindrical, branched and uninucleate.

Compare the structures of different typesof muscular tissues. Note their shape,number of nuclei and position of nuclei withinthe cell.

6.3.4 NERVOUS TISSUE

All cells possess the ability to respond tostimuli. However, cells of the nervous tissueare highly specialised for being stimulatedand then transmitting the stimulus veryrapidly from one place to another within thebody. The brain, spinal cord and nerves areall composed of the nervous tissue. The cellsof this tissue are called nerve cells or neurons.A neuron consists of a cell body with anucleus and cytoplasm, from which long thinhair-like parts arise (Fig. 6.12). Usually eachneuron has a single long part, called the axon,and many short, branched parts calleddendrites. An individual nerve cell may be upto a metre long. Many nerve fibres boundtogether by connective tissue make upa nerve.

Nuclei

Striations

(a)Spindle shaped

muscle cell

Nucleus

(b)

(c)

Striations

Nuclei

Fig. 6.11: Types of muscles fibres: (a) striatedmuscle, (b) smooth muscle, (c) cardiacmuscle

We can move some muscles by consciouswill. Muscles present in our limbs move whenwe want them to, and stop when we so decide.Such muscles are called voluntary muscles[Fig. 6.11(a)]. These muscles are also calledskeletal muscles as they are mostly attached

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Nerve impulses allow us to move ourmuscles when we want to. The functional

Nucleus

Dendrite

AxonNerve ending

Cell body

combination of nerve and muscle tissue isfundamental to most animals. Thiscombination enables animals to move rapidlyin response to stimuli.

uestions1. Name the tissue responsible for

movement in our body.2. What does a neuron look like?3. Give three features of cardiac

muscles.4. What are the functions of areolar

tissue?

QWhatyou havelearnt• Tissue is a group of cells similar in structure and function.

• Plant tissues are of two main types – meristematic andpermanent.

• Meristematic tissue is the dividing tissue present in the growingregions of the plant.

• Permanent tissues are derived from meristematic tissue oncethey lose the ability to divide. They are classified as simple andcomplex tissues.

• Parenchyma, collenchyma and sclerenchyma are three typesof simple tissues. Xylem and phloem are types of complextissues.

• Animal tissues can be epithelial, connective, muscular andnervous tissue.

• Depending on shape and function, epithelial tissue is classifiedas squamous, cuboidal, columnar, ciliated and glandular.

• The different types of connective tissues in our body includeareolar tissue, adipose tissue, bone, tendon, ligament, cartilageand blood.

• Striated, unstriated and cardiac are three types of muscletissues.

• Nervous tissue is made of neurons that receive and conductimpulses.

Fig. 6.12: Neuron-unit of nervous tissue

TISSUES 79

Exercises1. Define the term “tissue”.

2. How many types of elements together make up the xylem tissue?Name them.

3. How are simple tissues different from complex tissues in plants?

4. Differentiate between parenchyma, collenchyma andsclerenchyma on the basis of their cell wall.

5. What are the functions of the stomata?

6. Diagrammatically show the difference between the three typesof muscle fibres.

7. What is the specific function of the cardiac muscle?

8. Differentiate between striated, unstriated and cardiac muscleson the basis of their structure and site/location in the body.

9. Draw a labelled diagram of a neuron.

10. Name the following.

(a) Tissue that forms the inner lining of our mouth.

(b) Tissue that connects muscle to bone in humans.

(c) Tissue that transports food in plants.

(d) Tissue that stores fat in our body.

(e) Connective tissue with a fluid matrix.

(f) Tissue present in the brain.

11. Identify the type of tissue in the following: skin, bark of tree,bone, lining of kidney tubule, vascular bundle.

12. Name the regions in which parenchyma tissue is present.

13. What is the role of epidermis in plants?

14. How does the cork act as a protective tissue?

15. Complete the table: