Class 9 Science Chapter 3: Tissues in Action

Tissue made of actively dividing cells, and the source of all plant growth. Its cells are small, thin-walled, densely packed and generally without vacuoles, which is exactly what continuous rapid division requires.

The growth zone at the tips of roots and shoots. It is what makes a plant grow in length, which is why a root stops lengthening once its tip has been removed.

Actively dividing cells arranged as a ring within the stem. They produce new cells both inward and outward in a concentric fashion, so the stem increases in diameter, and the annual rings in a cut trunk record that work year by year.

Meristematic tissue at the base of an internode or just above a node. It accounts for grass growing back after mowing or grazing, and for a clipped hedge turning bushy.

The process by which cells produced by a meristem give up the ability to divide, change in structure and function, and become permanent tissue specialised for support, transport or storage.

Plants are mostly fixed in one place and need support to stay firm and upright, which the cell wall provides, while animals move and their cells, lacking a wall, change shape easily. They also feed differently: animals have tissues for digesting food obtained from other sources, while plants have tissues that use solar energy to synthesise food by photosynthesis.

A plant grows in length through the apical meristem at the tips of roots and shoots. It grows in girth through the lateral meristem arranged in a ring in the stem. It regrows after cutting or grazing through the intercalary meristem at the nodes.

Two bulbs are stood over jars of water and their root lengths measured for three days. On day three the root tips of one bulb are cut by about a centimetre while the other is left alone, and measuring continues. The cut roots stop growing while the untouched ones keep lengthening, showing that roots grow only from their tips.

Lateral meristem is a ring of actively dividing cells in the stem that produces new cells inside and outside in a concentric manner, increasing the diameter. Each year's addition appears as a ring on a cut trunk, and the width of a ring reflects whether that year's growing conditions were favourable or unfavourable.

Clipping removes the apical meristem, so the stems stop growing in length. New branches then arise from the nodes of the stem, driven by intercalary meristem, and the extra branching gives the hedge a bushy appearance.

They are small with thin cell walls, have a large prominent nucleus and dense cytoplasm containing many organelles, generally lack vacuoles, and are tightly packed with little or no intercellular space. Every one of these features supports continuous and rapid cell division, since storage space and thick walls would only get in the way.

Differentiation is the process by which meristematic tissue takes on specific jobs. Of the cells a meristem produces, some remain meristematic so division can continue, while others lose the ability to divide, change in structure and function, and become permanent tissues specialised for support, transport or storage.

Where the growth zone sits decides what the plant can withstand. Grass keeps its intercalary meristem at the nodes, low on the stem, so mowing or grazing removes the leaves but leaves the growth zone intact and the plant regrows. A plant relying only on apical meristem at the tips loses its growth in length the moment those tips are cut.

All plant growth depends on meristematic tissue, made of actively dividing cells, and it comes in three kinds matching the three ways a plant grows.

Apical meristem sits at the tips of roots and shoots and makes the plant grow in length. It shows itself when the root tips of an onion are trimmed: those roots stop lengthening while untouched roots beside them carry on growing.

Lateral meristem is arranged as a ring within the stem. Working in a ring, it lays down fresh cells both inward and outward, so the stem thickens. Its work is visible in the annual growth rings on a cut trunk, whose widths record the favourable and unfavourable years of the tree's life.

Intercalary meristem lies at the base of an internode or just above a node. It accounts for regrowth after cutting, and appears whenever grass grows back after mowing or grazing, or a clipped hedge turns bushy with new branches from its nodes.

Taken together the three explain everything a plant does as it gets bigger: apical adds height and root depth, lateral adds girth, and intercalary restores what has been removed.

The tissues of the two kingdoms differ because the organisms live in different ways, and each difference in living produces a difference in tissue.

Most plants are fixed in one place and cannot move away from anything, so they need support to stay firm and upright. The cell wall supplies that rigidity and strength. Animals, by contrast, generally move, though some such as sponges are immobile.

An animal cell has no rigid wall, so it changes shape easily. This cellular flexibility is ultimately what makes an animal body suitable for locomotion, since a cell unable to deform would be useless in a moving tissue.

The two also feed in quite different ways. An animal carries tissue for breaking down whatever it has eaten; a plant carries tissue for turning sunlight into the substances it needs. Each kingdom also has its own distinct tissues for transporting food and water around the body.

Finally, their growth patterns differ, because the tissues responsible for growth differ in structure and in function. In plants that means meristematic tissue concentrated at particular places, which is why a plant's growth is confined to tips, a ring in the stem, and the nodes.

Two glass jars are filled with water and an onion bulb stood on each so the root-bearing base is immersed. The roots are allowed to grow and their lengths measured on days one, two and three.

On day three the root tips of the bulb in Jar B are cut by about one centimetre, while the bulb in Jar A is left untouched. Measurement of both continues from day four onwards for four more days, and the readings are recorded in a table.

The results separate clearly. The roots in Jar A continue to grow in length throughout. The roots in Jar B stop growing once their tips have been removed.

Jar A is essential to the experiment even though nothing is done to it. It provides the comparison against which Jar B is judged, so that the difference can be attributed to the cutting rather than to the water, the temperature or the bulbs themselves. Without it, one could not say that cutting caused the change.

The conclusion is that roots grow only from their tips, because the tips consist of cells that divide continuously. Those growth zones at the tips of roots and shoots are the apical meristems, and they are what make a plant grow in length.

Cells of meristematic tissue share a characteristic appearance. They are small, with thin cell walls, a large prominent nucleus, and dense cytoplasm containing many organelles. There is rarely a vacuole to be seen, and the cells sit so close together that almost no gap is left between them.

These features are not accidental. Each one suits the tissue to continuous and rapid cell division: thin walls and small size make division quicker, dense cytoplasm supplies the machinery, and space that might have gone to a large vacuole is given over to working material instead.

Because it divides continuously, meristematic tissue keeps adding new cells to the plant body. What becomes of those cells then divides into two paths.

Some of the newly formed cells remain meristematic, which matters because it allows division to continue. If every daughter cell specialised, the meristem would exhaust itself and growth would stop.

The others lose the ability to divide. Form and duty both alter as they settle into permanent tissue, each kind thereafter doing one job — holding the plant up, carrying material about, or storing it. This process, by which meristematic tissue becomes specialised for particular work, is called differentiation.

A simple permanent tissue is built from one type of cell, as parenchyma, collenchyma and sclerenchyma each are. A complex permanent tissue is built from several types working together, which is what makes xylem and phloem complex.

A waxy layer of cutin covering the epidermis. It reduces water lost by transpiration, and also guards against mechanical injury and invasion by parasites. Plants of very dry habitats may carry a particularly thick one.

The evaporation of water vapour through the stomata. It is not simply loss: it creates the transpiration pull that draws water up the xylem, and it helps the plant eliminate wastes.

Two wall-thickening materials with opposite effects. Pectin thickens collenchyma corners and gives a rubbery flexibility, so the tissue bends. Lignin thickens sclerenchyma walls and makes them hard and strong, producing woody structure.

The larger grouping into which plant tissues are organised. There are three: dermal forming the outer covering, ground forming the main body between dermal and conducting tissues, and vascular consisting of xylem and phloem.

A simple permanent tissue is composed of only one type of cell, and parenchyma, collenchyma and sclerenchyma are the three examples. A complex permanent tissue is composed of more than one type of cell working together, and xylem and phloem are the two examples.

The epidermis wraps the entire plant as its outermost sheet, one cell deep, built of flat rectangular cells pressed close together. Over it lies a waxy coat of cutin, the cuticle. Between them they keep out physical damage, hold in water, bar harmful microorganisms and buffer the plant against harsh surroundings.

Transpiration is the evaporation of water vapour through the stomata in the leaf epidermis. It creates a transpiration pull in the xylem, which is what moves water upward through the plant, and it also helps in eliminating wastes from the plant body.

Parenchyma consists of living thin-walled cells, loosely packed with intercellular spaces, which store food and photosynthesise in green parts. Collenchyma consists of living cells with corners unevenly thickened by pectin, giving support with flexibility so stems and tendrils bend without breaking. Sclerenchyma has thick lignified walls making it hard and strong, and most of its cells are dead.

Xylem consists of tracheids, vessels, xylem parenchyma and xylem fibres. Xylem parenchyma is the only living component, since tracheids, vessels and xylem fibres are primarily sclerenchymatous.

Phloem is mostly made up of living cells and is built from sieve tubes, companion cells, phloem parenchyma and phloem fibres. Sieve tubes are long tubular cells joined end to end by perforated walls, and they transport food from the leaves to other parts of the plant. Companion cells regulate sieve tube function, phloem parenchyma stores food along with resin, tannins and latex, and phloem fibres provide strength.

The dermal tissue system forms the outer covering, protecting the inner parts and reducing water loss. The ground tissue system forms the main body of the plant between the dermal and conducting tissues, and includes parenchyma, collenchyma and sclerenchyma. The vascular tissue system consists of the conducting tissues, xylem and phloem.

In a desert, water is scarce, and a thick cuticle reduces the water lost as vapour through the stomata, which helps the plant survive. Underwater there is no shortage of water to conserve, so the thickness buys the plant nothing and only obstructs exchange with the surrounding water. A feature is an advantage only in the surroundings that suit it.

Parenchyma consists of living cells with thin walls, loosely packed so that intercellular spaces lie between them. Its main function is storing food, though in the green parts of a plant it also carries out photosynthesis, and in aquatic plants a specialised form creates the air spaces that help them float.

Collenchyma also consists of living cells, but their corners are unevenly thickened by deposits of pectin, a chemical that gives flexibility rather like rubber. The tissue therefore supplies support together with flexibility, letting parts such as stems and tendrils bend without breaking.

Sclerenchyma is different again. Its cells have thick walls laid down with lignin, which makes them hard and strong and produces the woody structure of the plant, and most of these cells are dead. It is found in stems, leaf veins and the hard coverings of seeds and nuts.

These differences explain why a fresh twig bends while a dry one breaks. The fresh twig is supported by flexible pectin-thickened collenchyma, whereas what remains in a dry twig is lignified sclerenchyma, hard and brittle.

The same contrast explains why the fibres of a coconut husk are hard and brittle while the leaf stalks of coriander are soft and flexible: the husk is sclerenchyma and the stalk is supported by collenchyma.

Both are conducting tissues, and both are called complex permanent tissues because each is made up of different types of cells working together, unlike the simple tissues which consist of a single cell type.

Water and dissolved minerals travel up from the roots through the xylem, which stiffens the plant into the bargain. Four kinds of cell make it up: tracheids, vessels, parenchyma and fibres. Tracheids and vessels are tubular and thick-walled, which suits them to conducting water while resisting collapse.

A detail worth keeping straight is that xylem parenchyma is the only living component of xylem, since tracheids, vessels and xylem fibres are primarily sclerenchymatous. So the tissue that lifts water up a tall tree is, for the most part, no longer alive.

Phloem, by contrast, is mostly made up of living cells. Here too there are four: sieve tubes, companion cells, parenchyma and fibres. Sieve tubes are long tubular cells joined end to end by perforated walls, and they transport food from the leaves to the rest of the plant.

The remaining components support that work. Companion cells, which are specialised parenchyma cells, regulate the functioning of the sieve tube cells and monitor the loading and unloading of sugars. Phloem parenchyma stores food materials along with resin, tannins and latex, and phloem fibres, being primarily sclerenchymatous, provide strength.

The epidermis forms the outermost layer of the plant body, a single tightly packed layer of flat rectangular cells covering all its parts. Being continuous, it shields the plant against mechanical injury, water loss, harmful microorganisms and extreme environmental conditions.

Over it lies a waxy layer of cutin called the cuticle, which reduces the water lost through transpiration and adds protection against mechanical injury and against invasion by parasites. In plants of very dry habitats this cuticle may be especially thick, cutting water loss further.

The epidermis is not uniform, however. In many plants hair-like projections arise from its cells, and in roots these are called root hair. They increase the surface area available for absorbing water and minerals from the soil, so a simple change in cell shape substantially increases what the plant can take up.

In leaves the epidermis carries pores called stomata. These permit gaseous exchange, and they are also where transpiration takes place, meaning the evaporation of water vapour from the plant.

Transpiration in turn does more than lose water. It creates a transpiration pull in the xylem, which is what moves water upward through the plant, and it also helps eliminate wastes from the plant body. The epidermis therefore protects, absorbs and exchanges all at once.

Plant tissues can be classified by function into protective, supporting and conducting tissues, but in a plant body these do not work alone. They are gathered instead into three larger assemblies, each called a tissue system.

The dermal system is the plant's outermost wrapping. It protects the inner parts and reduces water loss, and it is the epidermis with its cuticle that makes it up.

The ground tissue system forms the main body of the plant, lying between the dermal and the conducting tissues. It includes the three simple permanent tissues: parenchyma, collenchyma and sclerenchyma.

The vascular tissue system consists of the conducting tissues, namely xylem and phloem, which carry water and food respectively.

A transverse section of a sunflower stem shows all three at once. On the outside lie the epidermis, its cuticle and epidermal hair. Within that sits the ground tissue, where parenchyma, collenchyma and sclerenchyma can be told apart by their wall thickness and packing. Deeper still lie xylem and phloem, with lateral meristem between them, and once these can be named a stem section stops being a jumble of cells and becomes a readable map.

The tissue forming the body's outer covering and lining its internal organs. Its cells are closely packed with very little space between them, which keeps germs out and water in while still allowing absorption, secretion and the movement of substances.

Tissue that connects and supports other tissues. Blood, bone, cartilage, tendon and ligament all belong to this group despite looking nothing alike, because connecting and supporting is what they have in common.

The material in which the cells of a connective tissue sit. It explains why these tissues differ so much: blood's matrix is watery, soft and jelly-like, while bone's is hard, solid and rigid with calcium and phosphorus compounds.

Two connective tissues easily confused. A tendon connects muscle to bone and so brings about movement. A ligament connects bone to bone, providing stability, limiting movement and helping prevent dislocation.

The redness and swelling, sometimes with pus, seen at an infected area. It follows from white blood cells collecting there to deal with the infection.

It wraps the body as skin and continues inward as the lining of organs — mouth, lungs, blood vessels, intestine. Its cells sit hard against one another, leaving almost no gaps.

Because its cells are packed closely with almost no space between them, the tissue prevents the entry of germs and reduces water loss, while still permitting absorption, secretion and the movement of substances across it. A barrier letting nothing through would leave the body unable to feed or excrete, so the tissue has to do both jobs at once.

Exchange epithelium is a single layer of thin flat cells lining blood vessels and lungs. Protective epithelium has many layers with flat tightly packed outer cells and is found in skin, mouth and oesophagus. Absorptive epithelium is a single layer of tall pillar-like cells, often with hair-like structures, lining the small intestine.

A connective tissue is one that connects and supports other tissues. Blood joins the parts of the body by carrying nutrients, gases and hormones between them, while bones connect and support the body from head to toe. They differ in consistency only because of their matrix, which is watery and jelly-like in blood and hard, solid and rigid in bone.

Blood is 55 per cent plasma by volume, with the formed elements making up the remaining 45 per cent. The formed elements are red blood cells, which carry the iron-rich protein haemoglobin and live about four months, white blood cells, which collect at infected areas, and platelets, which help blood clot at an injury.

Bone has a rigid matrix containing calcium and phosphorus compounds, which gives it strength and rigidity along with support and protection. Cartilage has a soft, jelly-like matrix, which gives it flexibility and lets it cushion the ends of bones for shock absorption.

A tendon connects muscle to bone, and by carrying the muscle's pull it brings about movement. A ligament connects bone to bone, providing stability, limiting movement and helping to prevent dislocation. One makes movement happen and the other decides how far it may go.

A cut bleeds red and then clots, because haemoglobin in the red blood cells gives the colour and platelets bring about clotting at the injury. An infected area turns red and swollen because white blood cells collect there, causing pus and inflammation. During running the face reddens because working muscles need more oxygen, so breathing quickens and blood flow increases.

Epithelial tissue forms the outer covering of the body and lines its internal organs, and its different forms are structurally adapted to different work.

Exchange epithelium is a single layer of thin flat cells, found lining the blood vessels and lungs, where liquids and gases must diffuse rapidly. Protective epithelium instead has many layers, with the outer cells flat and tightly packed, and is found in skin, mouth and oesophagus, where mechanical injury, friction and microbes are the danger.

Secretory epithelium consists of cells specialised for producing and releasing substances, which may be cuboidal or columnar. It is found in salivary glands, sweat glands and the stomach lining, and produces mucus, enzymes, hormones, sweat and saliva.

Sensory epithelium is made of specialised receptor cells carrying hair-like cilia, and is found in the nostrils, taste buds and inner ear, serving smell, taste, sound and balance.

Absorptive epithelium is a single layer of tall pillar-like cells, often bearing hair-like structures, and lines the small intestine for efficient uptake of nutrients and water.

The pattern is consistent enough to reason from. Where material must cross quickly the barrier is one thin cell deep; where wear is the threat it is many layers thick; where uptake matters the cells are tall and carry projections that add surface area.

A connective tissue is one that connects and supports other tissues. Blood does this by transporting nutrients, gases and hormones between different parts of the body, and bone does it by connecting and supporting the body from head to toe.

This definition is about function rather than appearance, which is why tissues as unalike as blood and bone belong together. Blood is fluid and bone is hard, yet both connect and support.

The explanation for that difference lies in the matrix, the material in which a connective tissue's cells sit. In blood the matrix is watery, soft and jelly-like, which is what makes the tissue flow. In bone the matrix is hard, solid and rigid, containing calcium and phosphorus compounds, and that gives bone its strength and rigidity.

Cartilage makes the point again from a third direction. Its matrix is soft and jelly-like, so although it is a supporting tissue like bone, it gives flexibility and cushions the ends of bones, absorbing shock rather than bearing weight rigidly.

So the matrix is what allows one family of tissue to supply a skeleton with rigid parts that bear load, flexible parts where bones meet, and a fluid that reaches every corner of the body.

Blood divides by volume into plasma, which is 55 per cent of the total, and the formed elements, which are the remaining 45 per cent and consist of red blood cells, white blood cells and platelets.

The red colour of blood comes from haemoglobin, an iron-rich protein carried in the red blood cells. Those cells live for about four months and are replaced regularly, which is why the body must keep producing them throughout life.

When you get a small cut, red blood oozes out and a clot forms after some time. The clotting is the work of the platelets, which act at the site of the injury.

When you get a skin infection the area turns red and slightly swollen, and you may run a fever. This happens because white blood cells collect at infected areas, causing pus formation and the inflammation you can see.

When you exercise or run, you breathe faster and your face may turn red. Muscle at work calls for more oxygen, so the breath quickens and the blood moves faster; what shows in the face is that faster blood.

All four are connective tissues, and each makes a distinct contribution.

Bone provides strength, support and protection, its rigid matrix of calcium and phosphorus compounds letting it bear load without deforming. Without it there would be no framework for the joint at all.

Cartilage has a soft, jelly-like matrix and cushions the ends of the bones where they meet, absorbing shock and providing flexibility. Without it the bone ends would meet rigidly and wear against one another.

Tendons connect muscle to bone, and by carrying the pull of a contracting muscle across to the skeleton they bring about movement. You can feel this by wiggling your fingers and noticing the movement in your forearm, where the muscles actually lie. Without tendons the muscle's effort would reach nothing.

Ligaments connect bone to bone, providing stability, limiting movement and helping prevent dislocation. Raise your leg and the knee stops at a definite limit, which is a ligament at work. Without them the joint would move freely past safe limits and dislocate.

The joint therefore depends on four tissues doing four different jobs: a rigid frame, a cushion between the surfaces, a means of producing movement, and a restraint on how far that movement goes.

Voluntary movements are under conscious control, such as running or writing, and are carried out by skeletal muscle. Involuntary movements happen automatically without conscious control, such as food moving along the intestine or the heart beating.

The long cylindrical cell from which skeletal muscle is built. These fibres are unbranched, carry many nuclei, and show the light and dark banding described as striation.

Muscle found only in the heart. Its fibres are cylindrical and branched, carry a single nucleus and show faint striations, and it contracts tirelessly and rhythmically so the heart beats throughout life without tiring.

The body's control and coordination network. The brain acts as its control centre, coordinating activities, memory and responses, and muscles of every kind depend on instructions carried by this tissue.

The cell of nervous tissue, specialised to receive, process and transmit messages. It has a cell body holding the nucleus, dendrites that receive signals, and an axon that carries messages away to its terminals.

Voluntary movements are under conscious control, such as running, writing or lifting objects, and are carried out by skeletal muscles attached to the skeleton. Involuntary movements occur automatically without conscious control, such as the movement of food in the intestine or the beating of the heart.

Bundled cells, long and tube-like, go by the name of muscle fibres. They do not fork, they carry several nuclei each, and they show banding that alternates light with dark.

It is found in organs such as the stomach and intestines. Its cells are spindle-shaped, carry a single nucleus and lack striations. It produces slow, continuous movements such as those involved in digestion.

This muscle occurs in the heart and nowhere else. Its fibres are tube-like and fork apart, each holding one nucleus, with banding so faint it is easily overlooked. What sets it apart is stamina: beat after beat in steady rhythm for a whole lifetime, and it never tires.

Smooth muscle is found in organs such as the stomach and intestines while cardiac muscle is found only in the heart. Smooth muscle cells are spindle-shaped whereas cardiac fibres are cylindrical and branched. Smooth muscle lacks striations entirely while cardiac muscle shows faint ones.

Nervous tissue is the network through which the body is controlled and kept in step, and the brain sits at its centre, holding together what the body does, what it remembers and how it reacts. Snatching a hand off something hot is that network working at speed.

No muscle of either kind acts on its own account: the order always arrives from nervous tissue. Even the heart, which beats without any conscious decision, is instructed to beat faster by the brain during exercise so that the body's increased oxygen demand can be met.

The cell body contains the nucleus and controls the cell's activities. The dendrites receive signals from other neurons. The axon is a long fibre carrying messages away from the cell, ending at the axon terminals, which transmit those messages to other cells.

Skeletal muscle is attached to the skeleton and produces voluntary movements under conscious control, such as running, writing or lifting. Its cells are long cylinders, gathered in bundles, unbranched, carrying many nuclei apiece, and crossed by obvious bands.

Smooth muscle produces involuntary movements and is found in organs such as the stomach and intestines. Its cells are spindle-shaped with a single nucleus and no striations at all, and it is suited to slow, continuous movements such as digestion.

Cardiac muscle is found only in the heart. Its fibres are cylindrical and branched with a single nucleus and faint striations, and it works tirelessly and rhythmically so that the heart beats throughout life without fatigue.

A common misconception is that striation goes with voluntary control, so that any involuntary muscle would be unstriated. Cardiac muscle disproves this: it is involuntary and still shows faint striations. Only smooth muscle lacks them entirely.

The practical lesson is to check each feature separately rather than assuming they travel together. Branching, nucleus count and striation between them identify any of the three.

Nervous tissue forms the body's network for control and coordination. The brain acts as its control centre, coordinating activities, memory and responses across the whole body, which is why it accounts both for pulling a hand quickly from something hot and for remembering the words of a song learned long ago.

The cells of nervous tissue are called neurons, or nerve cells, and they are specialised to receive, process and transmit messages. Each has three main parts.

The cell body contains the nucleus and controls the cell's activities. The dendrites receive signals from other neurons. The axon is a long fibre that carries messages away from the cell and ends at the axon terminals, which transmit those messages on to other cells.

This arrangement is deliberately asymmetrical, with signals entering through the dendrites and leaving along the axon, and that asymmetry is what allows information to travel in one direction through a network of neurons.

Muscles depend on this tissue completely. Both voluntary and involuntary muscles cannot function independently and receive their instructions from nervous tissue, which is why the brain can signal the heart to beat faster during exercise to meet an increased demand for oxygen.

Three features settle the question between the three muscles, and each should be checked separately rather than assumed from the others.

First look at shape and branching. A sample of long cylindrical fibres that are unbranched points to skeletal muscle. Cylindrical fibres that branch point to cardiac muscle. Cells that are spindle-shaped, tapering at both ends, point to smooth muscle.

Next count the nuclei. Skeletal muscle fibres are multinucleate, carrying many nuclei in each cell. Both smooth and cardiac cells have a single nucleus each, so this feature separates skeletal from the other two at once.

Then look for striations. Skeletal muscle shows clear light and dark bands, cardiac muscle shows faint ones, and smooth muscle shows none at all.

Putting these together: unbranched, multinucleate and clearly striated identifies skeletal muscle, which came from a muscle attached to the skeleton. Branched, single-nucleus and faintly striated identifies cardiac muscle, which can only have come from the heart. Spindle-shaped, single-nucleus and unstriated identifies smooth muscle, which came from an organ such as the stomach or intestine.

It is worth noting that the two involuntary muscles differ in striation, so a sample cannot be identified from its control type alone.

The two tissues are studied side by side because neither accomplishes much alone. Muscle supplies force but chooses nothing, while nervous tissue decides and instructs but moves nothing itself.

Nervous tissue forms the body's control and coordination network, with the brain as its control centre, coordinating activities, memory and responses. Muscles of every kind depend on it, since neither voluntary nor involuntary muscles can function independently.

In a voluntary action the link is obvious. Deciding to lift something is a decision made in the nervous system, and skeletal muscles attached to the skeleton then supply the force that carries it out.

Involuntary action shows the same dependence rather less obviously. The heart beats without any decision on your part, yet during exercise the brain signals it to beat faster in order to meet the body's increased demand for oxygen. Involuntary, in other words, does not mean unsupervised.

A reflex such as pulling your hand from a hot surface makes the partnership clearest of all. The signal is received, processed and a response transmitted, and muscle carries out that response, all before any conscious decision could have been reached.

Bones, muscles, joints, cartilage, tendons and ligaments working together. It lets us stand upright, move, maintain posture and protect delicate organs, and it operates under the control of the nervous system.

A junction where two or more bones meet. Joints allow movement but cannot move the bones on their own; the force must come from muscles pulling through tendons.

A joint in which a rounded bone end sits in a shallow hollow, as the top of the upper arm bone does in the shoulder bone. It permits forward, backward, sideways and circular movement, the freest range of any joint.

A joint at which the bones cannot move, as between the flat bones of the skull. Immobility is the point: it keeps the brain safe even while the rest of the body moves.

The cushion lying between each pair of vertebrae in the backbone. It absorbs shock and permits bending and twisting without injury to the spinal cord running inside the column.

Six things make it up: bones, muscles, joints, cartilage, tendons and ligaments. Between them they hold us up, carry us about, keep us in position and shield the fragile organs inside — all of it directed by the nervous system.

Movement begins with a muscle hauling on a bone. It cannot grip the bone itself, so tough pliant straps called tendons make the connection, and as the muscle shortens the tendon passes that pull along to the bone. What shows on the outside is the joint moving.

Wherever two bones or more come together, the meeting is called a joint. Joints allow movement but cannot move the bones on their own, because they only determine what kind of movement is possible. The force must come from muscles contracting and pulling through tendons.

At the shoulder the upper arm bone ends in a ball-like head that seats in a shallow cup on the shoulder bone. The arm can therefore travel forward, back, out to the side and right round in a circle. Shoulder and collarbone together make the shoulder girdle, which is what anchors the arm to the skeleton.

A hinge joint bends and straightens in a single direction only, like a door hinge, and is found at the elbow and the knee, where the kneecap protects it. A pivot carries the skull on the backbone, turning the head left and right much as a knob turns in its plate.

Fixed joints lock the skull bones in place so that nothing shifts. Being unable to move is the whole point: the brain stays undisturbed however much the body moves. What encloses it is a rigid box of flat bones, guarding eyes and ears along with it.

A bendable column running down from the base of the skull, assembled from many small bones known as vertebrae, which props the body up and keeps us on our feet. A pad of cartilage between each pair lets the column bend and twist while the spinal cord threaded through it comes to no harm.

Twelve pairs of ribs form a protective cage around vital organs such as the heart and lungs. The ribs attach to the spine at the back and the breast bone, or sternum, in front, and are joined by flexible cartilage. That flexibility lets the cage expand and contract during breathing, increasing and decreasing the space in the chest so air can move in and out of the lungs.

Wherever two bones or more come together, the meeting is called a joint. Joints allow movement but cannot move the bones on their own, and what movement is possible at any joint is decided by its structure.

The shoulder is the ball and socket: the upper arm bone ends in a rounded head, and that head sits in a shallow cup on the shoulder. This arrangement permits forward, backwards, sideways and circular movement, the widest range in the body.

A hinge joint is found at the elbow and the knee. It bends and straightens in a single direction only, like a door hinge, and at the knee a small bone called the kneecap protects the joint.

A pivot carries the skull on the backbone, turning the head left and right much as a knob turns in its plate. You can feel this by shaking your head and placing a hand at the back of your neck.

Fixed joints connect the flat bones of the skull, and there the bones cannot move at all. This is not a limitation but the purpose: it keeps the brain safe even while the body moves.

So structure sets the vocabulary of movement. A single dance pose may use three of these joints at once, each contributing only what its own structure permits.

The musculoskeletal system is built from bones, muscles, joints, cartilage, tendons and ligaments, and it works under the control of the nervous system.

Movement begins with instruction. Muscles cannot act independently, so the nervous system determines when a muscle contracts.

The muscle then supplies the force, but it cannot grip a bone directly. Muscles are attached to bones by strong, flexible bands called tendons, and when a muscle contracts the tendon transmits that force to the bone.

The movement itself appears at a joint, the junction between two or more bones. The type of joint decides what movement is possible: a ball and socket at the shoulder permits circular movement, while a hinge at the elbow permits only bending and straightening.

Two more structures shape the outcome. Cartilage cushions the ends of the bones where they meet, absorbing shock, and ligaments connect bone to bone, providing stability, limiting movement and helping prevent dislocation.

So a single movement involves the nervous system deciding, muscle pulling, tendon transmitting, joint permitting, cartilage cushioning and ligament restraining, with each structure contributing something none of the others could.

The skeletal system is a framework of bones providing strength and protecting delicate internal organs, and it includes the skull, the vertebral column and the rib cage.

Running down from the underside of the skull, the column bends where it needs to, being built of a run of small bones, the vertebrae, which between them bear the body's weight and hold it erect; and the spinal cord is threaded down inside.

This sets a difficulty. A column stiff enough to support a body would not bend, and a column that bent freely might injure the cord inside. The solution is a cartilage disc between each vertebra, acting as a cushion and allowing flexibility so we can bend and twist without injuring the spinal cord.

The rib cage faces the same problem in a different form. Twelve pairs of ribs protect vital organs such as the heart and lungs, which calls for a firm enclosure, yet breathing requires the chest to change in volume.

Here the answer is flexible cartilage joining the ribs to the spine at the back and the breast bone, or sternum, in front. That flexibility lets the cage expand and contract during breathing, increasing and decreasing the space in the chest so air can move in and out of the lungs, which is also why an injury to the ribs makes breathing painful and difficult.

Bones, muscles and joints have to last a lifetime, and four habits keep them in condition: maintaining correct posture, proper nutrition, regular exercise and yoga.

Together these keep bones strong, muscles fit and joints flexible, and protect the body from stiffness. No single one of them is sufficient on its own, since nutrition without movement leaves joints stiff, while exercise with poor posture can do harm.

Yoga, set out in the old Indian writings, covers posture, the control of breath and meditation, so its reach goes well past the muscles.

Studies credit it with suppler joints, a better carriage, steadier breathing, less strain, and some protection against the diseases that follow how we live. The breadth of those findings is notable: a practice aimed at the body turns out to affect stress and long-term health as well.

Its significance is recognised internationally. Every year the 21st of June is observed as International Yoga Day, to promote the role of yoga in health and well-being.

Plants are mostly fixed in one place and need support to stay firm and upright, which the cell wall provides, while animals move and their cells, lacking a wall, change shape easily. They also feed differently: animals have tissues for digesting food obtained from other sources, while plants have tissues that use solar energy to synthesise food by photosynthesis.

A plant grows in length through the apical meristem at the tips of roots and shoots. It grows in girth through the lateral meristem arranged in a ring in the stem. It regrows after cutting or grazing through the intercalary meristem at the nodes.

Two bulbs are stood over jars of water and their root lengths measured for three days. On day three the root tips of one bulb are cut by about a centimetre while the other is left alone, and measuring continues. The cut roots stop growing while the untouched ones keep lengthening, showing that roots grow only from their tips.

A simple permanent tissue is composed of only one type of cell, and parenchyma, collenchyma and sclerenchyma are the three examples. A complex permanent tissue is composed of more than one type of cell working together, and xylem and phloem are the two examples.

The epidermis wraps the entire plant as its outermost sheet, one cell deep, built of flat rectangular cells pressed close together. Over it lies a waxy coat of cutin, the cuticle. Between them they keep out physical damage, hold in water, bar harmful microorganisms and buffer the plant against harsh surroundings.

Transpiration is the evaporation of water vapour through the stomata in the leaf epidermis. It creates a transpiration pull in the xylem, which is what moves water upward through the plant, and it also helps in eliminating wastes from the plant body.

It wraps the body as skin and continues inward as the lining of organs — mouth, lungs, blood vessels, intestine. Its cells sit hard against one another, leaving almost no gaps.

Because its cells are packed closely with almost no space between them, the tissue prevents the entry of germs and reduces water loss, while still permitting absorption, secretion and the movement of substances across it. A barrier letting nothing through would leave the body unable to feed or excrete, so the tissue has to do both jobs at once.

Exchange epithelium is a single layer of thin flat cells lining blood vessels and lungs. Protective epithelium has many layers with flat tightly packed outer cells and is found in skin, mouth and oesophagus. Absorptive epithelium is a single layer of tall pillar-like cells, often with hair-like structures, lining the small intestine.

Voluntary movements are under conscious control, such as running, writing or lifting objects, and are carried out by skeletal muscles attached to the skeleton. Involuntary movements occur automatically without conscious control, such as the movement of food in the intestine or the beating of the heart.