Tissues in Action: Class 9 Science · Chapter 3

Leave a few soaked moong seeds on a wet cloth and, within two days, a white root pushes out of each one. A newborn baby weighs about 3 kg, and every gram of it, from eyelashes to toenails, came from a single fertilised cell. One cell becomes billions. How do those billions end up so different from one another?

Two things happen together. Cells divide again and again, so their number grows. Then groups of the new cells change: they take on particular shapes and settle into particular jobs. Think of the flat cells lining your mouth, the hard-walled cells of your thigh bone and the thread-like cells of your nerves. All of them descend from that first cell, yet they could hardly look more different. Knowing how cells specialise lets doctors grow skin for burn patients and lets farmers raise thousands of healthy plants from a few cells.

Back in Chapter 2 you studied the cell, the basic unit of life. An Amoeba is a single cell that does everything for itself: it takes in food, moves, breathes, gets rid of waste and divides. A many-celled organism shares this work out. Cells that are alike in structure gather into groups, and each group takes charge of one task. Such a group is a tissue (ऊतक).

Tissues are only one rung of a ladder (Fig. 3.1). Several tissues join to make an organ, such as a leaf or the stomach. Organs that work on the same task form an organ system, such as the shoot of a plant or our digestive system. All the organ systems together make the organism.

Splitting up the work like this is division of labour, and it pays off. A tissue that has only one job can be built perfectly for that job, so the whole body runs more efficiently. Messages race through your body along nervous tissue while muscle tissue does the pulling. A plant shares out its carrying between two tissues, xylem for water and minerals and phloem for food.

3.1 Why are Plant and Animal Tissues Different?

Think of a neem tree and a goat grazing under it. The tree spends its whole life in one spot. It must stand upright against wind and monsoon rain and hold its leaves up to the light. In Chapter 2 you saw that plant cells have a stiff cell wall, which helps here. Much of a plant's body is supporting tissue, and a large part of that tissue is made of dead cells with thick walls. Wood is the best-known example.

The goat lives differently. It walks to find grass, runs from danger and turns its head to look around. Its cells have no wall, so they can change shape, and its tissues can stretch, bend and contract (Fig. 3.2). Most animal tissues are living. Not every animal roams, of course: a sponge spends its adult life stuck to a rock.

Their growth follows different rules too. The neem keeps adding height and thickness for as long as it lives, but only from a few special growing regions. The goat grows all over its body until it reaches adult size, and then it mostly stops. Their food comes from different places, so their tissues differ again: leaves hold tissues that trap sunlight and make food, while the goat's gut holds tissues that digest grass. Both must carry water and food around inside, yet each uses its own kind of transport tissue.

3.2 Tissues for Growth in Plants

A peepal seedling that sprouts in a crack in a wall can, over the years, grow into a tree whose roots split the wall apart. Watch plants closely for a year and you will catch them growing in three different ways:

• in length: a bean shoot climbs its pole while its root pushes deeper into the soil;
• in girth: a banyan trunk gets wider year after year;
• after damage: a hedge sprouts again after trimming, and a field of grass recovers after goats have grazed it.

None of this can happen without fresh cells, and fresh cells come only from cells that divide. Plants keep such cells in special patches of meristematic tissue (विभज्योतक ऊतक), or meristems for short. There are three kinds of meristem, one for each kind of growth.

3.2.1 Apical meristem — How do plants grow in length?

Does a root lengthen evenly along its whole length, or only at one end? An onion standing over water makes a good test, because its roots grow quickly and are easy to measure.

The same is true of shoots. The tip of every shoot and every root holds a small cluster of dividing cells, the apical meristem (apex means tip). As these cells divide, they leave new cells behind them; the new cells then lengthen and push the tip forward. Mark a growing root with ink dots at equal gaps. Within a day or two the gaps near the tip spread apart, while the gaps further back stay the same (Fig. 3.3).

3.2.2 Lateral meristem — How do plants grow in girth?

Put your arms round a young gulmohar sapling and your hands easily meet. Twenty years later they will not. Many plants, dicots especially, widen as they lengthen. A round wooden coaster, or the end of a sawn log, carries a record of that widening: a set of rings, one inside the next.

The widening comes from a meristem that runs as a thin ring inside the stem, between the wood and the bark (Fig. 3.5). Because it lies along the sides of the stem rather than at a tip, it is called the lateral meristem. Its cells divide in two directions. The new cells formed on the inside become wood, which is mostly xylem; those formed on the outside become part of the bark, which carries phloem. Layer after layer, the stem gets thicker.

Why rings? In most of India, a tree grows fast during and after the monsoon and slowly in the dry or cold months. Wood made in the good season has large cells and looks pale; wood made in the poor season has small, tightly packed cells and looks darker. One pale band plus one dark band is one year's growth, an annual ring. Counting the rings gives the age of the tree. The width of each ring tells a story too: a broad ring means plenty of water that year, and a narrow one points to drought. Scientists read old trunks this way to learn about the climate of past centuries.

3.2.3 Intercalary meristem — How do plants grow after being cut?

A gardener trims a mehendi hedge flat with shears. Three weeks later the hedge is thicker and bushier than before. Cows graze a field down to stubble, yet a fortnight later the grass is back. To see how, look at where the leaves sit. They are not attached just anywhere: they grow from slightly swollen points on the stem called nodes. Each bare stretch of stem from one node to the next is an internode (Fig. 3.6).

Cut off the tip of a young stem and you remove its apical meristem, so that stem cannot grow any longer from its tip. Something else happens instead. Each node carries small buds, which the growing tip had been holding back. With the tip gone, the buds at the top nodes wake up and grow into side branches. This is why a trimmed hedge turns bushy and why gardeners pinch the tips of tulsi plants.

Grasses have one more trick. They keep patches of dividing cells near the base of their internodes, just above the nodes, and at the base of each leaf blade. These patches are the intercalary meristem (intercalary means 'inserted in between'). When a mower or a cow removes the top of a grass blade, these cells carry on dividing and push the blade up again from below. That is why a lawn needs mowing every week, and why grazing does not kill a pasture. Bamboo and sugarcane, which are giant grasses, have intercalary meristems too.

All meristems are built from the same kind of cell. Meristematic cells are tiny dividing machines, and their structure suits the job:

• they are small, with thin, stretchy cell walls;
• each has dense cytoplasm packed with organelles and a large, prominent nucleus;
• vacuoles are absent or very small;
• they are tightly packed, with almost no spaces between them.

Why no vacuole? A cell that divides every day or so must keep building fresh cytoplasm and copying itself. A large vacuole would fill most of the cell with stored sap that does no building work. Storage is left to other cells.

A meristem works like a nursery for cells, and most of its young cells do not stay young for long. Within days they stop dividing, swell, reshape their walls and settle into one job for good. A cell just behind a root tip may stretch into a long water pipe, while its neighbour fills up with stored starch. This settling-in is differentiation (विभेदन), and the mature tissues it produces are called permanent tissues (Fig. 3.7). A few cells are always held back unchanged, so the meristem never runs out.

3.2.4 Permanent tissues

Slice a young pumpkin or balsam stem across, as thinly as you can, and put the slice under a microscope. You will not see one kind of cell but several: a neat outer row, large rounded cells, cells with thick walls, and cells grouped in bundles. Each group is a permanent tissue doing its own job (Fig. 3.8). Some permanent tissues contain just one kind of cell; these are simple tissues. Others, the complex tissues, combine several kinds of cell into one working unit.

Protective tissue: the epidermis

What keeps a leaf from drying out in the May sun, or germs from walking straight into a stem? Every young plant part wears a skin, the epidermis, and this skin has three jobs. First, it is a barrier. Its outer walls carry a waxy film, the cuticle, made of a substance called cutin. The wax keeps water in and fungi out and takes the knocks of wind-blown grit. Desert plants such as aloe and cactus wear a thick cuticle, which is why their leaves feel leathery.

Second, it has doorways. A leaf's epidermis is dotted with tiny pores, the stomata, each between two guard cells. Carbon dioxide for photosynthesis comes in through them and oxygen goes out. Water vapour leaves by the same route; this loss, transpiration, sets up a pull that draws water up the xylem from the roots, and it carries away some wastes. Third, below ground it works as a sponge. Many root epidermal cells grow out into long, thin root hairs, which together give the root an enormous surface for soaking up water and minerals. Structurally, the epidermis is simple: usually one layer of flat cells fitted together without gaps.

Supporting tissue: simple permanent tissues

Bite into a guava and you may crunch on tiny gritty grains. Fold the leaf stalk of a pumpkin and it bends without breaking. Squeeze the swollen leaf stalk of a water hyacinth and it feels spongy, like a float. Three simple permanent tissues are behind these: parenchyma, collenchyma and sclerenchyma (Fig. 3.9).

Cut a potato and nearly everything you see is parenchyma, its cells crammed with starch. It is the plant's all-purpose tissue. In leaves and young stems the same kind of cell holds chloroplasts and makes food. In the water hyacinth and the lotus it is riddled with large air spaces, which is how those plants float. Parenchyma cells are alive, with thin walls, and they sit loosely together with small gaps between them.

The bendy strength of that pumpkin leaf stalk comes from collenchyma, a band of tissue just under the epidermis of young stems and leaf stalks. Look at its cells and you see the secret: their walls are extra thick only at the corners, where pectin, a rubbery material, has built up. The thick corners give support, while the thin stretches of wall between them allow some give. So the stalk sways in the wind and springs back, and a tendril can curl round a stick. Collenchyma cells are living.

The gritty grains in a guava belong to sclerenchyma, the plant's hard tissue. Its cells build walls so thick with lignin, the substance that makes wood woody, that there is hardly any room left inside. Most of them die once the wall is finished. Long sclerenchyma fibres run through stems and around leaf veins as rigid props. Short, stony cells make the stone around a mango seed and the shells of many nuts.

Conducting tissue: complex permanent tissues

A coconut palm can be 25 m tall. Water taken up by its roots has to reach the leaves at the very top, and sugar made in those leaves has to reach the roots and the ripening coconuts. This two-way traffic runs through the plant's transport tissues, xylem (जाइलम) and phloem (फ्लोएम). Neither is a single kind of cell: each is a team of four, which makes both of them complex tissues (Fig. 3.10).

Think of xylem as the plant's plumbing, carrying water and dissolved minerals one way only, upwards, from the roots to the leaves. The pipes are vessels and tracheids: long, thick-walled cells that are dead and empty when they start work. Vessel cells stack end to end and lose the walls between them, forming open tubes that can run a long way. Xylem fibres, a kind of sclerenchyma, act as props. That is why wood, which is mostly xylem, is so strong. Scattered among them are a few living helpers, the xylem parenchyma cells, which store food and pass water sideways.

Phloem is the delivery service. It takes food, mostly sugar, from the leaves to wherever it is wanted: down to the roots, up to a growing bud, or into a ripening mango. Its main channels are sieve tubes, chains of long cells whose end walls are pierced with holes like a sieve, so sugary sap can flow from cell to cell. Each sieve tube cell has a slim partner beside it, a companion cell. This specialised parenchyma cell keeps the sieve tube alive and controls how much sugar is loaded in or taken out. Phloem parenchyma stores food, along with resins, tannins and latex, and phloem fibres add strength. Apart from the fibres, all of these cells are alive.

Tissue systems

In a living plant these tissues never work alone. They are organised into three tissue systems, which run continuously through the roots, stems and leaves (steps 2 to 4 of Fig. 3.8):

1. Dermal ('skin') tissue system: the epidermis, and cork on older stems. It is the plant's protective coat and also keeps water in.
2. Ground tissue system: everything packed between the coat and the transport tissues, that is parenchyma, collenchyma and sclerenchyma. It stores food, makes food in green parts and holds the plant up.
3. Vascular tissue system: xylem and phloem, the plant's transport network for water, minerals and food.

3.3 Animal Tissues

Try these four things right now:

• wiggle your toes;
• rub a tulsi or mint leaf and sniff it;
• breathe in slowly and hold it for five seconds;
• press a fingernail until it goes pale, then let go and watch the pink return.

Four small actions, and a different tissue starred in each. Muscle tissue curled your toes. A special lining inside your nose caught the smell. An extremely thin layer of cells in your lungs let oxygen slip into your blood, and blood itself, which is also a tissue, flowed back under the nail. Behind all of them, nerves were carrying messages. Animal tissues belong to four main families: epithelial, connective, muscular and nervous tissue, and in each the build of the cells suits the work.

3.3.1 Epithelial tissues — Structure and functions

Every surface of your body, outside or inside, is faced with epithelial tissue (एपिथीलियमी ऊतक). It forms the outer skin, and it lines the gut from mouth to intestine, the air passages and the air sacs of the lungs, and the inside of every blood vessel. Its cells sit on a thin base layer and fit together edge to edge, with no gaps. A germ, a drop of water or a molecule of food therefore cannot slip between the cells; to get in or out it has to pass through them. That gives epithelium control over what is absorbed, what is secreted and what is kept out.

Epithelial cells come in different shapes, and each shape suits a different job (see the table and Fig. 3.11).

3.3.2 How are various parts connected in our body?

Here is a puzzle: what can a drop of blood and a piece of chicken bone possibly have in common? One flows, the other could stop a bullet-sized pebble. Yet biologists put them in the same family, connective tissue (संयोजी ऊतक). What unites them is their job: both join, support or link the other tissues of the body. Blood ties every organ to every other by carrying food, oxygen, hormones and wastes between them, and bone gives the whole body a frame to hang on.

Unlike epithelial cells, the cells of a connective tissue are not packed together. They are scattered through a matrix, the material that fills the space between them, and the matrix gives each tissue its character. Blood's matrix is plasma, a watery liquid, so blood can flow. Bone's matrix is stiffened with minerals, compounds of calcium and phosphorus, so bone can bear your weight.

Let a sample of blood settle (doctors spin it in a machine to speed this up) and it separates (Fig. 3.12). The straw-coloured liquid on top, about 55% of the volume, is plasma, carrying dissolved food, salts, proteins, hormones and wastes. The rest, about 45%, is cells and cell pieces. Red blood cells (RBCs) are packed with haemoglobin, an oxygen-carrying protein that contains iron and makes blood red. An RBC wears out after roughly 120 days, and the body makes new ones all the time. White blood cells (WBCs) defend the body against germs. Platelets, tiny fragments of cells, start the clotting that seals a wound.

Blood and bone are not the only connective tissues. Try the next activity to find more of them on your own body.

Cartilage is easy to find: it shapes your ear flap and the tip of your nose, caps the ends of bones inside joints, and forms the discs in your backbone. Its matrix is firm yet rubbery, not stiffened with minerals as bone's is, so it can be squashed or bent and then spring back. Tendons and ligaments are both tough bands, and the easiest way to tell them apart is by what they join. A tendon (कंडरा) runs from a muscle to a bone. A ligament (स्नायु) runs from one bone to another, holding a joint together.

3.3.3 Can we control movement in our body?

Your body has three kinds of muscle tissue, and a good first question to ask about each is: can I control it? Wave, wink, or tap your foot to a song: you chose each of these, so they are voluntary movements. The muscles behind them are hooked onto bones, so they are called skeletal muscles. Under a microscope a skeletal muscle turns out to be bundles of very long, cylindrical cells, the muscle fibres. Each fibre has many nuclei, no branches and a pattern of light and dark bands across it. Because of these bands it is also called striated (striped) muscle.

Other movements go on whether you think about them or not. The pupils of your eyes widen when the lights go out, your heart keeps time all night, and your last meal is being squeezed along your gut right now. These are involuntary movements, and two kinds of muscle carry them out:

• Smooth muscle forms part of the walls of hollow organs: the stomach and intestines, blood vessels, the bladder. Each cell is shaped like a spindle, pointed at both ends, with one nucleus and no stripes. It works slowly and steadily, pushing food through the gut in gentle waves.
• Cardiac muscle exists only in the heart. Its cells are cylindrical but branched, linking into a network; each usually has one nucleus and the stripes are faint. It beats about 70 times a minute when you are resting, and keeps going for a lifetime without tiring.

3.3.4 How does the body sense, communicate and respond?

A cricket ball flies towards you. In a fraction of a second your eyes spot it, your brain works out where it will land, and your hands move to meet it. Spotting, deciding and acting are all the work of nervous tissue, which makes up the brain, the spinal cord and the nerves that branch into every part of the body. The brain is the headquarters, collecting reports, storing memories and issuing orders. No muscle, voluntary or involuntary, acts on its own: each waits for nerve signals. A sudden fright, for example, sends signals that set your heart racing within a moment.

Nervous tissue is built from neurons (तंत्रिका कोशिका), or nerve cells, and the easiest way to learn their parts is to follow a message through one (Fig. 3.14). The message arrives at the dendrites, short branching extensions that collect signals from other cells. It passes into the cell body, the part that holds the nucleus and runs the cell. It then leaves along the axon, a single long fibre, and at the far end the axon terminals hand it on to another neuron, a muscle or a gland. Some axons are extraordinarily long: those running from the lower spinal cord to the muscles of the foot can be about a metre.

3.4 The Musculoskeletal System

You have now met the parts one by one: bones, cartilage, ligaments, tendons and skeletal muscles, plus the joints where bones meet. In the body they act as a single team, the musculoskeletal system. The team holds you up against gravity, keeps your posture, moves you about and guards your brain, heart and lungs. The nervous system is its boss: no skeletal muscle contracts without a signal from a nerve.

Here is the key fact about muscles: a muscle can only pull, by contracting and getting shorter; it can never push. When a muscle contracts, its tendon passes the pull to a bone, and the bone turns at a joint (Fig. 3.15). To move the bone back, a different muscle on the other side of the joint must pull. So muscles work in pairs: the biceps at the front of your upper arm bends the elbow, and the triceps at the back straightens it again.

Bone is lighter than most people guess: in an adult the whole skeleton is only about one-eighth of the body's mass, roughly 12–15%. Muscle makes up a much bigger share. The figures change from person to person with age, sex, build and how active someone is.

3.4.1 The musculoskeletal system in action

Your table shows a pattern: the shoulder and hip can go almost anywhere, while the elbow and knee only fold and unfold. What decides this is the joint, the meeting point of two or more bones, and in particular the shape of the bone ends there. A joint is only a meeting point, though. The force that swings the bones always comes from muscles, pulling through their tendons.

3.5 Types of Joints

The shape of the bone ends at a joint, and the ligaments around it, decide how far and in which directions it can move (Fig. 3.16).

3.5.1 Ball and socket joint

Watch a fast bowler: the arm whirls round in a complete circle. Inside the shoulder, the top of the upper arm bone is shaped like a ball and rests in a shallow cup on the shoulder blade. It can therefore roll in any direction: forwards, backwards, sideways or round. This is a ball and socket joint. The shoulder blade and the collarbone together make the shoulder girdle, the frame that attaches each arm to the trunk. Your hip is built the same way, but with a deeper cup: it gives up a little freedom in return for the steadiness needed to carry your weight.

3.5.2 Hinge joint

A door swings open and shut but never up or down. Your elbow works the same way, folding and unfolding in a single plane, so it is called a hinge joint. Knees are hinges too, guarded at the front by a small, flat bone, the kneecap. So are the many little joints in your fingers and toes.

3.5.3 Pivot joint

Keep your shoulders still and look over your left shoulder, then your right. The turning happens at the very top of the backbone. The first vertebra, which carries the skull, is shaped like a ring. It swivels around an upright peg that sticks up from the vertebra below, much as a ring turns on a finger. A joint that lets one bone rotate around another like this is a pivot joint.

3.5.4 Fixed joints

Not every joint moves. Your skull is made of several curved plates of bone, knitted together along wavy seams so tightly that they cannot shift at all. Joints like these are fixed joints. They turn the skull into one rigid helmet around the brain. A newborn baby's skull plates have not yet locked together, which leaves soft spots on the head; they close up during the first two years.

3.6 Skeletal System

An adult's skeletal system has about 206 bones. Between them they give the body its shape, carry its weight, guard soft organs and give muscles something to pull on. This section looks at two parts you can feel for yourself, the backbone and the rib cage (Fig. 3.17); the third great protector, the skull, you met among the fixed joints.

Run a finger down the middle of a friend's back and you can feel a row of bumps. Each bump belongs to a vertebra, one of the small bones stacked up to form the backbone, or vertebral column. The stack holds the upper body up and keeps it upright, and the spinal cord passes down a tunnel through it. A single solid rod would be strong but unbendable. Instead, a disc of cartilage sits between each vertebra and the next, like a rubber washer. The column can then bend and twist while the bones stay cushioned and the spinal cord stays safe.

Now put your hands on the sides of your chest and take a slow, deep breath. You can feel the rib cage rise and widen under your palms. It is built from 12 pairs of ribs curving round from the backbone, and it protects the heart and lungs. Yet it cannot be rigid, or you could not breathe. The trick is at the front, where most ribs reach the breastbone (sternum) through strips of bendable cartilage. Breathing in, muscles between the ribs swing them up and out, the chest grows roomier and air rushes into the lungs. Breathing out, the ribs settle back.

Think as a scientist: one cell, a whole plant

Can a fully grown, specialised plant cell go back to being a beginner? In 1958 the American plant scientist F. C. Steward showed that it can.

Why was this so surprising? A phloem cell is a finished, specialised cell that would normally never divide again. In Steward's flasks it was as if a retired specialist went back to school. The cells first dedifferentiated, shedding their special features and starting to divide again into a mass of unspecialised cells. Then, guided by the nutrients and growth substances, the mass redifferentiated, forming roots, shoots and in the end a whole plant. A cell that can rebuild a complete organism in this way is said to show totipotency. You started life as one such cell: a fertilised egg, or zygote, which divided and differentiated into your entire body.

This discovery is the basis of plant tissue culture. Laboratories now grow thousands of identical, disease-free banana, potato, sugarcane and orchid plantlets from tiny pieces of a single good plant. They need only a small space and can work at any time of year. The same method helps to save rare plants from extinction.