Class 6 Science Chapter 10: Living Creatures: Exploring their Characteristics

A Shell on the Morning Walk

Avadhi and Aayush are out walking with their parents when Avadhi spots some shells and reaches for one. Her mother stops her. That shell may be the home of a living snail, and it is actually a part of the snail's own body.

The Question That Follows

The two of them are puzzled, and it is a fair puzzle. The shell does not move at all, so how can something alive be inside it? They take the question to their teacher, who turns it into a class discussion about the living and the non-living.

Start by Looking Around

Look about the room and list what you can see. A pencil. A book. A pigeon at the window. Mark each one living or non-living, and, more importantly, write down your reason. The reasons are where the interesting part is.

The First Guess: Movement

Most people reach for movement first. Living things move, non-living things do not. But cars move along a road all day, and nobody calls a car alive. So movement on its own cannot settle the question.

And Plants Break It the Other Way

Plants never travel from one place to another, and nobody doubts that they are alive. So the test fails on both sides at once, which is worth noticing. One characteristic is not going to be enough.

Plants Do Move, Though

They simply do not go anywhere. A flower opening is movement. A climber winds itself around whatever object is placed close to it. And Drosera, an insectivorous plant, has saucer-shaped leaves fringed with sticky hairs of unequal length that move inward to trap an insect that lands there.

Growth

Try on a dress you wore four years ago and it will not fit, because your body has grown since. Plants and every other living being grow too. Nothing in the non-living column does this on its own.

Nutrition

Growing takes food. Living beings need food for their growth and their development, and you can list five of them without thinking hard. A pencil has never needed a meal.

Respiration

Count your breaths for a minute after a walk, then after a run, then after a few dance steps, and the three numbers will differ. Watch a dog or a cow resting and you can see the abdomen move. Breathing is one visible part of the larger process called respiration.

How Plants Breathe

They have no nose, so it is easy to assume they do not. The surface of a leaf carries tiny pores called stomata, and air passes in and out through those. Ask older pupils to show you some under a microscope. Every living being respires.

Excretion

White patches show up on shirts around the armpits in summer. They come from sweat, which is water and salts the body is getting rid of as waste. Getting waste products out of the body is what we call excretion, and urine is another product of it.

Plants Excrete Too

Look at grass early in the day, or at a rose. Plants push out excess water and minerals as small droplets on their leaves. So this is not something only animals do.

Stimulus and Response

Step barefoot on a thorn, or touch a hot cup of tea by accident, and your body reacts before you have decided anything. A thorn and a hot cup are stimuli. Anything, or any event, that prompts a living being to react is a stimulus.

Plants Answer Too

Touch a chhui-mui, the touch-me-not, and its leaves fold at once. Certain plants fold their leaves after sunset, with facing leaves coming together, which you can watch in the amla tree. Every living being responds to stimuli.

Reproduction, and the End of the List

Cats have kittens and dogs have puppies, but no pencil, chair or bulb has ever had a young one. Reproduction means making new individuals of one's own kind, and life continues because of it. Where a living being can no longer do these things, although food, air and water are all there, it is said to be dead.

Scenario:

A class sorts twelve things into living and non-living. A car causes an argument. Rohit calls it living because it moves and uses fuel. Their teacher asks the class to check it against the full list of eight characteristics instead of one. The car moves and consumes fuel, so two fit. It does not grow, reproduce, excrete, respond to stimuli, respire or die. The class counts two out of eight and agrees that is not enough. Sana writes the rule the class will use from now on. A single shared feature proves nothing, because the list has to be met as a whole rather than in part.

Analysis:

Movement is the feature people reach for first and it is the weakest on its own. Counting against all eight is what stops a car being alive.

Activity:

Pick three things that move but are not alive. Check each against all eight characteristics and write how many it meets.

Title:

Test ten things against the list

Safety:

Do not pick up shells, snails or insects to test them. Watch them where they are, and ask before touching a plant that is not yours.

What You Notice:

Almost everybody leans on movement for the first guess, and almost everybody gets a plant wrong because of it. A houseplant is the useful row: it fails your first test and passes every other one. If you listed anything made of wood, cotton or leather, that row is worth arguing about with somebody else.

Drosera is an insectivorous plant whose saucer-shaped leaves carry sticky hairs that move inward to trap an insect.

The leaves of the amla tree come together after sunset, which is called sleeping.

Grasses and roses push out excess water and minerals as small droplets on their leaves.

The eight characteristics — Move, feed, grow, breathe, excrete, respond, reproduce, die — Eight in a row, and all eight have to be there.

Why movement alone fails — A car moves, a tree does not — One counter-example on each side of the test.

Living or Not?

Here is a question worth arguing about. A seed sits in a packet doing nothing at all, month after month. Would you call it living or non-living, and what is your reason? Hold on to your answer and see whether the rest of this topic changes it.

The Word for It

Put a seed in the right conditions and something happens. A seed that has turned into a sprout is said to have germinated. So the question becomes a practical one. What conditions does a seed need before it will do that?

Design It Properly

Guessing is easy; testing is better. Fill four matching pots from the same heap of garden soil, and put four bean seeds into each of them. Since pots, soil and seeds are alike all through, anything that turns out differently must come from how they are kept.

Pot A: Dry and Sunny

Give this one direct sunlight and no water at all. Do not water the soil at any point over the fifteen days. This pot is asking one question: can seeds manage without water? Nothing else about it is unusual, which is what makes the answer readable.

Pot B: Drowned and Sunny

Add so much water that it always stands above the soil, and top it up whenever the level drops. Keep this pot in direct sunlight too. It is asking whether more water is simply better. Plenty of people expect this to be the best pot of the four.

Pot C: Damp and Dark

Keep the soil slightly moist with a moderate amount of water, added regularly, and put this pot somewhere completely dark. The question here is whether sunlight is needed. Everything else about it matches the pot that does best.

Pot D: Damp and Sunny

Same moderate watering, same slightly moist soil, but standing in direct sunlight. This is the comparison pot, the one the other three are measured against. Whatever happens here is what ought to happen when nothing has been taken away.

Predict Before You Look

Now fill in a table. For each pot, mark whether air, sunlight and water are available, and write down whether you expect the seeds to germinate. Writing predictions first is what stops you quietly adjusting them later.

Then Watch for Ten Days

Look at the pots regularly for seven to ten days and record what has happened in each. When all four are in, compare your predictions with your observations, row by row, and mark every place they disagree.

What You Find

Pot A does nothing, which surprises nobody. Pot B does nothing either, which surprises almost everybody. Pot C germinates in the dark. Pot D germinates as expected. So the seeds that succeeded are the ones that had both air and the right amount of water.

Why Pot B Failed

It had more water than any other pot, so the failure needs explaining. Seeds use the air held in the spaces between soil particles. Water standing above the soil fills every one of those spaces, and the seeds are left with water and no air.

What the Water Does

Seeds need water because it lets them carry out the processes their growth depends on. There is a second job as well. A seed's outer covering is called the seed coat, and water softens it so that the tiny embryo inside can develop into a plant.

What the Soil Does

The soil is doing more than holding the seed upright. The spaces between its particles store the air the seeds need, and those same spaces let the roots push through easily once growth begins. A tightly packed pot with no spaces in it would defeat a seed twice over.

And the Light?

Pot C settles this one. For bean seeds, light is not needed for germination, and most seeds are the same. That does not make light unimportant, though: once a seed has germinated, the seedling needs sunlight for its further growth.

Not Every Seed Agrees

There are exceptions worth knowing. Seeds of Coleus and Petunia need light to germinate, so burying them under soil stops them sprouting. Seeds of Calendula and Zinnia need darkness instead, and have to be covered with enough soil. Now go back to the first card. Living, or not?

Scenario:

A class sets up four identical pots of seeds. One gets water and light, one gets water and no light, one gets light and no water, and one gets neither. After a week the first has sprouted well, the second has pale weak shoots, and the last two show nothing. Imran says the test proves seeds need light. Their teacher asks him to look again at the second pot. It sprouted without any light at all. The class works out what each pot can and cannot tell them. Water is needed for germination, and light is needed for what comes after, which is why four pots were set up rather than two.

Analysis:

Four pots separate two conditions that two pots would have confused. The dark pot sprouting is what distinguishes germination from growth.

Activity:

Set up four pots of seeds varying water and light. Record what happens each day for a week and say what each pot tells you.

Title:

Four jars instead of four pots

Safety:

Use garden soil rather than anything from a drain, wash your hands afterwards, and keep glass jars away from the edge of the sill.

What You Notice:

Jar C is the one that changes minds. Almost everybody predicts that nothing will happen in a dark cupboard, and it sprouts. Jar B smells sour after a few days, which is worth noting in the table. The seedlings in jar C come up pale and leggy compared with those in jar D, which is the first hint that light matters after germination even though it did not matter before.

Coleus and Petunia seeds need light to germinate, so covering them with soil stops them sprouting.

Calendula and Zinnia seeds need darkness, so those have to be covered with enough soil.

Roots find their way through the very same spaces between soil particles that hold the air.

What germination needs — The right amount of water, and air with it — Too little water fails, and so does too much.

Light and germination — Dark to start, light to grow on — Most seeds germinate without light but a seedling needs it afterwards.

Two Questions Worth Asking

A plant has grown out of its seed, and now it has a direction. Does sunlight decide which way its parts grow? And if you turned a whole seedling upside down, which way would the root and the shoot go then? Both can be tested.

Start With Seedlings

Set some bean or gram seeds to sprout on a damp cloth, or else on damp tissue paper. Leave them until each one has become a seedling with a small root and a small shoot, so that both parts can be watched. A seed that has only just split open will not show you enough.

Mounting Them

Take three glass plates and fasten a sheet of stout blotting paper down one face of every plate, using thick soft cotton thread. Tie one seedling to each plate with the same soft thread, taking care not to damage the plant.

The Beakers

Label three glass beakers or tumblers A, B and C, and pour water into all three. The water level matters: each seedling must stay above it. The bottom of the blotting paper dips in and soaks, so the plant gets its moisture from the wet paper.

Beaker A: Upright, Light All Round

Stand the glass plate upright in beaker A, with the seedling the natural way up, and put the beaker in sunlight. Nothing unusual is being done here. This is the case the other two are compared with. Whatever it does is what ought to happen when nothing has been interfered with.

Beaker B: Upside Down, Light All Round

In beaker B, arrange the plate so that the shoot of the seedling points downwards and the root points upwards. Put this beaker in sunlight as well, so that light reaches it from every direction.

Beaker C: Upright, Light From One Side

Stand this plate upright as in A, then place a cardboard box over the beaker. Cut one small circular hole in the back of the box, so that the seedling gets light from a single direction only. Everything else about this beaker matches beaker A exactly.

Predict Before You Watch

Fill in a table with a row for each beaker and a line each for the shoot and the root. Write what you expect before you look. Then record what actually happens, and mark every place where the two columns disagree.

Beaker A: What Happens

The straightforward case comes out as you would guess. Stand the plant the right way up and its root heads downwards while its shoot heads upwards. No surprises, but it sets the pattern the others are tested against.

Beaker B: What Happens

This is the interesting one. The plant was set upside down, and it does not stay that way. The root bends round and grows downwards. The shoot bends round and grows upwards. Both of them return to the directions beaker A showed.

What Beaker B Proves

If a shoot simply grew away from the seed, the upside-down shoot would have carried on downwards. It bent instead. So the direction is not fixed by where the seed happens to be pointing. Something outside the plant is deciding it instead.

Beaker C: What Happens

Now light comes from one side only. The shoot grows in the direction of the light, leaning across towards the hole in the box. The root takes no notice at all and continues to grow downwards, exactly as in the other two. One part of the plant answers the light and the other does not.

Putting the Three Together

Read the three results as one. Shoots of plants grow upward and move towards sunlight. Roots of plants grow downwards. That holds however the plant is placed and wherever the light is coming from.

Why This Matters

Recall the list of things all living beings do. This experiment shows two of them in one plant at the same time: growth, and movement in answer to a stimulus. The stimulus here is the light, and the leaning shoot is the response.

A Scientist Who Measured It

Jagadish Chandra Bose, an Indian scientist of 1858 to 1937, spent years on some remarkable work with plants. The machine he built for it, the crescograph, kept a record of the way a plant answers light, heat, an electric current and gravity. It could also measure growth far too slow for the eye, and it settled the point that plants do sense what is around them and do react.

Scenario:

A class germinates seeds and then turns one seedling upside down in its jar. After two days the root has bent and is heading down again, and the shoot has turned and is heading up. Meera says the plant must be able to see which way is up. Their teacher asks whether the jar had any light. It was kept in a dark cupboard. The class works out that sight cannot be the answer. Something other than light tells the root and shoot which way to go, and whatever it is, the two parts answer it in opposite directions. Rohit notes that the plant responded, which is one of the eight characteristics.

Analysis:

The dark cupboard rules out light, and the opposite bends rule out one single instruction. Root and shoot answer the same pull in contrary ways.

Activity:

Germinate a few seeds, invert one in a dark place, and check it after two days. Draw what the root and the shoot have done.

Title:

Give a seedling one window

Safety:

Keep the jars away from the edge of the sill, and ask before cutting the box. Do not put the jars anywhere they could fall on somebody.

What You Notice:

The upside-down seedling is the one worth sketching daily, because the bend is visible within about three days and finishes within a week. The boxed jar takes longer and the lean is gentler than most people expect. In both jars the root stays stubbornly downward, which is easier to see than the shoot because it does not have leaves to confuse the eye.

Jagadish Chandra Bose lived from 1858 to 1937 and did some remarkable experiments with plants.

His crescograph could record how a plant answers light, heat, electricity and gravity.

The same machine could measure how fast a plant grows, which is far too slow to watch.

The rule that holds in every case — Shoot up, root down, whatever you do — Turn the plant over and both of them turn back.

What the box adds — One hole, and the shoot leans — Light from a single side pulls the shoot across.

One Seed, Three Months

Here is an activity that cannot be hurried. Plant a bean seed, give it the conditions it needs, and then watch it regularly for three months. The point is not one dramatic moment but the whole sequence of them. Nothing here happens quickly enough to notice on any single day.

Keep a Record

Rule a table with columns for the date, what you observed and a sketch. Fill in a row whenever a change becomes visible, and note the date every time. The dates are what turn a set of drawings into a record of how long each stage took.

Questions to Answer as You Go

Write the answers down as you find them. How long before any change appears at all? How many days go by before a first flower shows? Once parts of the flower have dried, is there any further growth?

Stage One: The Seed

It begins with a single bean, doing nothing visible. You know from the four-pot experiment what it is waiting for: the right amount of water together with air. Give it those and the wait ends. Your table has one row filled in and nothing drawn in it yet.

Stage Two: Germination

The seed turns into a sprout, sending a root down and a shoot up, exactly as the three-beaker experiment showed. This is the first change your table records, and it happens sooner than most people expect.

Stage Three: Leaves Appear

The shoot lengthens and leaves open along it. The plant now looks like a plant rather than a sprout, and it starts to grow steadily. Several rows of your table will be filled during this stage alone, because the change from week to week is easy to see.

Stage Four: Flowers Appear

After some weeks the first flower opens. Note the date carefully, because this is the stage most people misjudge when they guess in advance. From here on, the changes come more quickly.

Watch What Happens to the Flower

The flower does not simply fall off and end there. Some parts of it dry up, and this is the moment to look closely rather than assume the show is over. Growth is still going on in what is left.

Stage Five: The Fruit

The parts of the flower that remain develop into the fruit. In a bean plant, that fruit is a pod. Open one and you find the seeds sitting inside, which is the whole point of the exercise.

And Then the Plant Dies

Keep watering it and watch anyway. After the fruits containing seeds have formed, the plant turns yellow and dries up. This is not neglect on your part; the plant has finished what it was doing.

When a Plant Is Dead

It is worth putting precisely. A plant that stops growing, with all its life activities gradually coming to an end, is considered dead. That holds even when every necessary condition is still available to it, which is the part that matters.

Now Sow the New Seeds

Take the seeds out of the pod your plant produced and sow them. They germinate, and a new generation of bean plants begins. Nothing was brought in from outside to make that happen. Everything the new plants needed was already inside the pod your plant made.

The Name for the Whole Thing

A seed grows into a young plant, matures, and produces flowers and fruit. The fruit holds seeds that give rise to a new generation. The whole journey, from a seed to a plant and on to the next crop of seeds, is a plant's life cycle.

Why It Is Called a Cycle

Look at where it starts and where it ends. It begins with a seed and it finishes by producing seeds. The last thing it makes is the first thing it was, which is exactly what a cycle means.

Two Characteristics, Shown Together

Go back to the list of eight. This one activity shows growth, over three months of sketches, and reproduction, in the pod at the end. It also shows the eighth item, death, which is why the list ends where it does.

Scenario:

A class plants bean seeds and keeps a diary for three months. They record the date of each stage: germination, first leaves, flowers, pods, and the plant drying out. Arjun is surprised by the last entry and says the plant failed. Their teacher asks the class to count the seeds inside the pods. There are more than were planted. The class compares the beginning and the end of the diary. The plant did not fail; it finished, and it left behind more seeds than it started from. Nita writes that the diary is a circle rather than a line, because the last entry is also the first.

Analysis:

Dying looks like failure until the pods are counted. The diary ends where it began, with seeds, and more of them.

Activity:

Plant a bean seed and keep a dated diary of every stage. At the end, count the seeds you have and compare with what you planted.

Title:

Keep a dated diary of one bean

Safety:

Ask before taking soil or a pot, keep the pot off a window ledge where it could fall, and wash your hands after handling soil.

What You Notice:

Germination arrives sooner than most people predict and flowering much later, so the two guesses go wrong in opposite directions. Pages go by with no new row at all during the leafy stage, which is itself worth noticing. The pod is the satisfying part: counting the seeds in it tells you how many plants one plant could produce.

A bean plant will take you through its whole life cycle in about three months of watching.

The pod you eat as a vegetable is the fruit of the bean plant, and the beans inside it are its seeds.

A plant that has finished fruiting yellows and dries even if you keep watering it faithfully.

The order of the stages — Seed, sprout, leaves, flowers, pod — Five steps, and the pod holds the seeds that start it again.

Why it is called a cycle — It ends where it began — The last thing it makes is the first thing it was.

Why Stagnant Water Is a Problem

You will have seen the advice on a school noticeboard or in a campaign: do not let water stand anywhere around you. Female mosquitoes are bloodsucking insects that transmit diseases such as malaria, dengue and chikungunya. The advice and the disease are connected through a life cycle.

Go and Look

Carry out a safety audit at school or at home, taking a hand lens if you have one. Standing water usually collects in a desert cooler, in the tray under a planted pot, or in any container left open to the sky. In it you may find two different worm-like creatures. Neither of them looks in the least like a mosquito.

Larva and Pupa

Those two creatures are the larva and the pupa, two distinct stages in the development of a mosquito. Look at the difference in their shapes. If you find them, report it to your teacher and discuss what can be done.

A Puzzle Worth Solving

Which of the two comes first? You cannot tell by looking. Separate four or five larvae and four or five pupae into two containers of the same water, watch both daily, and see which one changes into the other. Keep watching afterwards to see which container produces a mosquito first.

The Four Stages

A mosquito lives through four separate stages, and they run egg, larva, pupa, adult. Each one gives way to the next: out of the egg comes a larva, the larva turns into a pupa, and the pupa finally becomes the flying insect. Four names, and each one a genuinely different creature to look at.

And Then It Starts Again

Out of the pupa comes the adult, which settles on the water for a moment before taking off, and it may go on living another ten to fifteen days. The female of that generation then puts her own eggs down on water or close beside it, and the whole round begins once more.

Why They Keep Surfacing

Watch larvae and pupae in water and they come up to the surface again and again. The reason is simple. They live in water but they need air in order to respire, so they have to come up for it.

How the Cycle Is Broken

That fact is exactly what kerosene oil exploits. Sprayed on stagnant water it forms a thin layer over the surface. The layer separates the water from the air, so the larvae and pupae cannot take air in, and they die.

How Different the Stages Look

Significant changes happen in appearance, body shape and structure from one stage to the next. The egg looks nothing like the larva, the larva looks nothing like the pupa, and the pupa looks nothing like the adult mosquito.

A Walk to a Shallow Pond

Now to a different animal. After a week of rain, a class walks out to a shallow pond ringed with trees and tall grasses, with instructions to watch everything from a distance and disturb nothing. Everybody has come in full sleeves and full trousers.

Spawn

At the edge of the water, and sometimes stuck to plants growing in or around it, is a white jelly-like substance. It is a cluster of frog eggs, and it has a name of its own: spawn.

The Stages of a Frog

There are four. The egg stage, which goes on to the embryo. The tadpole stage, first with a tail and no legs, and later with hind legs. Then the froglet stage. Then the adult frog. Some of the drawings in a textbook are grouped together, so the six you see may belong to only four stages.

What the Tail Is For

Tadpoles develop legs while still keeping their tails, and the tail is not left over from anything. It is what helps them swim, which is the only way of getting about that is open to them at that stage.

Froglet to Frog

Little by little the tadpoles take on the look of small frogs, and at that point they are called froglets. Water is still their home, though they now begin spending part of the day out of it. They keep growing, the tail goes altogether, and the legs turn strong enough to jump with and land on.

A Change of Home

Notice what has happened besides the change of shape. A tadpole lives only in water; an adult frog lives both in water and on land. The silk moth is worth adding here: it also passes through egg, larva, pupa and adult, and its larvae wrap themselves in the threads that silk fabric is made from.

Scenario:

Four open drums stand behind a school kitchen, catching rainwater for washing. After a fortnight of rain the class inspects them with a hand lens and finds wriggling creatures in three of them. Three suggestions follow. Nandini wants kerosene oil poured on all four drums at once, since that is what her mother does. Pravin says the creatures are harmless, because they are not mosquitoes and cannot bite anybody. Sumitra says the school should simply cover the drums and empty the fourth one, which is cracked and leaking anyway. The teacher asks one question: what are you actually trying to stop?

Analysis:

All three suggestions touch something real, but they aim at different points in the same cycle, and one of them rests on a mistake about what the creatures are.

Activity:

Walk round your school or your home with a notebook and list every container that could hold standing water, marking which of them actually does. Rule a column for what could be done about each one, and a column for who would have to do it. Then check the same list again a week later and note what has changed.

Title:

Audit the standing water around you

Safety:

Take an adult with you, do not put your hands into any standing water, and do not try to move heavy containers yourself. Wash your hands afterwards.

What You Notice:

Most people find more containers than they expected, and the ones that matter are rarely the obvious ones. Coolers and the saucers under planted pots are the usual offenders. If you find two different shapes moving, you have found both the larva and the pupa, and that is worth reporting to a teacher.

An adult mosquito rests briefly on the water surface after emerging, then flies away, and may survive ten to fifteen days.

The silk moth passes through the same four stages, and its larvae wrap themselves in the threads that silk fabric is made from.

Several centres for rearing silk have been opened around the country by the Khadi and Village Industries Commission.

The mosquito stages — Egg, larva, pupa, adult — Four in order, and the silk moth follows the same four.

The frog stages — Spawn, tadpole, froglet, frog — The tail goes and the legs come, in that order.