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

On a school trip to a guava orchard, Kavya reached for a thin brown twig lying on a leaf. The twig stood up on six legs and walked away! It was a stick insect, an animal that looks just like a dry stick.

That evening, her cousin Rohan showed off his new robot dog. It trotted across the room, wagged its tail and yapped when he clapped. Kavya laughed. “The twig that looked dead was alive. The dog that looks alive is not. So how do we tell?”

In this chapter we hunt for the signs that every living thing shows. Then we watch seeds wake up and see roots and shoots pick their direction. Last, we follow mosquitoes and frogs as they change shape while growing up.

10.1 What Sets the Living Apart from the Non-living?

What do a sparrow, a neem tree and you share that a bicycle, a cloud and a stone do not? It cannot be moving, because a bicycle rolls and a cloud drifts. It is not size or colour either. The answer is a whole set of things that living bodies do.

Scientists list these signs, or characteristics, that all living things share. Let us look at them one by one. We will test each sign on a puppy and on a ceiling fan (Fig. 10.1).

Moving

Animals walk, fly, swim, crawl and slither from place to place. But movement alone can fool us. A ceiling fan turns, a river flows and a bus races down the highway. None of them is alive.

Plants seem to stay still. A plant cannot walk to a better spot. Yet parts of a plant do move, only more slowly than animals (Fig. 10.2).

• Flowers open and close. A lotus opens in the morning sun and closes towards evening. The four o’clock plant (गुलअब्बास gul-abbas) opens its flowers late in the afternoon.
• Climbers hold on. A bitter gourd (करेला karela) vine has thin, curly threads called tendrils. A tendril slowly winds around any stick or wire it touches.
• Some plants catch insects. An ant crawls onto a tiny sundew plant, drawn by what looks like sparkling dew. But each shiny drop is glue, on the tip of a red bristle. The ant is stuck! Over the next hour or so, the bristles bend in over it like closing fingers. Then the plant slowly digests its meal. Plants that feed on insects like this are called insectivorous plants. Sundews grow on wet, sandy ground, as in parts of the Western Ghats.

Food and growth

In many homes, a child’s height is pencilled on a door frame every birthday. Year after year, the marks climb higher. Your body has grown. The mango sapling your grandparents planted may now be taller than your house.

Living things grow from the inside, by building more of their own body. A heap of sand gets bigger only when someone tips more sand on it. That is not growth.

To grow, living things need food. Food gives them energy, and the material to build their bodies. Getting and using food is called nutrition (पोषण). Green plants make their own food in their leaves. Animals eat plants, or other animals.

Breathing and respiration

Put a hand on your chest and sit still. Feel it rise and fall? That is breathing. You do it more than twenty thousand times a day without a single thought. Does it always go at the same speed? Count and find out (Fig. 10.3).

Watch a pigeon perched on a ledge: its feathery chest puffs out and sinks back, again and again. The soft throat of a house lizard pulses in the same way. All animals take in air somehow. A fish takes in air dissolved in water, through its gills.

Why does a busy body need more air? Your body uses air to release energy from the food you have eaten. That energy lets you run, think and even dream. This use of air to get energy from food is called respiration (श्वसन). Breathing is the job of bringing fresh air in and sending used air out, so that respiration can carry on. It never stops, whether you are awake or asleep.

A plant has no nose and no lungs. So how does air get inside it? Roots and seeds take in air from the gaps between the grains of soil. Leaves have doorways of their own: thousands of tiny pores, far too small to see without a microscope (Fig. 10.4). These pores are called stomata (say STOH-ma-ta). Air moves in and out through them, and the plant respires all day and all night, just as you do.

Getting rid of wastes

Every busy body makes wastes, and it has to get rid of them. Your body filters wastes out of your blood and sends them out in urine. On a hot day your skin helps too. After a cricket match in the May heat, lick your upper lip: it tastes salty! Sweat is water carrying salts that your body does not need. Scientists call this clearing out of wastes excretion (उत्सर्जन).

Plants have wastes to clear too. Early on a cool morning, look at the pointed tip of an arbi (colocasia) leaf. A single bright drop may hang there. Dew would wet the whole leaf, but this drop sits only at the tip. Tomato leaves may wear a row of tiny drops along their saw-toothed edges. The plant is pushing out extra water with minerals dissolved in it. Some trees also store wastes in old leaves, which later fall off.

Responding to changes

A mosquito lands on your arm, and your other hand slaps it before you can think. A firecracker bursts nearby, and you jump. The smell of hot jalebis makes your mouth water. Each time, something around you changed, and your body reacted.

The change that sets off a reaction is called a stimulus (उद्दीपन). When there is more than one, we say stimuli. What the body does in reply is its response. A street dog lifts its ears at the crackle of a biscuit packet. The crackle is the stimulus, and the lifted ears are the response.

What about plants? Run a finger gently along a leaf of the touch-me-not (छुई-मुई chhui-mui or lajwanti). Within seconds its tiny leaflets snap shut in pairs, and the whole leaf droops (Fig. 10.5). Plants react to light and darkness as well. At sunset, the leaflets of the tamarind (imli) and the big rain tree close up in pairs. The tree seems to be settling down to sleep. Morning light opens them again.

Making more of their own kind

Hens hatch chicks, cows give birth to calves, and a fallen mango seed can grow into a new mango tree. Each time, living things have made young ones like themselves. This is called reproduction (जनन). A scooter, however old it gets, never leaves behind a little scooter!

Imagine if no kitten, calf or seedling were ever born again. Every living thing lives only for a limited time. So one day there would be no cats, no neem trees and no people. Reproduction keeps each kind of living thing going, generation after generation.

When life ends

Signs of life do not last for ever. Think of a butterfly lying still on a windowsill. Air, water and flowers are all around it, but its body can no longer use them. When all the signs of life stop for good, the living thing has died. A dry fallen leaf and a grain of cooked rice were once parts of living things. Now they cannot grow, respire or respond.

Now go back to your table in Activity 10.1 and fill in the last two columns. Did any of your first guesses change?

10.2 Essential Conditions for Germination of a Seed

Soak a handful of moong in water at night. By morning the seeds are fat and soft. Wrap them in a damp cloth, and a day later tiny white tails poke out. At home we call them sprouts. Each seed has started to grow into a new plant. This start is called germination (अंकुरण).

Yet a jar of dry moong can sit in the kitchen for months without sprouting. So what exactly does a seed need before it starts to grow? Let us test some ideas, one at a time.

How water, air and warmth help

A dry seed is small and hard. Inside it, a tiny baby plant called the embryo (भ्रूण) lies resting, with a packed lunch of stored food. The tough outer skin around it is the seed coat. Add water, and the seed drinks it up and grows fat. Its coat turns soft and cracks open. Now the embryo can wake and begin to grow (Fig. 10.8).

What about light? Deep in the soil it is pitch dark, yet most seeds sprout there happily. The need for light begins when the shoot pokes above the ground, because leaves need sunlight to make food. Sprouts kept in a dark cupboard grow pale yellow, thin and floppy. On a sunny windowsill they turn green and sturdy.

A waking seed also needs air, because it respires to get energy from its stored food. That is why the seeds in Cup 4 hardly sprouted: water had pushed the air out of every gap around them. In a field, the soil is full of tiny air pockets between its grains. Farmers plough before sowing to loosen the soil. Loose soil holds plenty of air, and young roots push through it easily.

Seeds also need the right warmth. Most seeds will not sprout inside a fridge, and great heat can harm them. That is one reason why farmers sow each crop in its own season.

10.3 Growth and Movement in Plants

Drop a sprouting chana seed on the soil any way up. A few days later, its root is heading down into the soil and its shoot is heading up. How does the seedling know which way is which? And what does light have to do with it?

Look at the roots first. In all three cups, every root tip ended up pointing down, even the one that began by pointing at the sky. It simply curved round in a U-turn.

Now the shoots. The shoots in Cups A and B both ended up pointing up, so the upside-down one had to turn round too. Only the shoot in Cup C leaned sideways, towards the window of its box.

So a root follows the pull of the Earth, called gravity, downwards. This takes it deep into the soil, where water and minerals are. A shoot grows against that pull, and towards light. Its leaves need the light to make food. The bending that we see is a slow kind of movement, made by growing more on one side than on the other.

10.4 Life Cycle of a Plant

A seed sprouts, and the seedling grows. What happens after that? Does a plant simply keep on growing for ever? To find out, you need patience, and a plant diary that runs for about three months.

Your diary tells a story (Fig. 10.12). The seed sprouts in about a week. Leaves unfold and the plant grows taller. After about six weeks, pale yellow flowers open. Each flower lasts only a day. Its petals fall, and the part left behind swells into a green pod: the bhindi we cook. A pod left on the plant turns brown and dry, then splits open to show rows of round seeds.

Look at the first and last pages of your diary. The story began with one seed, and it ends with dozens of seeds in every dry pod. Seeds make plants, and plants make seeds, so the story loops round like a circle. This loop is the life cycle (जीवन चक्र) of the plant. And the old plant? Once its pods are dry, it yellows and wilts, however carefully you water it. Its life has ended, but its seeds carry life on.

Not every plant dies after making seeds. A mango tree lives for many years. It flowers late in winter and fruits in summer, year after year. Each mango carries a seed that can start a new tree. Still, even a mango tree grows old and dies one day, while its life cycle carries on through its seeds.

10.5 Life Cycle of Animals

Animals too are born, grow up, have young ones and die. Many baby animals look like small copies of their parents. A kitten is a tiny cat, and a calf is a small, wobbly cow. Some babies, though, look nothing like their parents, as you will soon see.

Sketch two baby animals you have seen, next to their parents. Which babies look like small adults, and which look quite different?

10.5.1 Life cycle of a mosquito

Every monsoon, health workers in many towns go from house to house. They tip out the water standing in flowerpot saucers, coolers and old tyres. Why fuss over a little still water? Bzzzz! The answer is whining around your ears at night. Only female mosquitoes bite: they need a meal of blood to make their eggs. As they feed, some kinds pass on germs that cause malaria, dengue or chikungunya.

Here is the link. A mosquito lays her eggs on still water. Some kinds lay them on the damp inside wall of a pot or tank, just above the water line. The young ones that hatch can live only in water.

Peer into a bucket or a drum of water that has stood for a few days. Thin, wriggly creatures may be hanging just under the surface, heads down. These are mosquito larvae (लार्वा), often called wrigglers. A few comma-shaped ones may tumble towards the bottom when you tap the side. These are pupae (प्यूपा), or tumblers.

Watch a wriggler for a minute and you will see it keep coming back up to the top. It is coming up for air! Each time, it pokes a short breathing tube at its tail end out into the air. A tumbler comes up to breathe too, through two tiny tubes on its back, like a pair of little trumpets (Fig. 10.13).

This need for air is their weak spot. Picture a wriggler swimming up for a breath. If a very thin film of oil covers the water, its breathing tube pokes into oil instead of air. Soon it suffocates. Tumblers meet the same fate. Health workers sometimes use this trick in drains, ditches and ponds that cannot be emptied. At home, the simplest way is to empty, scrub or cover every container of water once a week.

Add the egg to the jar diary, and you have a mosquito’s four stages: egg, larva, pupa and adult (Fig. 10.14). In warm weather, the whole journey from egg to flying adult takes only about a week to ten days. The last change happens right at the surface. The pupa’s skin splits along its back, and the adult pulls itself out. It balances on the empty skin, like a sailor on a tiny raft, until its wings dry and harden. Then it is off.

A female mosquito may live for about two weeks, laying batch after batch of eggs. She often picks a fresh patch of still water each time, and every batch can start the whole cycle again.

Each stage has a body built for its own job. The tiny dark egg simply floats and waits. The bristly larva is an eating machine that swims and grows. The curled-up pupa neither eats nor grows: inside it, the adult is being built. The adult has wings, long legs and a needle-like mouth. It flies off to feed and find a mate, and each female lays new eggs.

10.5.2 Life cycle of a frog

After the first heavy monsoon showers, the night fills with the croaking of frogs. The males are calling to the females. Soon the ponds, puddles and paddy fields hold something new.

Frog eggs are easy to miss. At first they look like a lump of clear, wobbly jelly. Look closer and you will see it is packed with hundreds, sometimes thousands, of tiny black dots. Each dot is an egg. A clump of frog eggs in jelly is called spawn. The jelly holds the eggs together and keeps them wet. It also makes them too slippery for many hungry fish to grab. Some frogs leave their spawn floating in the shallows; others wrap it around water weeds.

Inside each egg, an embryo grows. In about a week, a tiny tadpole (टैडपोल) wriggles out. It looks more like a little fish than a frog. It has a big head, a long tail for swimming and no legs at all. It breathes under water, as a fish does, and nibbles soft water plants.

Over the next few months, watch three things change: the legs, the tail and the way the young animal breathes (Fig. 10.15). The back pair of legs sprouts first and the front pair later. The tail stays long for weeks, then slowly shrinks. The animal also starts rising to gulp air at the surface.

By about three months, the young frog is a froglet: four legs, a stubby tail and lungs ready for air. Now it is time to move house. It hops onto the wet bank for short trips, then longer ones. Its body slowly absorbs the last bit of its tail. The grown-up frog splits its time between two homes. It hunts insects in the grass, and slips into the pond to cool off, hide or lay eggs.

Each stage of a frog suits its home. A tadpole’s tail and gills work in water, while an adult’s strong legs and lungs work on land. The shortest stage is the egg and embryo, which lasts only about a week. The longest is the slow change from tadpole to frog.

This chapter followed three journeys: the bhindi’s, the mosquito’s and the frog’s. Each one ends where it began, with a seed or an egg ready to start again. That is what makes them cycles. Filling in a pond or cutting down a tree breaks such cycles for many living things. That is one more reason to protect them.