Diversity in the Living World: Class 6 Science · Chapter 2

Look around: so many lives

Sit still on a school bench for five minutes and simply look. A line of ants marches along a crack in the wall. A pigeon struts on the parapet. A squirrel races up the neem tree, flicking its bushy tail. Tiny purple flowers peep out of the grass, and a dragonfly zips past your nose. How many kinds of living things did you spot?

Now picture the same five minutes beside a village pond, on a snowy Himalayan slope or on a beach in Goa. You would meet a completely different crowd! In this chapter we explore this huge variety of life. We also learn how scientists sort it into groups, and why each plant and animal suits the place where it lives.

2.1 Diversity in Plants and Animals Around Us

People who study living things do not start with a book. They start outdoors, with sharp eyes, open ears and a notebook. They look, listen, sniff and write things down. You can do exactly the same.

Look closely at where leaves join a stem. On a money plant they take turns: one leaf on the left, the next a little higher on the right. This is an alternate arrangement. On a guava twig, leaves grow in pairs facing each other: an opposite arrangement. On a kaner branch, three leaves often sprout from one point, like the spokes of a wheel. This is a whorl.

Who needs whom? Watch a bee dive into a flower. It sips nectar, and pollen sticks to its fuzzy body, ready for the next flower. Under your feet, earthworms turn fallen leaves into rich soil for roots. Up in a big tree, birds find nest holes, shade and fruit. In return, the birds and bats that gobble the fruit drop its seeds far away, where new trees can sprout. Plants need animals, and animals need plants. Follow one tree and its visitors in Fig. 2.2.

2.2 How to Group Plants and Animals?

Walk into a busy kirana shop and ask for a packet of jeera. The shopkeeper grabs it in a second. How? All the spices sit together on one shelf, the soaps on another and the biscuits near the counter. Things that share a feature are kept together in one group.

We can sort living things the same way. First we choose a feature, then we put together everything that shares it. The feature we choose is the rule, or basis, of the grouping. Change the rule, and the groups change too.

In Fig. 2.3, the same eight living things are sorted by two different rules. Notice what happens when both rules are used together.

Why do scientists bother? Grouping turns a huge, confusing crowd into a few groups that are easier to study and remember. Better still, once you know which group a plant belongs to, you can guess many of its other features without even looking. You will see this trick at work very soon. You will also meet grouping again, this time for objects, in the chapter 'Materials Around Us'.

2.2.1 How to group plants?

Where should we start sorting plants? We could use flowers, leaves, roots or seeds, and we will try some of these soon. The easiest start is the whole plant, with your own body as the measuring tool. Three checks do the job: how tall the plant is, what its stem is like, and where its branches begin.

Start small. Pinch a pudina stem gently between two fingers. It is green and juicy, and it bends without snapping. Plants with soft, green stems like this rarely grow taller than your waist. They are called herbs (शाक). Dhania and palak in a kitchen garden are herbs, and so is the wheat in a field.

Next, crouch beside a mehendi hedge and peep at its base. You will not find one trunk. Instead, a bundle of hard, brown stems, each about as thick as a finger, fans out just above the soil. The whole bush reaches about the height of a grown-up. This is a shrub (झाड़ी). Kaner and chandni grow the same way.

Last, wrap your arms around a jamun or a neem. You are hugging its trunk: one thick, woody stem that holds up the whole plant. Look up, and the branches begin well above your head, so you can walk beneath them. A plant built like this is a tree (वृक्ष). Peepal and teak are trees too.

Try to stand a lauki (bottle gourd) vine upright on its own. It flops! Plants with floppy stems manage in one of two ways. A climber clings to something tall and works its way up. The lauki wraps thin, springy threads called tendrils around a trellis. A money plant grips walls with little roots that grow from its stem. A creeper does not bother to climb at all: it sprawls across the ground, like pumpkin, watermelon and sweet potato.

Sort nine more plants on the Herb, shrub or tree? tab of Fig. 2.4. It has a tray for climbers and creepers too.

Veins in leaves

Hold a leaf up to the sunlight and look through it. A fine design appears, a bit like the lines on your palm. Each line is a vein, a tiny pipe that brings water into the leaf and carries food away from it. Botanists call the design these veins make the leaf's venation.

Your two piles of rubbings have scientific names. The road-map pile shows reticulate venation (जालिकावत शिराविन्यास). Reticulum is Latin for a little net. Mango, peepal, rose and tulsi leaves belong in this pile. The notebook-lines pile shows parallel venation (समानांतर शिराविन्यास). Grass, maize, bamboo, sugarcane and onion leaves belong there.

Roots

Have you ever pulled weeds out of a flowerpot? Some slide out easily with a soft tuft of roots. Others hold on tight, as if pegged into the soil. Half of a plant's story is hidden underground.

So a plant can have a taproot or fibrous roots. Here is a puzzle. You cannot see roots without digging, but leaves are easy to see. Could the leaves give away the secret hidden in the soil?

So a leaf can tell you about the roots hidden below it. If a plant's leaves have a net of veins, it most likely has a taproot, as pea and sunflower do. If its veins run side by side, it most likely has fibrous roots, as sugarcane and bamboo do.

Seeds

Crack a roasted peanut out of its shell and rub off its thin red skin. Now press gently along its edge. It falls neatly into two halves! Does every seed do this?

Scientists name plants after this difference. Di means two, so a plant with two seed leaves is a dicotyledon, or dicot for short (द्विबीजपत्री). Mono means one, so a plant with a single seed leaf is a monocotyledon, or monocot (एकबीजपत्री). Peanut, pea, moong and mango are dicots. Maize, rice, wheat and sugarcane are monocots.

Now for the trick promised earlier. Seeds, leaves and roots are three separate clues, yet they point the same way. Watch the clues line up in Fig. 2.10.

Now try the Dicot or monocot? tab of Fig. 2.4. Each card gives you just one clue: a seed, a leaf or a root.

2.2.2 How to group animals?

Animals are an even wilder mix. Some are smaller than a grain of rice, while a big elephant can outweigh a whole busload of people. Some wear feathers, some fur, some scales and some shells. Any of these differences could be a rule for sorting them. Let us try one of the most eye-catching rules: how they move.

So movement makes a handy rule: fliers here, swimmers there, walkers, hoppers and crawlers somewhere else. It is only one rule of many, though. A zoo keeper might sort animals by what goes in their feeding bucket. Someone else might sort them by what covers their bodies: feathers, fur, scales or a shell. Every clear rule helps us see order in the crowd. Try the How does it move? tab of Fig. 2.4.

2.3 Plants and Animals in Different Surroundings

A fish washed up on a sandy beach gasps for breath. A squirrel that tumbles into a pond paddles frantically for the edge. Each living thing is built for its own kind of place. A fish has a smooth, pointed body that slips through water, and fins to push and steer. A squirrel has sharp claws that grip tree bark, and a bushy tail that keeps it balanced on thin branches.

Does the place decide which plants and animals live there? Let us collect some evidence.

Imagine you are a plant in the Thar desert. Nobody waters you, and a good shower may come only a few times a year. You also cannot move into the shade at noon, or find a blanket when the night turns chilly. A cactus copes by being its own water bottle. Its thick, fleshy stem soaks up water after each rare shower and stores it for the long dry weeks. Its leaves have shrunk to sharp spines, so very little water escapes, and thirsty animals keep their distance.

Now travel north to the hills of Himachal Pradesh. Wet snow is surprisingly heavy: the snow on one wide branch can weigh as much as several buckets of water. In winter a Himalayan tree must hold up load after load of it. The deodar's trick is its shape. It is a tall cone, like a half-closed umbrella, and its branches slope downward and bend without breaking. Snow slips off before it can pile up. Compare it with a broad, flat-topped tree in Fig. 2.13.

Change the conditions and you change the cast. Scorching sand and freezing snow each suit their own special set of living things. That is why a list of living things from the Thar looks nothing like one from Himachal. Each region, with its own heat, cold, rain and soil, ends up with its own mix of life.

Animals show the same pattern. India has camels in two very different deserts. The hot, sandy Thar desert is home to camels with one hump. The cold, rocky desert of Ladakh is home to camels with two humps. Watch how their bodies differ in Fig. 2.14.

On a hot May afternoon, you might sweat through your shirt and gulp glass after glass of water. A camel plays it differently: it holds on to almost every drop it drinks. It barely sweats, its dung comes out as dry pellets, and it passes only a little urine. That is how a camel can walk for days between drinks. Its humps are a store of fat, used as food when grazing is poor.

Sikkim is so fond of its rhododendrons that one of them is the state tree. In Sikkim's damp mountain forests, many rhododendrons grow into tall trees that blaze with flowers in spring. Now travel south to the bare, wind-blasted hilltops of the Nilgiris in Tamil Nadu. Here the rhododendrons are short and stocky, with small, tough leaves. Up there, gales would tear big leaves and topple tall trunks. Same kind of plant, different build, each suited to its home.

Adaptations and habitats

The cactus's fleshy stem, the deodar's sloping branches, the camel's wide feet and the fish's fins are all adaptations. So is the short, sturdy shape of a Nilgiri rhododendron. Each one fits its owner to a particular kind of place. Flip the cards below to test yourself.

Think of a habitat as a home address with everything included. It has a roof (shelter), a kitchen (food), a tap (water), fresh air and a safe corner to raise young. A village pond is one such address. Lotus, rohu fish, frogs, snails and dragonflies all live there together, like families in one housing colony. A yak's address is a cold mountain meadow, and a hornbill's is a tall forest.

Scientists often sort habitats with one simple question: land or water? Follow a raindrop to meet both kinds. It lands on a Himalayan slope, trickles through a sal forest and runs across a grassy maidan. These are homes on land, or terrestrial (स्थलीय) habitats, from terra, Latin for earth. Then the drop slips into a stream, rides down the Ganga and reaches the salty Bay of Bengal. These are homes in water, or aquatic (जलीय) habitats, from aqua, Latin for water.

A few animals, such as frogs and toads, spend part of their lives in water and part on land. A frog starts life as a tadpole in a pond. As a grown-up it hops on land but returns to water to lay eggs. Such animals are called amphibians (उभयचर). Their name means 'two lives'.

When habitats are harmed

Imagine waking up to find your house, your kitchen and your water tap gone overnight. That is what habitat loss means for a plant or animal. A kingfisher whose pond is filled in must search for a new home. So must a hornbill whose forest is cut down. If they cannot find one, they will not survive. Destroy enough habitats, and whole kinds of living things grow rare and then vanish. Biodiversity shrinks.

Ask your grandparents about the house sparrow, the gauraiya. Many will tell you it once nested in almost every home. Old houses had gaps under tiled roofs, open ventilators and cracks in the walls: perfect nest spots. Newer buildings are sealed tight, and many gardens are now paved. In many cities sparrows have become rare. The good news is that nest boxes and native plants are helping them return in some places.

Protecting biodiversity is not only a job for governments and scientists. You can plant a native tree like neem or jamun. You can keep a bowl of water out for birds in summer and leave a wild corner in the school garden. And you can promise never to pluck, poke or trap. When habitats stay healthy, the living world keeps its wonderful variety.