My Science School

          Some people take a holiday each year in warmer parts of the world. Some animals do so, too, although for them it is not for pleasure but for survival. These animals go on migrations—long-distance, usually seasonal, journeys. They stay in an area where there is enough food, water, shelter or other needs. When conditions change, the animals travel on to find better surroundings, especially a place to breed.

          In some cases, the journeys can be fairly random. Herds of wildebeest (gnu) and zebra wander the African plains. They stay in an area while there are plants to eat. Once these are gone, they set off to find an area where recent rains have produced new plant growth. There is usually a yearly pattern to their movements. But sometimes an extra-long dry spell drives them far away.

          For most migrating creatures the journeys are at a regular time each year and usually follow the same routes as well. In the polar lands of the far north, the summer is short but the long hours of daylight and warmth allow plenty of plant growth. There are few resident animals to eat the plants. So birds such as geese fly up from the south in the spring to feed and raise their young in the Arctic. Then in the autumn, before the long, dark, icy winter grips the Polar Regions. They return south to temperate Europe, Asia and North America.

          Another group of migrating birds, such as swallows and swifts spend spring and summer in Northern temperate lands, feeding and breeding. Then, in autumn, they fly south to the warmth of the tropics.

          Birds are the main group of migrating animals because their power of flight allows them to cover long distances rapidly. Some land mammals migrate too, such as caribou. In Australia, herds of kangaroos and flocks of emus travel hundreds of kilometres across deserts in search of areas where rain has brought fresh plant growth.

OCEAN WANDERERS

          Animals migrate through oceans as well as across land. Some are regular to-and-fro migrants with the seasons, like grey whales. Their total yearly journey is more than 20,000 kilometres, making this whale the longest-travelled mammal. Green turtles are probably record-holders for reptiles. Some groups feed off the tropical coasts of South America and then swim over 2000 kilometres to breed on lonely Ascension Island in the middle of the Atlantic Ocean.

          Some ocean migrations are less regular. Salmon grow up in European and North American rivers, then swim out to sea. They wander the oceans for 10,000 kilometres or more before returning to their home rivers.

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          Many animals, from earthworms to whale sharks, lead simple lives. Their behaviour is limited and they only encounter others of their kind briefly to mate. Other creatures have much more complex behaviour. They form groups, have contests for group supremacy, mates and territories, help each other when feeding and even look after each other’s offspring.

          Living things strive to survive so that they can breed and pass on their genes to their offspring. An animal must choose its breeding partner carefully. Courtship behaviour and mating displays pick out a partner of the same species, the opposite sex, sexually mature, strong, fit and healthy. This increases the chances of the offspring being fit and healthy, too.

          In some animals, survival depends on having a territory. This is a patch of land or water where the owner can live and feed without being in direct competition with rivals of its own kind. Owners often mark their territories by rubbing scents, spraying urine and leaving piles of droppings around the borders. They defend the territory against other of their kind by calls and songs, and also by visual displays.

HELPING OTHERS

          Life in the wild is a battle to stay alive. But sometimes helping others can increase an individual’s own chances of survival. Some living things form partnerships with other, quite different species where both partners gain. This is known as symbiosis. Cleaner fish are small fish, such as wrasses, that tend to larger fish. The big fish could easily eat the cleaner. But the cleaner nips fish lice and other pests from its body, mouth and gills. The big fish is relieved of these parasites and the cleaner fish gets its meal.

          The hermit crab forms a symbiotic partnership with the calliactis sea anemone. The anemone protects the crab with its stinging tentacles. In return the crab carries the anemone to new places to catch victims. Also each partner may share in the leftovers of the other’s meal.

          Many group-living animals produce alarm calls and actions if they spot danger. This warns others in their group. Likewise if one group member finds a plentiful supply of food, the others gather round to share it. A few kinds of animals even work together to hunt prey. These co-operative killers include wolves, lions and African wild dogs.

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          Meat-eating predators, or carnivores, obtain their food by hunting and attacking other creatures—their prey. Most prey have some form of selfdefence and so the predator must overcome this. Many predators have strong and agile bodies, quick reactions, keen senses and hunting weapons such as sharp teeth and long claws. Some use speed to race after their prey. Others lurk hidden among leaves or long grass—perhaps also disguised by camouflage—then ambush their victims.

 

 

 

 

          Owls use silence and stealth to swoop on prey. The owl’s feathers have very soft edges so they make hardly any sound as the bird flaps and glides. The owl can see well at night with its huge eyes. It can hear even better and pinpoint a mouse’s footsteps or chewing noises in the darkness.

 

 

 

 

          Many animals use chemical weapons rather than physical ones. Some snakes and spiders have poisonous bites, while wasps and scorpions have venomous stings. The poison can also be used for selfdefence as well as subduing prey. These predators usually give warning that they are about to bite or sting an attacker. For example, a rattlesnake shakes its tail, while a poisonous spider rears up to show its fangs. This is because the supply of venom is limited and predators need it to hunt, so they try to avoid using it in defence unnecessarily.

 

 

 

DEFENCE

          Plant-eating animals need to defend themselves from carnivores. One defensive strategy is to fight back. Elephants, wild boar and warthogs can slash with their tusks. Gazelles, antelopes and wild cattle like musk oxen jab enemies with their sharp horns. Some, like zebras, can kick out powerfully with their hard hooves. Safety in numbers also helps—many eyes and ears are more likely to detect approaching predators. Musk oxen form a circle around their young to keep away wolves.

 

SPINES AND POISONS

          One type of physical protection is a hard body case, as in turtles, tortoises, armadillos, snails and beetles. Another strategy is prickles, spikes or spines, as in hedgehogs, porcupines and porcupine fish. Many animals, from mice to deer, rely on their sharp senses, speed and agility to escape as they dodge or outrun their predators.

 

 

 

          Certain fish, beetles, caterpillars, moths and butterflies have horrible-tasting or poisonous flesh. Predators soon learn to avoid them because they advertise this form of defence with bright body patterns called warning colours. Some animals puff up to look bigger, such as puffer fish, toads and lizards. Another strategy is suddenly to flash bright colours and patterns at the enemy, especially eye spots which resemble or mimic the eyes of an even bigger predator!

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          The process of evolution ensures that only living things that are suited, or adapted, to their environment will survive. An animal or plant that is poorly adapted to its surroundings soon loses out in the struggle for survival. Animals are adapted to three major features of their environment: the climate (including temperature and rainfall), the food sources available, and avoidance of predators.

          Life thrives in its greatest diversity in warm and moist or wet conditions, such as tropical rainforests and coral reefs. Places that are very cold or very dry provide the greatest challenge to animal survival. Yet some creatures can live in even the driest deserts. They include large animals such as camels and oryx, and smaller creatures like jerboas, lizards, scorpions and insects.

          In hot deserts, smaller animals often hide from the scorching sun by day and come out in the cooler night. They produce very little sweat, urine or other body liquids, thus saving valuable water.

          Desert creatures have adapted in various ways to moving over soft sand. The camel has very wide feet so it does not sink in. Jerboas and gerbils hop and leap on their large back feet. The sidewinder snake moves its body diagonally in a series of Z-shaped stages, pushing sideways against the loose grains. Some desert-dwellers, like the water-holding frog, burrow underground and sleep through the worst of the drought.

 

 

 

        Only warm-blooded birds and mammals are able to live on land or in the air in the Earth’s coldest regions, the Arctic and Antarctic icecaps. Reptiles and amphibians would simply be too cold to move. Musk ox, yak, seals and polar bears have thick furry coats to keep out the chill.

 

 

 

 

          Marine polar mammals like walruses and whales have almost no fur at all. But they do have a thick layer of fat under the skin, called blubber, to keep in body warmth. Birds such as penguins also have blubber. Other birds, like the ptarmigan and snowy owl, have extra-thick plumage. They can fluff out their feathers to trap a layer of air which keeps out the cold.

 

 

 

 

          One of the most varied habitats is the coast. The main change here is the twice-daily rise and fall of the tide. Animals such as crabs, worms, shrimp and shellfish are adapted to being active when they are covered by the seawater—whether it is day or night, winter or summer. Shellfish like limpets have strong outer cases to prevent them drying out at low tide and also being smashed by the waves. Soft-bodied worms and fish burrow in sand or hide under rocks for protection against winds and waves.

          One of the most constant habitats is the bottom of the sea. It is always dark and cold, with few water currents. The main problem at great depths is the enormous pressure of the water. Deep-sea fish, starfish and sea cucumbers would go soft and floppy (and die) if brought to the surface.

 

          One of the key features of an animal is that it moves. It moves parts of its body when it opens its mouth, bends its neck or curls its tentacles. Most creatures also move about in their surroundings. They run, walk, jump, hop, slither, swim or fly. A few animals do not move about, at least as adults. Barnacles and mussels are stuck to seashore rocks. But they were mobile during their young, or larval, stages.

 

 

MOVING IN WATER

          Many animals live in either the sea or in rivers and lakes. Water is much denser than air and so resists movement more. To travel through water quickly, creatures must be smooth and streamlined so the water slips past them easily (submarines are a similar shape for this reason). Fish such as sharks swish their tails from side to side to provide the forward propulsive force for swimming. A fish’s fins and a dolphin’s flippers provide control for steering, slowing and going up or down. Penguins flap their wings and “fly” through the water.

 

 

 

MOVING IN AIR

          Only three groups of living animals truly fly in a sustained, controlled way—birds, bats and insects. Air is so thin it produces little resistance to movement, but it provides very little buoyancy either. Fliers flap their wings down to create a lifting force as well as back to push themselves forwards.

          Birds and bats have very thin, light bones to reduce body weight and so save on the energy needed to stay airborne. Even so, they must take in almost twice the amount of energy as food, compared to their ground-dwelling counterparts. The down flapping wing muscles in the chest are by far the largest muscles in the body of a bird or bat. Insects and bats control their movement by tilting or twisting their wings. Tiny insects like gnats and midges flap their wings nearly 1000 times each second to stay aloft. Birds control their flight by fanning and twisting their feathers. Hummingbirds are the fastest-flapping birds, with up to 80 wing beats each minute.

          All animal movement is made possible by muscles. A gorilla has about 640 muscles, making up about three-fifths of its body weight. A fast fish has 30-40 large muscles along each side of its body. Forming nine tenths of its body weight, they pull on the backbone to swish the tail from side to side.

 

MOVING ON LAND

          Large land animals move in a way suited to their habitat. On open plains, cheetahs, gazelles and antelopes have long legs and run very fast. Forest animals like deer are slower in straight-line speed, but more agile as they zigzag between trees. Other animals, such as squirrels and sloths, are adapted to moving in the trees themselves. They have long, sharp claws to grip the bark. Central and South American monkeys have long tails, called prehensile tails, that curl around branches like a fifth limb. Limbs are not vital for movement. Many snakes are limbless, yet they can slither on the ground, swim in water, and climb trees— and even glide!

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          Most animals move around as they search for food, shelter or mates and avoid danger. So they have senses to detect what is going on around them. We have the same five main senses as many animals—sight, hearing, smell, taste and touch. Our main sense is sight. Compared to many animals, our eyes see clearly, in detail and in an especially wide range of colours. However, some animals have much better sight and other senses than we do. Some can even detect what we cannot, like tiny pulses of electricity.

          Some animals are nocturnal or active at night. They include cats, mice, bats, owls and moths. Their large eyes pick up as much of the faint light as possible. Animals that live in total darkness, like moles and cave salamanders or fish at the bottom of the sea have tiny eyes or none at all.

          The eye contains specialized nerve endings that detect patterns of light and send information about them to the brain. Other senses work in a similar way. In the ear, the eardrum is a thin piece of skin that vibrates when sounds hit it. Again, nerve endings detect these vibrations. Mammals, birds, lizards and frogs have eyes and ears on the head. However, some animals have them in other places on the body. A snail has eyes on flexible stalks. A clam has a row of small, simple eyes in the fleshy frill or mantle along the gaping edge of its shell. A grasshopper has eardrums on its knees.

 

 

SMELL AND TASTE

          Smell and taste are chemosenses. They are based on the presence of chemical substances, called odorants for smells and flavorants for tastes. We smell airborne odorants with the nose and taste flavorants in food and drink when they touch the tongue. Some animals have chemosensors on other parts of the body, too. A fly can taste with its mouthparts, its antennae (feelers) and its feet. A male moth’s feathery antennae can detect special floating chemicals given off by the female moth even if she is two or three kilometres away.

          A dog can smell scents up to 10,000 times weaker than we could detect.

          For animals in water, smell and taste are much the same. A shark has groups of chemosensors (taste buds) all around the inside of its mouth and also on the front of its snout. They are especially sensitive to blood and body fluids. A catfish has so many chemosensors in the skin all over its body that it is like a “living tongue”. Some fish including sharks, rays and elephant-snout fish (mormyrids) can detect the tiny electrical pulses given off by the active muscles of other animals. They use their electrosense to find prey in cloudy water or hiding in sand and mud.

 

 

     

 

         Electricity travels well in water, so many water animals have evolved to sense it. Electricity does not travel through air so land animals do not sense it. There are other senses that we lack and that we find difficult to imagine. Some animals migrate vast distances across featureless oceans with amazing accuracy. They may be able to sense Earth’s natural magnetic field or the way our planet’s downward pull of gravity varies slightly from place to place.

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          Of the five great kingdoms of living things, the animal kingdom is by far the largest. Scientists have identified at least two million different kinds or species, from tiny worms smaller than a full stop, to huge elephants and blue whales. There are almost certainly at least the same number of species still to be identified – mainly insects in tropical forests. The total number of animal species may be far higher: 10 million, or even more. The key features of an animal are that it has a body made of many microscopic cells and it gets its energy and nutrients by eating - that is, it takes in or consumes food. Most animals can also move about during all or part of their lives.

          Most animals ingest their food. This means they take food items into their bodies through an opening, the mouth, to be digested and absorbed inside. However a few creatures feed in more unusual ways. A tapeworm inside another animal’s gut is surrounded by ready-digested food. Also the tapeworm has no mouth. It simply soaks in or absorbs the nutrients through its very thin body surface.

 

MOUTHS

          There are almost as many different sizes and designs of animal mouth as there are different kinds of food. Most mammals like ourselves have teeth that can bite smaller pieces from a large food item and then chew the pieces into a soft pulp that is easily swallowed. But some mammals have few teeth—or none at all. The anteater collects its tiny food items of ants and termites by flicking out its long, sticky tongue. The vampire bat has front teeth like razor blades but it only uses them to slice a slit in its victim’s skin. Then it uses its tongue to lap up its meal of blood.

          Birds lack teeth, although they can peck powerfully with their strong, horn-covered beaks. Some birds have long, thin beaks like tweezers for probing into cracks or mud for small food items. Others have deep, short, powerful beaks that work like nutcrackers, for splitting seeds and nuts. Frogs also lack teeth for biting and chewing. They grab their food and gulp it down whole.

 

FOOD

          Most animals eat mainly plants or plant parts such as leaves, fruits, seeds, shoots and roots. They are herbivores. Some animals eat mainly the flesh of other creatures (sometimes their whole bodies). They are called carnivores. Other animals eat a wide range of both plant and animal food and are known as omnivores. A few animals, like dung beetles and crabs, eat dead or rotting food and wastes. They are detritivores.

          Food is broken down by the digestive system. This is usually a long tube coiled inside the animal’s body. The first part is the bag-like stomach with its muscular wall. It stores and squashes the meal. Next is the long intestine that absorbs the nutrients. Wastes pass out of the other end, the anus.

 

 

PREDATORS AND PARASITES

          A predator is an animal that hunts down and catches its victims, known as the prey. Some predators are large and fierce, like lions, wolves, crocodiles and sharks. Others are small but just as fierce, like shrews, newts, rag worms and diving beetles.

A parasite is an organism that takes its nourishment or shelter from another, the host. It may harm the host in the process but does not necessarily kill it. Fleas, lice, ticks and tapeworms are animal parasites. Mistletoe is a plant parasite of other plants.

          A few kinds of animals do not feed at all. A young mayfly leaves the water, splits its old skin and flies off as a winged adult. But it has no mouth and cannot eat. It mates and dies within a day.

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          A tree is a large plant with a woody stem or trunk, covered with a layer of bark. There are two main groups of trees: broadleaves and the conifers. Broadleaved trees are flowering plants that produce fruits with seeds inside. Conifers produce cones, which carry seeds on the face of each of their scales.

          Many broadleaved trees are deciduous: their leaves drop in autumn, or, in hot countries, during the dry season. Some broadleaves and nearly all conifers are evergreen. Their leaves do fall, but not all at the same time. The palm tree, which grows in hot countries, is a different type of tree. It usually has no branches and only a few large leaves at its tip.

          Trees are a valuable resource. They give us fuel, timber, medicines, food, paper, rubber and even soap. Even more importantly, they take in carbon dioxide and give off oxygen, so maintaining the balance of gases in the atmosphere.

HOW A TREE LIVES

          Like all plants, a broadleaved tree has roots and a shoot. The shoot is made up of a trunk (its stem) and branches bearing leave buds and flowers. The trunk holds up the tree while the branches and twigs spread out the leaves so that they receive as much sunlight as possible. The leaves themselves grow in a spiral pattern to avoid shading. Water (blue arrow) is drawn up from the soil to the leaves through the sapwood. The leaves use the water and sunlight, as well as carbon dioxide in the air to make food by photosynthesis. This food (red arrow) passes from the leaves to all other parts of the tree through the inner bark.

          At the base of the tree, a network of roots spreads outwards, anchoring it into the ground. Behind the root tips lie the root hairs which soak up water and nutrients from the soil. A large tree may take up several hundreds of litres of water every day.

A TREE’S YEAR

          As spring arrives, the buds of the horse chestnut tree open, the shoots lengthen and the leaves unfold. Flowers blossom, ready for pollination. In summer, the leaves are fully open. The fruits, made up of a spiny casing with a large seed or “conker” inside, ripen and fall to the ground. During autumn, the leaves turn brown as food drains from them into the trunk. A scar forms at the base of each stalk and the leaves fall off. In winter, the tree is protected by its waterproof bark. The buds, next year’s leaves and flowers, are covered by scales.

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SEEDS AND FRUITS

          After male pollen grains have been carried to the female parts of a flower, the male and female cells join and begin to develop into the baby plant. The flower parts are no longer needed and they shrivel away, to be replaced by the developing seeds in the seed-head. A seed is usually accompanied by a food store for its early growth, neatly packaged inside a casing. Some plants, like orchids, produce many thousands of tiny seeds. An ear of wheat is a head of wheat seeds or grains. We grind them up to make flour.

          Seeds have a better chance of growing if away from the parent plant. If they fall next to the parent, they would be in its shade and would also compete with it for soil nutrients. For these reasons, seeds have many ways of being spread far and wide.

 

FRUITS AND NUTS

          A fruit is the protective case around a seed. Some fruits are very light, like the feathery “parachutes” of the dandelion. They blow away in the wind. Some fall into water and float to a new place, like the coconut. A nut has an especially tough outer case. Animals may crack some nuts and eat the seeds within, but they also drop many as they feed. A squirrel buries nuts such as acorns, but may forget to dig them up, so in effect it has planted new oak trees! Some fruits have juicy, tasty flesh. These are known by the everyday name of “fruits”. The fruity part attracts animals to eat it. The seeds are spilled or pass through the animal’s guts to emerge unharmed and far away.

 

 

     

    A seed germinates, or begins to grow, only when conditions are suitable. This usually requires moisture of some kind, the right temperature, and perhaps darkness, which means the seed, is buried in the soil. Some seeds like those of the ironwood tree do not germinate unless they have been scorched by fire. This usually means many plants have burned away so the ground is bare and ready for new life. Other seeds do not germinate until after they have been cracked by frost and then warmed slightly, that is, when winter is over and spring has arrived. When conditions are right, the baby plant begins to grow using its store of food in the seed-leaves, or cotyledons. It splits its case, sends roots down into the soil and grows its shoot up towards the light.

 

FUNGI

          Mushrooms, toadstools, brackets, yeasts, moulds and mildews are all fungi. They form one of the five great groups or kingdoms of living things. Fungi are rotters. They grow networks of thin, pale threads, called hyphae, into the bodies of dead and dying plants and animals. The threads cause the body to decompose. They then absorb the released nutrients through their surface. Like bacteria, fungi are nature’s recyclers. They return the nutrients in dead animal and plant matter or animal droppings back into the soil.

 

          A fungus’s network of threads is known as the mycelium. It is usually hidden in the soil, inside a dead animal’s body or under a dying tree’s bark. So we rarely notice fungi at work. We are more likely to notice them when they reproduce. They do this by growing fruiting bodies. Many of these are shaped like umbrellas—we call them mushrooms and toadstools. The presence of a mushroom indicates a network of hyphae in the soil below, rotting down and absorbing nutrients. The mushroom’s top, or cap, releases millions of tiny fungal spores that blow away in the wind.

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          A plant may not look lively and active. But inside its millions of microscopic cells, thousands of chemical changes take place as part of the plant’s life processes. Like an animal’s body, a plant’s body has many specialized parts for different jobs. The roots take in water, minerals, salts and other substances from the soil in which the plant grows. The stiff stem holds the main parts of the plant above the surface, away from animals on the ground that might eat it, and above other plants so that the leaves can catch more sunlight.

 

          A plant’s leaves are “light-powered food factories”. They are broad and flat so that as much light as possible falls on them. A green substance called chlorophyll in the leaves catches or absorbs the energy in light. It uses this energy to make a chemical reaction. Water, taken up from the soil, and carbon dioxide, taken in from the air, join together to form sugar, which contains lots of energy in chemical form. The plant then uses the sugar to power its life activities. The process is called photosynthesis —a word meaning “making with light”.

          The carbon dioxide for photosynthesis comes from the air. It seeps into the leaf through tiny holes in its lower surface, known as stomata. In addition to sugar, photosynthesis also produces oxygen, which seeps out into the air. Living things including ourselves need oxygen to survive. Plants help to top up its level in the air.

FLOWERS AND POLLEN

          A plant’s flower is designed to reproduce—make seeds which grow into new plants. A typical flowering plant has both male and female parts. The male parts make tiny particles, pollen grains, which look like fine yellow powder. Each grain contains a male cell. Pollen is produced in bag-like anthers on stalks, called filaments. The female cells or ovules (eggs) are in the ovary, a fleshy part at the flower’s base. A taller part, called the style, sticks up from this, with the stigma at its top. Pollen must travel from the anthers of one flower to the stigma of another of the same kind, so the male and female cells can join and develop into seeds.

          The transfer of pollen is called pollination. Some pollen grains are light and balloon-like and are blown by the wind. Others are sticky and carried by animals. To attract animals, the flower has colourful petals and a strong scent and makes sugary liquid called nectar. When animals come to drink the nectar, the pollen sticks on them. It brushes off at the next flower on to the stigma. A tube grows from the pollen grain down the style to the ovary. The male cell moves down this to join the ovule.

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          The second largest kingdom of living things after animals is the plants. The key feature of a plant which sets it apart from other living things is that it obtains energy from light by the process of photosynthesis. Most plants have broad, flat surfaces, such as leaves or fronds, where this happens. Just as there are many groups of animals, from simple worms to complicated mammals, so there are many groups of plants. However they are divided into two main kinds—the simpler types without flowers, and those with flowers.   

 

NON-FLOWERING PLANTS

        The simplest non-flowering plants are algae. They nearly all live in water, although a few kinds can survive in damp places, like Pleurococcus alga which grows as a green powder on shady tree trunks. Nearly all seaweeds and some types of pondweeds, such as the green, hair-like spirogyra, are algae. An alga has no proper roots, stem or leaves, although it may have a stem-like part and leaf-like blade. It absorbs water and nutrients through its body surface.

        Mosses and liverworts are known as bryophytes. A moss has small green leaflets but no proper stem or roots. It absorbs water and nutrients through its leaflets so it can only live in damp places. Liverworts grow in similar places. Each has a low, flattened body known as a thallus.

        Ferns, or pteridophytes, are also non-flowering. A fern has roots which absorb water and minerals from the soil, and a stiff stern to hold up its much-branched fronds. The stem, like the stem of a flowering plant, contains tiny pipes or tube-like vessels to carry the water and other substances from the roots to the fronds. Plants with these vessels are known as vascular plants.

        All of these non-flowering plants reproduce by making tiny, dust-like spores which grow into new plants. Conifers, also called gymnosperms, reproduce by seeds. The seeds form in hard, scaly structures known as cones. Pines, firs, spruces, larches, redwoods and cypresses are all conifers.

 

BACTERIA

          The commonest living things are bacteria. They are too small to see without a microscope. Most are about one to five microns (0.001 to 0.005 millimetres) across. A quarter of a million would fit on the head of a pin. Bacteria are all around us in their billions. They float in air and live on icy mountain-tops, in the scalding water of hot springs, in dark caves and on the bottom of the sea. There are more than 4000 known kinds, and probably many more yet to be identified. They vary in form but there are three main shapes. These are: spheres or balls known as cocci, cylinders or rods, called bacilli, and corkscrew-like spirilli. Most bacteria reproduce simply by splitting in two.

 

          Bacteria belong to the main kingdom of living things known as monerans. A typical bacterium has a tough outer skin, or cell membrane, which contains jelly-like cytoplasm. Tiny blobs, known as ribosomes, float in the jelly and make various substances for the bacterium’s life processes. Also floating in the cytoplasm is a long, coiled-up chemical called DNA, which unravelled would be more than 1000 times longer than the bacterium itself. This is the bacterium’s genes, a “manual” containing every structural detail of the organism. Some bacteria get their energy from light, like plants. Others absorb nutrients through their cell membranes.

            Some bacteria are harmful. They get into other living things, including humans, and cause diseases such as anthrax and typhoid. But most bacteria are harmless. Many kinds live in the soil and play a vital role in nature because they cause the decay or rotting of dead plants and animals.

 

 

 

VIRUSES

          The smallest living things are viruses. They are “alive” only because they can produce more of their kind if they invade another living thing. Viruses cannot reproduce on their own. They get into another living cell, the host cell, and take over its life processes to make more copies of themselves. In the process they destroy the host cell.

 

          A typical virus has an outer shell or coat made of proteins. Inside is a length of genetic material, usually DNA. Different viruses are shaped like bricks, rods, golf balls and even space rockets. Many can exist in their non-living form for years and be frozen solid, boiled or made into crystals - yet still come alive when host cells are available. Viruses cause diseases in plants, animals and people. These include the common cold, measles and AIDS (caused by the Human Immunodeficiency Virus, HIV).

 

 

 

 

 

 

PROTISTS

           Like Bacteria and other monerans, protists are microscopic single cells. But unlike monerans, each protist has its genetic material (DNA) wrapped inside a bag-like membrane to form the nucleus or control centre of the cell. Protists live mainly in water and damp places. Some are like tiny plants, absorbing their energy from sunlight and their raw materials for growth from the water around them. Others move around and consume food particles such as bacteria.

 

           Some protists have a rigid, case-like cell wall around them. The types known as foraminiferans and radiolarians make shells with beautiful shapes and patterns. Others have no rigid case and can take up any shape. A few protists cause diseases, such as plasmodia, which produce malaria.

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          A Fossil comes from an organism (living thing) that survived long ago. After the organism died, parts of it were buried in sand or mud, preserved in the rocks and turned to solid stone. Hard, tough body parts form the best fossils because they do not rot away quickly after death and so have more chance of being preserved. They include animal bones, teeth, horns, claws and shells, and plant wood, bark and cones.

          Fossils show that many kinds of animals, plants and other organisms have lived during the hundreds of millions of years that make up the Earth’s past. Most of these organisms, such as ammonites, trilobites, dinosaurs and mammoths, are no longer alive. Other types have survived almost unchanged for millions of years. Sharks and turtles are examples of these. Some have appeared quite recently, such as human beings. The study of fossils, known as palaeontology, is one part of the evidence for evolution—the way that living things change through time.

         Why does evolution happen? Why don’t living things simply stay the same? Life is a continual struggle to avoid predators and bad weather, to find food and shelter, and to breed. Living things that survive the struggle are those best suited or adapted to the conditions. However the conditions change naturally with time. Some kinds of food may become more scarce. The climate may become colder or warmer. New diseases may appear. Living things must evolve to suit new conditions or die out.

 

   

 

 

          Besides studying fossils, we can see evolution at work by observing living things today. Some types of animals are very similar to each other. Hawaiian finches for example, differ only in small ways. They probably all evolved from one original species. Their beaks changed, or adapted, to eat different foods.

 

 

 

 

EVIDENCE FOR EVOLUTION

          An embryo is a living thing at an early stage of its development, like a human baby during the first few weeks of life growing in its mother’s womb. At this early stage, a developing human embryo looks very similar to the embryo of any other mammal, such as a monkey or cow. It is also similar to the embryo of a bird, a reptile like a turtle, and even a fish. The simplest explanation as to why these very young organisms are so similar to each other is evolution. Over millions of years they have evolved from the same ancestors. They are now different as adults. But they have kept the similarities to their ancestors, and so to each other, during the early stages of their development.

 

 

 

 

 

 

 

 

NATURAL SELECTION

          Evolution happens by the process of natural selection. In the struggle for survival, some living things adapt better to the conditions. These individuals are more likely to survive and produce offspring. If the offspring inherit the same features, they too have more chance of survival. It is as if nature chooses who will survive and who will not.

 

          Evolution by natural selection explains the bodily features and behaviours of living things—even those that seem to be a hindrance. The long, colourful feathers of a male bird of paradise may seem a drawback. They make him more obvious to predators and less able to escape from them. But they also attract females for breeding, and so this feature is passed on to his offspring.

 

          Look at your surroundings. There may be walls, windows, chairs, tables and similar objects around you. Perhaps there are also machines, cars and gadgets. There may be other people too, and pets and plants. Which ones are alive? You can probably tell at a single glance if an object is living or not. For example, a dog is alive but a book is not.

          But exactly how did you decide which things are alive and which are non-living? Perhaps you watch them to see if they move. A person or animal moves. Even a sleeping cat breathes softly. But a toy electric car moves and it is not alive, while a plant does not seem to move yet it is a living thing. Perhaps you look for signs of breathing. But the snails and plants in an aquarium do not seem to breathe, and they are alive. The giant panda is just a picture, but you know from looking at it that a real panda would be alive. How?

          Living things are called organisms. We know if something is a living organism, rather than non-living, from several features. First, an organism grows and develops at some stage, usually changing its shape and getting bigger. Second, life processes happen inside the organism that change chemical substances from one form to another and which use up energy. Third, an organism must take in raw materials for its growth and also take in energy to power its life processes. Fourth, an organism reproduces—it produces more of its own kind.

 

 

ORIGINS OF LIFE

          How did life begin? Scientific studies show that planet Earth formed about 4600 million years ago, from a massive ball of cloud, dust and gases whirling through space. At first, the rocks of Earth were far too hot for life. But gradually they cooled and massive rainstorms lasting many thousands of years filled the lakes, seas and oceans with water.

          These seas contained all kinds of salts, minerals and other chemical substances. By chance, some of them joined to each other—perhaps helped by the energy of lightning flashes from the storms that raged across the globe. A few simple chemicals gathered as blobs. Other chemicals joined around them. These others then broke off to form blobs of their own. The first very simple living things had reproduced. This may have happened as long as 3000 million years ago. Life stayed as simple microscopic organisms for another 2000 million years.

 

 

 

 

 

 

GROUPS OF LIVING THINGS

          To understand how living things have changed or evolved in the past, and how they work and survive today, it helps to know which ones are similar to each other. So organisms are classified or put into groups. There were once only two main groups or kingdoms, plants and animals.

 

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