My Science School

          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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          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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