Everywhere you look, you will see plants—even if you live in a city. As well as the trees, grass and flowers that grow in the ground, there are things made from plants all around you. The jeans you wear, the books you read, wooden chairs and tables, headache pills, car tyres and, of course, most of the food you eat, all come from plants. Plants are not just useful—they are essential. We really could not live without them.
A PLANET FULL OF PLANTS
From space, the Earth’s continents look mainly green. That is because of the trillions and trillions of plants that cover most of the Earth’s land area, in forests, grasslands, farmers’ fields, parks and gardens. All these plants are essential to life on Earth, and make our planet work the way it does. It is because of plants that the Earth has an atmosphere we can breathe, food for animals to eat and soil for more plants to grow in. So what exactly are plants?
GREEN LIFE
A plant is a type of living thing. Unlike humans and other animals, most plants cannot run around, eat, chase each other or talk. They are simpler life forms that spend most of their lives in one place, rooted in the soil. Instead of having to look for food, they soak up gases from the air, water from the ground and light from the Sun, and use these to make food inside their leaves. They do this using a special chemical called chlorophyll. Chlorophyll is bright green, so that is why most plants are green.
Plants come in a huge variety of shapes and sizes. They range from tiny rootless duckweed, less than 1 millimetre across, to sequoia trees, which grow up to 85 metres tall and are the biggest living things on Earth. In between are thousands of different species, or types, of plants, including mosses, ferns, seaweeds, pond plants, vines, cactuses, bushes and trees. Scientists have counted about 260,000 different plant species altogether.
WHERE DO PLANTS LIVE?
Most plants need soil to grow in. Soil is made from a mixture of water, gases, bits of rock and the rotten remains of dead plants and animals, and it builds up naturally wherever there is land and moisture. So plants are found all over the world, wherever they can find enough soil, water and sunlight. Plants usually do not grow in dark places such as caves, in very cold places such as the North and South poles or high, snowy mountaintops or in the driest deserts.
Many plants live in water—in ponds, rivers, lakes and seas. They cling to rocks, root themselves in river beds or just float around. And a few plants, called epiphytes or air plants, can take all the moisture they need from the air, so they do not even have roots. They can grow clinging to rocks, walls or other plants.
HOW DO PLANTS MAKE THEIR FOOD?
Plants need sunlight to survive because they use it to make their food. They do this inside their leaves, using a process called photosynthesis. The leaves contain tiny food factories called chloroplasts. They are full of green chlorophyll, which soaks up sunlight. The energy from the sunlight is used to power a chemical reaction, which turns chemicals from water and air into plant food. The plant uses the food to make new plant parts and grow bigger.
THE MAIN PARTS OF A PLANT
A typical plant has roots, a stem, branches, leaves and flowers. Inside the plant are tubes that carry water and food chemicals around to different parts of the plant. The tubes are made of two types of plant tissue called xylem and phloem.
Plants that have xylem and phloem are called vascular plants, and they include almost every type of plant. But there are a few non-vascular plants, such as mosses and liverworts. They do not have delivery systems for carrying water and food around. Instead, the water and food they need simply soak through them.
PLANT CELLS
Like all other living things, plants are made up of tiny units called cells. Plant cells are different from most animal cells because they have a strong, rigid cell wall. Under a microscope, plant cells often look like tiny bricks or square rooms.
MAKING MORE PLANTS
As with other living things, plants reproduce, or make copies of themselves. A typical flowering plant, such as a poppy, makes two different types of cells, called pollen and ovule cells, inside its flowers. The two types of cell join together to make seeds, which can grow into new plants.
Other plants can reproduce more simply. Some grasses, for example, can grow sideways roots called rhizomes. They grow into new plants that eventually break off from their parent plant.
HOW LONG HAVE PLANTS EXISTED?
Scientists think plants first developed in the sea, around 3,000 million years ago. Much later, roughly 500 million years ago, some plants began to grow on land. As old plants died and rotted, more and more soil built up, making it easier for new plants to grow. Gradually, plants spread across large land areas, forming the first forests. Some types of prehistoric plants have been preserved as fossils, giving scientists clues about how plants have changed over time. And coal, which we use as a fuel, is made from the remains of millions of prehistoric plants, squashed and hardened underground over millions of years.
BREATHING WITH PLANTS
Plants help to maintain the atmosphere—the blanket of air that surrounds the Earth. When humans and other animals breathe, they take in a gas called oxygen from the air, and release another gas, carbon dioxide, as waste. But when plants make food from sunlight, they do the opposite. They use up carbon dioxide, and release oxygen. The huge mass of plants in forests, grasslands and oceans make it possible for the Earth’s animals to go on breathing.
PLANTS AND THE FOOD CHAIN
A food chain is a sequence of living things in which each creature feeds on the next one down the chain.
Plants are vital to other life on Earth, as they are at the bottom of the food chain for almost all the world’s animals. They also provide food for many other living things such as fungi and bacteria. Even carnivores, which are meat-eating animals, rely on plants, because somewhere along the line, the animals that they eat are plant-eaters.
FOOD FOR HUMANS
Plants are at the bottom of the human food chain too. Directly or indirectly, they provide us with almost everything we eat. We consume huge amounts of cereals such as rice, wheat and corn, fruit such as apples and bananas, and vegetables such as cabbage. We also eat lots of plant products such as sugar, coffee and chocolate, as well as meat from animals such as cows and sheep, which feed on plants.
Until about 10,000 years ago, people simply collected berries, roots and other plant food from the wild. Today, we mainly eat plants that we have grown on farms. Over the centuries, farmers have selected and bred the most useful plants to grow as crops, creating new varieties of plants that were once wild. Modern farm wheat, for example, was bred from a wild grass called einkorn. It has bigger seeds and contains more food than its wild relative.
WHAT OTHER USES DO PLANTS HAVE?
Besides eating plants, we use them in thousands of other ways. Many modern medicines, such as aspirin, morphine and quinine, were first discovered in plants. Most recreational drugs, including caffeine and tobacco, come from plants too. Cotton cloth starts life as fluffy hairs that grow on the seeds of the cotton plant; rubber is made from a milky liquid that flows from rubber trees; and cork is a type of tree bark. Flowers are sold as gifts or made into expensive perfumes. We use bamboo, reeds, rushes and wood from trees to make furniture, buildings, boats and household objects. Paper is made from trees too. Because of all these important uses, many countries rely on plants such as cotton, tobacco, conifer trees or coffee bushes for most of their jobs and wealth.
Did you know?
• Some bristlecone pine trees can live to be 5,000 years old.
• Some plants get extra food by eating insects. The Venus flytrap, for example, has mouth-shaped leaves that can snap shut in a second to catch flies.
• Pebble plants look just like pebbles. They are desert plants that camouflage themselves as pebbles to stop animals from eating them.
Microsoft ® Encarta ® Premium Suite 2005. © 1993-2004 Microsoft Corporation. All rights reserved.
Workbook 5 page 12 number 3
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Chapter 2: States of Matter
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Chapter 3: Light
Light is all around us. It is part of our lives from the day we are born. We rely on it for many of the things we do. Yet what is light? We have only recently begun to understand it.
WHAT IS LIGHT?
For a long time, scientists argued about the nature of light. Isaac Newton thought that it was a stream of particles, but Christiaan Huygens, a Dutch scientist, did not agree. He thought light must be a wave. We now think it can behave both as a particle and a wave.
It is simplest to understand light as a wave. Light waves are made up of electric and magnetic fields that are changing (vibrating) all the time. It is a form of electromagnetic radiation. Light waves are different from sound waves. Sound needs something to travel through, such as air or water. But light waves do not. They can travel through empty space—a vacuum.
Light waves travel very fast indeed. They travel fastest through a vacuum. Their speed in a vacuum is 299,792,458 metres per second. According to Albert Einstein’s theory of relativity, nothing can travel faster than the speed of light.
The wavelength of a wave is the distance from one peak to the next, or from one trough to the next. The light we see does not have just a single wavelength, but a whole spread of wavelengths. That is because white light is made up of all the colours of the rainbow mixed up together. Newton showed this by splitting up the different colours in light using a prism. Each colour is a different wavelength. A spread of light at different wavelengths is called a spectrum. A rainbow is a spectrum in the sky.
The wavelengths of the light we see are tiny. Red has the longest wavelength, at about 70 millionths of a centimetre, while violet has the shortest, at about 40 millionths of a centimetre.
THE PROPERTIES OF LIGHT
Light has various properties including shadows, reflection, refraction, interference and diffraction.
Shadows
Light travels in straight lines (except in special circumstances). That is why we see shadows. Shadows are areas where the light rays are blocked by something. When the Sun is high in the sky, the sunlight comes down at a steep angle, and your shadow is small. Stand facing away from the Sun when it is low in the sky and you will find that your shadow becomes very long, because of the low angle of the sunlight.
As the Sun seems to move across the sky through the day (as the Earth turns), the direction of shadows changes. A sundial uses the changing direction of the Sun’s shadow to show the time.
Shadows only appear if light cannot pass through an object. We say the object is opaque. If light can pass through an object, we say the object is transparent. If only some light passes through, we say the object is translucent.
Reflection
We see things because light bounces off them and into our eyes. The bouncing of light off a surface is called reflection. Surfaces also absorb some light. We see a surface as blue if it reflects blue light but absorbs all the other colours. We see reflected images in glass and polished metal because they have very smooth surfaces that reflect a lot of light. The light rays are all reflected at the same angles. Other surfaces are rough and cause light to scatter off in different directions.
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