5. REPRODUCTION AND LIFE CYCLES - jsbasicscience.comjsbasicscience.com/Downloads/Chap5.pdf · slide under the microscope. Use low power first, then try high power. Compare what you

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5. REPRODUCTION AND LIFE CYCLES

5.1 REPRODUCTION AND LIFE CYCLES All living things reproduce their own kind. In fact, reproduction is one of the most important characteristics of living things.

Tomato plants and pawpaw trees make seeds that grow into new tomato plants or new pawpaw trees. Coconuts palms make coconuts that grow into new coconut palms. New banana plants grow from the base of old banana plants and new yams grow when we plant old yams.

Animals reproduce too. Dogs have puppies and human beings have babies. Hens lay eggs that hatch into chicks, and lizards lay eggs that hatch into baby lizards. Insects lay eggs too. The eggs of most insects hatch into grubs, and after some time, the grubs turn into young insects. Because of reproduction, all the different kinds of living organisms continue to exist. They do not die out. As the old ones die, the younger ones replace them. The process of being born, growing and developing into an adult, and then reproducing more young, is called a life cycle. The idea of a life cycle is shown in the diagram on the right. In this chapter, you will learn how to use a microscope to look at the tiny cells of which all living organisms are made. You will learn how cells are involved in reproduction and growth. After that, we will study the life cycles of a number of different animals including amphibians, insects, birds and mammals. Finally, we will study the reproduction and growth of plants, especially flowering plants.

1. Why is reproduction so important for living organisms?

2. What is a life cycle? Why is the word cycle used?

3. For most plants, the life cycle could be summarised: PLANTS make SEEDS grow into NEW PLANTS. Summarise, in the same way, the life cycles of (i) hens, (ii) dogs, (iii) flies.

reproduction

ADULT YOUNG

growth and development

Pawpaw trees make seeds that grow into new pawpaw trees

All living things reproduce

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5.2 HOW TO USE A MICROSCOPE Remember the microscope from Chapter 1? A microscope is used to make very small things look bigger. In this chapter, we study very small things called living cells. To observe living cells, we use a microscope. Study the diagram carefully. Even if the microscopes at your school look a little different, they all work in the same way. The specimen. The object we want to look at is called the specimen. The specimen must be very thin so that light can shine right through it. The specimen is placed on a thin glass rectangle called a slide. The specimen is usually protected by covering it with a small cover slip of glass or plastic. The slide is placed on the stage of the microscope and is held in place by two clips. Light from a window or lamp is reflected through a hole in the stage, onto the bottom of the slide, by a small mirror. Some microscopes have their own lamp instead of a mirror. Magnification. We observe the specimen on the slide by looking through a tube that has an eyepiece at the top and an objective at the bottom. The tube is moved very slowly up and down by turning the adjusting knob. There are usually two or more different objectives that can be screwed or clicked into place. A low power objective is often marked ×10. This means that it makes the specimen look ten times bigger. A high power objective may be marked ×40, meaning that it magnifies 40 times. There are usually two different eyepieces that fit into the top of the tube. A low power eyepiece may be marked ×4 and a high power eyepiece may be ×10. The total magnification of a microscope is obtained by multiplying the power of the objective by the power of the eyepiece. If the low power objective is used, with the low power eyepiece, the total magnification will be 40 (10 × 4). The high power objective with the high power eyepiece will magnify the specimen 400 times (40 × 10). This is usually the highest magnification we can get with a school microscope. Magnifications of many thousands are possible with more expensive microscopes. Using the microscope. First place the slide on the stage and hold it in place with the two clips. Always use the low power objective first. Carefully lower the tube with the adjusting knob, until the objective is very close to the slide. Watch from the side and be very careful that the objective does not touch the slide. That might damage the objective (and the slide). Now look down the eyepiece and tilt the mirror to obtain the brightest possible light. Slowly raise the tube, using the adjusting knob, until you can see the specimen clearly. You may need to move the slide to find exactly what you want to look at. You will find that the slide, and the image you are looking at, move in opposite directions! When you change to the high power objective, you may be able to see the specimen clearly without moving the adjusting knob very much. If not, you must take your eye away from the eyepiece and start again. Carefully lower the tube until the objective is very close to the slide. Now look down the eyepiece again. Slowly raise the tube with the adjusting knob until you can see the specimen clearly. Never lower the tube while looking down the eyepiece - the objective can easily hit the slide and be damaged.

1. In a microscope, what are each of the following, and what are they used for: (i) slide, (ii) cover slip, (iii) stage, (iv) clips, (v) mirror, (vi) adjusting knob?

2. A school microscope has two objectives marked ×10 and ×40, and two eyepieces marked ×6 and ×10. Work out all the magnifications that we can obtain with this microscope.

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5.3 LIVING CELLS All living things are made up of tiny "building blocks" called cells. The word cell means the smallest unit of something. Living cells have nothing to do with the electrical cells we studied in the last chapter, they are completely different. Living cells vary in size and shape but they are very tiny. Your body contains billions of cells of many different kinds. At least ten ordinary cells could fit on this full stop. The box on the right explains how you can prepare microscope slides of two different kinds of cells. The sketches below show what you might see. Onion skin cells Cheek skin cells The structure of cells. The diagrams below show a typical plant cell and a typical animal cell. All cells have a cell membrane, cytoplasm and a nucleus (plural nuclei). The cell membrane is like a very thin skin around the cell. It holds the contents of the cell and controls the solutes that pass in and out. The cytoplasm is a sort of living jelly inside the cell and the nucleus is the cell's control centre. Plant cells usually have three things that animal cells do not have. Look at the sketches and the table on the right. Plant cells always have a thick cell wall, outside the cell membrane. The cell wall supports the cell and helps to keep its shape. Solutions can pass freely through cell walls, but the cell membrane controls the solutes that enter and leave the cell. Inside plant cells, there are usually large spaces called vacuoles. These are filled with a watery liquid called cell sap. Finally, many plant cells contain small green objects called chloroplasts. You will learn more about chloroplasts in Chapter 6.

1. Why do we often stain cells before we study them under a microscope?

2. What is a cell membrane and what does it do?

3. What is a vacuole? If you see a cell with a small vacuole and no cell wall, is it a plant cell or an animal cell?

Plant cells Animal cells

Thick cell wall No cell wall

Large vacuoles Usually no vacuole

Usually has chloroplasts No chloroplasts

Preparing slides of cells Many cells are transparent, so it is difficult to see clearly what is inside. We usually stain cells with a coloured dye to make it easier to study them. Onion skin cells. Cut open an onion and break off a small segment from inside. From your segment, remove a small piece of the very thin, transparent skin that covers it. Place this skin on a microscope slide. Add one drop of a stain, such as iodine, red ink or any food colouring. Cover the specimen with a cover slip. Study your slide under the microscope. Use low power first, then try high power. Compare what you see with the picture of onion skin cells on the left. Cheek skin cells. Wash out your mouth with clean water, then scrape the inside of your cheek with a spoon. Put a bit of the stuff from your cheek onto a glass slide and add a drop of any stain. Cover the sample with a cover slip. Study your slide under the microscope. Compare what you see with the picture of cheek cells.

A plant cell An animal cell

Differences between animal and plant cells

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5.4 CELLS, REPRODUCTION AND GROWTH Unicellular organisms. Some tiny living organisms consist of one cell only. They are called unicellular organisms. There are many different kinds of unicellular organisms and a few are shown below.

These organisms are not plants or animals. They belong to the Protista kingdom. Spirogyra is like a simple plant. It has cell walls, chloroplasts and large vacuoles. Amoeba and Paramecium are like simple animals. They have no cell walls or chloroplasts and only small vacuoles. They move around and catch their own food. Euglena is like both an animal and a plant. It has no cell wall and it swims around like an animal by waving its tail. But it also has chloroplasts and vacuoles like a plant. Protista is quite a mixed up kingdom! Binary fission. Amoeba reproduces in a very simple way. It feeds and grows by surrounding and absorbing tiny scraps of food. When it reaches a certain size, it splits in half and becomes two amoeba. The nucleus splits first, then the cytoplasm and membrane. This kind of reproduction is called cell division or binary fission ( binary means two parts, and fission means splitting).

Bacteria are unicellular organisms that reproduce by binary fission. When they have food and warmth, they may divide every 20 minutes. We can easily calculate what happens if just one bacterium gets onto food in a warm place. After 20 minutes there will be two bacteria, after 40 minutes there will be four, and after an hour there will be eight. The number doubles three times every hour. After three hours there will be 512 bacteria, and after half a day there will be 68719476736. Check it yourself! Some bacteria cause diseases, so you can see why it is dangerous to eat old scraps of discarded food. Multicellular organisms and growth. Ordinary plants and animals are made of many cells, usually billions of cells. An organism with more than one cell is called a multicellular organism. In multicellular organisms, the cells are of many different kinds. The cells of our skin, our muscles, our blood and other parts of our bodies are all quite different. When a multicellular organism feeds, the food is shared amongst all the cells. Each cell grows and eventually splits in two. The two new cells remain together, side by side. As the organism grows, so does the number of cells it contains. We can say that a multicellular organism grows by cell division.

1. What are the 5 kingdoms of living things?

2. What is (i) a micro-organism, (ii) a unicellular organism and, (iii) a multicellular organism?

3. In Amoeba, cell division leads to reproduction. But in larger organisms, cell division just makes them grow bigger. Explain why the results of cell division are different in these two cases.

Looking at some unicellular organisms with a microscope

Instructions for growing micro-organisms in water were given in Section 3.13. Prepare some water as instructed there. After a few days, place one drop on a microscope slide. Cover the drop of water with a cover slip and observe it using the high power of your microscope. You will probably see Paramecium, and you may find Amoeba, Euglena and other organisms too. If there is any green scum on the water, make a slide and look at that - it may be Spirogyra or a similar organism. Some unicellular organisms

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5.5 SEXUAL REPRODUCTION Most animals and plants produce their young by sexual reproduction. Two special cells, called sex cells or gametes, come together and grow into a new animal or plant. One of the cells is a male gamete and the other cell is a female gamete. Biologists use special symbols for these two cells. The symbol ♂ represents the male gamete and the symbol ♀ represents the female gamete. The arrow shows that, during reproduction, the male gamete moves around seeking a female gamete. When it finds one, it enters through the cell membrane and the nuclei of the two cells join together, or fuse. The fusion of the male and female gametes is called fertilisation. The fertilised cell is called a zygote. The cells of the zygote divide and change, and it grows and develops into a new animal or plant. Sexual reproduction in animals. In animals, the male gamete is called a sperm. All male animals produce sperm cells. They are very tiny, even for cells. Each sperm cell has a long tail, which it uses for swimming. Sperm cells are very active. Their mission in life is to find a female cell and fuse with it. The female gamete in animals is called an egg cell or ovum (plural ova). It is quite big for a cell and can not move on its own. Its mission in life is to attract a sperm cell to fuse with it and fertilise it, so it can become a zygote and grow into a baby animal. To reproduce, animals must find a way for sperm cells from the father to meet egg cells from the mother. The process used to bring these cells together is called mating. When they mate, some animals use external fertilisation and some use internal fertilisation.

External fertilisation is used by most fish and amphibians, which reproduce in water. When they mate, the male and female release large numbers of gametes into the water at the same time. The sperms swim around trying to find ova. Some of the gametes may be swept away by the water and never meet. And fish may eat many of the zygotes before they are big enough to look after themselves. So external fertilisation is usually quite a wasteful process. Internal fertilisation is used by reptiles, birds and mammals, and by many invertebrates. When they mate, the male

places his sperm cells inside the body of the female. The sperm cells swim around inside the female seeking egg cells to fertilise. Internal fertilisation is much more efficient than external fertilisation. Usually all the ova present are fertilised. In birds, and in most reptiles and invertebrates, the fertilised ova develop into eggs. The female lays the eggs in a suitable place where they develop into young. In mammals, the fertilised ova remain inside the body of the female. They develop into young inside her womb. When they are ready, they are born alive from their mother's body. 1. What is meant by each of the following: (i) gamete, (ii) sperm, (iii) ♀, (iv) mating, (v) zygote? 2. Why do female fish produce thousands of ova when they mate, but female birds produce only a few ova? 3. What happens to the zygote of (i) a fish, (ii) a bird, and (iii) a mammal?

♂ ♀

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5.6 THE LIFE CYCLE OF AN AMPHIBIAN - THE FROG A common amphibian is the frog. Unless you live in a very dry region, you are probably familiar with the croaking of frogs, especially in the evenings and after rain. Like all amphibians, the frog is at home in the water as well as on the land. When they mate, frogs use external fertilisation. First they find a pond or a swamp. The male frog sits on the back of the female frog and she releases her eggs cells into the water. At the same time, the male releases his sperm cells on top of the egg cells. The sperms swim to the egg cells and fuse with them. The egg cells are fertilised and become zygotes.

At first, the zygotes (fertilised eggs) are protected by a slimy jelly and the whole mess is called frog spawn. After a few days, the eggs hatch into tiny tadpoles that swim around and feed on tiny scraps of plants. At first, the tadpoles breathe through gills outside their bodies, but after a week or two the gills disappear inside. After a few weeks, some extraordinary changes take place. First the tadpoles grow a pair of back legs. A bit later, front legs appear too. Slowly the tail gets shorter and shorter, and the tadpole starts coming to the surface to breathe. After two or three months, the tadpoles have changed into tiny frogs! We can easily observe all these changes if we collect some frog spawn and keep it at home or at school. Just follow the instructions in the box above. The change from a tadpole into a tiny frog, is an example of metamorphosis. "Metamorphosis" is an ancient Greek word that means "transformation". Scientists now use the word metamorphosis to describe the complete change of shape that occurs when the young of certain organisms develop into the adult form. Can you think of any other organisms where the young undergo metamorphosis?

1. Why do you think that frogs use ponds or swamps when they reproduce, and not streams and rivers?

2. List, in order, all the changes that take place as a tadpole gradually changes into a tiny frog.

Observing the development of frog spawn

First you must collect some fresh frog spawn. A good time to look for frog spawn is when it has just rained after a long dry spell. The frogs will tell you where they are by their loud croaking! Collect some water and some green water plants too. Check with an adult before you go in case there may be any local dangers. Keep the frog spawn in a bowl of water in a cool shady place. A wide bowl is better than a narrow jar. Make sure there are always fresh green water plants in the bowl. The plants provide food for the tadpoles and help to keep the water fresh. Observe the tadpoles regularly and keep a record of all you observe. When the tadpoles start to turn into frogs, put a sloping stone in the bowl so they can crawl out of the water to breathe. When they are ready to leave the water, return them to the place where you found them. Frogs have an important place in natural food chains. They help to control insects by eating them and they provide food for snakes and other predators. If we remove too many frogs, we may get a plague of insects, and snakes may come into town looking for other small animals to eat.

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5.7 THE LIFE CYCLES OF THREE INSECTS The diagrams below show the life cycles of three insects - a mosquito, a house fly and a butterfly. When they mate, they all use internal fertilisation. They are all examples of metamorphosis too.

All these three insects lay eggs. The eggs hatch into very active grubs or larvae (singular larva). The larvae eat a lot and grow fat, and then turn into pupae (singular pupa). The pupa is a resting stage. Inside the pupa a wonderful change takes place. The larva slowly turns into the completely different form of the adult insect. When the adult is fully developed, it breaks out of the pupa, and flies away. 1. What is metamorphosis? Briefly describe four

examples. 2. What are the larvae and pupae of a butterfly

called?

3. Describe two things we might try doing to stop mosquitos reproducing near our homes. Explain how you think each method might stop them.

Observing life cycles To observe these life cycles, you have to find the eggs of these insects in water, on decaying meat or on leaves. Keep the eggs in a glass jar with some of the water, meat or leaves. Cover the top with something that allows air to get in and make sure the larvae have plenty to eat. Record your observations regularly. You may have to wait some weeks for adults to emerge from the pupae, but this is the most exciting part.

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5.8 THE LIFE CYCLE OF A BIRD - THE DOMESTIC FOWL All over the world, people keep fowls for their eggs and their meat. There are many different kinds of domestic fowls and they may have different names in different countries. The female is called a hen, the male is called a cock and the young are called chicks.

When fowls mate, sperms from the cock swim up the egg passage of the hen. They look for an ovum to fuse with and fertilise. The fertilised ovum forms a shell and becomes an egg, which the hen lays in a safe, warm place. She usually lays one egg every day. After a few days, if we do not take them away to eat, she will sit on her eggs to keep them warm. This is called incubating the eggs. After three weeks, the incubated eggs will hatch and baby chickens will break out through the shells. The diagrams show how the chick embryo develops inside the egg. An embryo is an organism that is still growing and developing from a zygote, before it is hatched or born.

A hen may go on laying one egg almost every day for quite a long time, especially if we keep taking her eggs away. However, unless she keeps mating, her eggs will not be fertilised. An unfertilised egg cannot hatch into a baby chick. 1. Do fowls use internal or external fertilisation? 2. Give the meaning of these words: embryo,

incubation.

3. While it is still in the egg, how does a chick embryo (i) obtain oxygen from the air for respiration, (ii) the food it needs for its growth and development?

How to look inside an egg The diagram shows how to remove part of the shell from an egg. You will need scissors and plenty of care and patience! First draw a line around the egg as shown. Gradually break and remove the shell, and cut away the membrane, down to the line.

If you can, try to look inside one fresh egg and one that has been incubated for a week.

Both the birds have openings under their tails. When they mate, these openings touch. Sperms from the cock swim into the egg passage of the hen.

cells which grow into chick

air passes through shell

yoke

membrane

strand supporting yoke

New laid egg

yoke

After 10 days

embryo chick

After 3 weeks

wool or tissue paper

small dish

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5.9 THE LIFE CYCLE OF A MAMMAL - THE DOG Dogs have been our helpers and friends for thousands of years. Dogs guard our homes, herd our domestic animals, and help us to hunt food and pests. In many cultures, dogs are also kept as pets and companions. Like us, dogs are mammals. They are a good example of how mammals reproduce. In mammals, the young develop in a special place called the womb or uterus inside the female’s body. Like reptiles and birds, mammals use internal fertilisation. When dogs mate, the male places his sperms in the birth passage of the female. The sperms swim to the uterus and into the tubes where the female releases her egg cells (ova). The sperms meet and fertilise several ova. The fertilised eggs (zygotes) become embryos, which grow and develop inside the uterus for about nine weeks. When the embryos have developed into puppies, they are born alive through the birth passage of their mother.

Dogs mating Embryos developing

When they are born, puppies are blind and helpless. They do not open their eyes for several days. Their mother looks after them and feeds them on milk from special glands called mammary glands. The puppies obtain the milk by sucking at teats on these glands. Feeding in this way is called suckling. As puppies grow up, they gradually become independent and learn to find their own food. Dogs have several puppies at the same time. But in human beings, and some other large mammals, the female usually releases only one egg cell at a time, so only one baby is born.

As you may have guessed, the word mammary comes from the word mammal. Only mammals have mammary glands for suckling their young. A good way to find out if an animal is a mammal is to see if the female has mammary glands. A summary of the life cycles of animals. We have seen how several different classes of animals reproduce. Adult male animals produce sperm cells and adult females produce egg cells. When a male and a female mate, the sperm cells swim to meet the egg cells. The sperms fertilise the egg cells by fusing with them and forming zygotes. The zygotes grow and develop by cell division. In different ways, they develop into young animals. Eventually the young grow up and become adults themselves. Then it is their turn to reproduce. So long as the life cycle keeps going round and round like this, an organism cannot die out.

mating fertilisation zygote ADULT YOUNG

growth and development

1. List all the ways in which people use dogs. 2. What is the other name for the womb of a female? 3. Compare the life cycles of a reptile, a bird and a

mammal. 4. If we see a baby animal drinking milk from its

mother, what class of animals does it belong to? 5. Try to guess what unusual things might happen,

just occasionally, to cause the birth of human twins.

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5.10 THE STRUCTURE OF A SIMPLE FLOWER The biggest division of plants is the flowering plants. Flowering plants use flowers for sexual reproduction. In this section, we will identify and name the different parts of some simple flowers. The male and female parts are usually found in the same flower. The outer parts of a flower are the sepals and the petals. Before the flower opens, while it is still a bud, it is protected by the sepals. These are usually green and look like small leaves. When the flower opens, the sepals can be found under the petals. The petals are usually brightly coloured. It is the petals that make flowers look so attractive. The diagrams below show four different flowers. Some of the sepals and petals have been removed so we can see all the parts inside.

The female part of a flower is called the carpel. The carpel is right in the centre of the flower. In simple flowers there is only one carpel, or a few joined together so that they look like only one. The carpels of the four flowers above are illustrated on the right. At the base of each carpel is an ovary that contains the female gametes ( ♀ ) or ovules.

The male parts of a flower are called stamens. There are usually several stamens - at least one or two for each petal and sometimes many more. The stamens surround the carpel(s). Each stamen has a thin stem called the filament that supports an anther. The stamens of the four flowers above are illustrated on the left. When the anther ripens, it splits open and releases tiny grains called pollen. The tiny grains of pollen are the male gametes ( ♂ ) of the plant.

A few plants such as pawpaw (papaya) and corn (maize) have separate male and female flowers. The male flowers have only stamens inside, and the female flowers have only carpels.

1. List the four main parts of a simple flower - list the parts in order, starting from the outside and working inwards.

2. Name the male and female gametes of a flowering plant. Where do we find each gamete and which one is the bigger?

Examining a simple flower. Collect and examine several different flowers. Choose simple flowers with six petals or less. Try to find flowers that are fully open with ripe anthers and pollen. With each flower: (i) Carefully remove about half the sepals and petals. Draw a simple sketch of the remaining half of the flower. Identify and label the sepals, the petals, the stamens and the carpels. (ii) Remove a stamen and look for pollen on the anther. Examine the stamen and the pollen with a hand lens or the low power of a microscope. Make labelled drawings of the stamen and pollen. (iii) Remove the carpel and carefully split open the ovary with a sharp blade. Observe the ovules inside with a hand lens. Make labelled drawings of the carpel and ovules.

© D.G.Mackean

Guava Hibiscus Morning glory Bauhinia

Carpels from the same flowers

Stamens from same flowers

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5.11 POLLINATION AND FERTILISATION When a flowering plant is ready to reproduce, male gametes (pollen) must meet and fuse with female gametes (ovules) from the same kind of plant. Pollen from one kind of plant can not fertilise ovules from a different kind of plant. As with animals, it is the very tiny male gametes that move. There are two steps - first pollination and then fertilisation. Pollination means the transfer of pollen from an anther to a carpel. At the top of each carpel, there is a sticky part called the stigma. The stigma is supported on a stem called the style, which joins it to the ovary at the base of the carpel. When an anther is ripe, it splits open and exposes the pollen inside. During pollination, the pollen is carried from an anther and deposited on a stigma. The pollen may be carried by insects, by birds or by the wind.

Insect pollination. Insects are attracted to flowers by their bright petals and fragrant scent. The insect goes into the flower to feed on a sweet, sugary solution called nectar. Nectar is found at the base of the petals. Pollen from the anthers brushes off onto the body of the insect as it enters the flower. Later, some of the pollen on the insect brushes off onto the stigma in another flower of the same kind. Many different insects pollinate flowers, but one of the most important insects for pollination is bees. Bird pollination. Birds such

as sun birds, honeyeaters, and humming birds are also attracted to flowers to feed on the nectar. Their long, thin beaks are adapted to reach right inside flowers. While these birds are feeding, pollen from the anthers sticks onto their beaks. In the next flower they visit, some of this pollen is transferred onto a stigma.

Wind pollination. The pollen of some flowers is carried by the wind. These flowers do not need colourful petals, fragrant scent and nectar to attract insects or birds. They are often very tiny and have stamens and carpels only, with no petals at all! Most grasses and cereal crops are wind pollinated. When the anthers ripen, clouds of pollen blow away and pollinate neighbouring flowers of the same kind. Wind pollinated flowers often have enlarged, fluffy stigmas to help them catch the pollen in the air.

Fertilisation means the fusing of a male gamete with a female gamete. When a flower is pollinated, pollen sticks onto the stigma at the top of a carpel. Each grain of pollen starts to grow a tunnel down the style towards the ovary. The tunnels are called pollen tubes. In a few hours, the pollen tubes reach the ovary. The pollen grains fuse with the ovules and fertilise them, forming zygotes. After fertilisation, the ovary develops into a fruit, and the zygotes develop into seeds.

1. Explain the difference between (i) pollination and fertilisation, and (ii) self-pollination and cross-pollination.

2. (i) What is nectar? (ii) How are some birds

adapted for feeding on nectar? (iii) How are flowers adapted for attracting sun birds? (iv) Which one benefits - the sunbird or the flower?

3. Why might we starve if we poisoned too many insects?

4. Two students argued about why flowers smell

sweet and have bright colours. One said to attract insects to help plants to reproduce. The other said to make the world around us more beautiful. Which student do you agree with?

Self-pollination is when pollen is carried from the anthers of a flower to a carpel of the same flower.

Cross-pollination is when pollen is carried from an anther of one flower to a carpel of another flower of the same kind. Cross-pollination is normal in most flowers.

Male and female flowers of corn

♀ ♂

stamens

stigmas

© D.G.Mackean

pollen

Fertilisation

pollen ♂

ovules ♀

stigma

pollen tubes

style

ovary

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5.12 FRUITS AND SEEDS All flowers make fruits and seeds. After pollination and fertilisation, all parts of the flower except the sepals and ovary fall away. The ovary swells and develops into a fruit. The fertilised ovules (zygotes) develop into seeds. If they find suitable conditions, the seeds can grow into new plants.

Fruits. The diagrams above show the development and the structure of a few fruits. In biology, the word fruit refers to the swollen ovary that develops after pollination and fertilisation. Examples include all the common fruits that we eat such as mangoes, oranges and pawpaws (papaya). Some of the "vegetables" we buy in the market are really fruits too. Tomatoes and pumpkins are fruits, and so are the pods that contain peas or beans. The peas and beans are the seeds inside the fruit. A head of corn is a fruit that has the seeds on the outside. Although we eat many fruits and seeds, there are also many that are not edible. The dispersal of seeds. If seeds fell straight to the ground, the new plants would be crowded together close to the parent plant. There would not be enough space, soil, water and light for all of them. Many would not grow well. Fortunately, the seeds of most plants are dispersed far and wide in various ways. This gives the new plants a better chance. Sometimes the whole fruit travels, sometimes just the seeds. Dispersal by wind. The seeds of some plants are very light. Some have papery wings or feathery hairs that make them float in the air. When the fruits of these plants are ripe, they split and release the seeds. The seeds may be blown a long way by the wind before they come to earth and start growing. Dispersal by water. The fruits and seeds of a few plants that live close to water, may be carried to new places by the water. The fruits of mangrove trees are dispersed by water, and coconuts are sometimes carried to distant places by the sea.

Dispersal by animals. Some fruits and seeds have hooks that cling onto the fur of mammals. These "hitch hikers" may be carried long distances by the mammal before they fall to the ground. Other seeds travel inside mammals and birds! When animals eat fruit, the seeds often pass right through without being digested. When the animal defecates, the seeds reach the ground and can grow. Self-dispersal. Plants with pods

often disperse their own seeds. When the pod is ripe, it may split open violently. The seeds are thrown away from the parent plant.

1. (i) What is a fruit? (ii) Describe how a fruit forms. 2. What do plants gain by dispersing their seeds?

3. (i) List as many fruits as you can that grow in your region. (ii) List some plants in your region whose seeds are dispersed by (a) the wind, (b) animals.

Milkweed and Jacaranda seeds are dispersed by wind

Mangrove seed is

dispersed by water

Self-dispersal

Burrs and black-jack are “hitch hikers”

Tomato seeds pass through animals

Self-dispersal

Ovary of guava flower turns into fruit Tomato fruit

ovary

ovary

sepal

© D.G.Mackean

Ovary of bauhinia flower turns into fruit

sepal

fruit (pod)

swollen ovary

fruit

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5.13 SEEDS AND GERMINATION The structure of a seed. The diagram on the right shows the structure of a bean seed. Seeds have a tough outer coat called the testa. The micropyle is a tiny hole in the testa that lets water and air pass in and out. The hilium is a scar where the seed was attached to the fruit. The lower diagrams show the insides of a bean seed and a corn seed. The embryo is the part that grows into a new plant or seedling. The radicle becomes the root of the seedling, and the plumule becomes the shoot that carries the first leaves. The cotyledons or "seed leaves" provide a store of food for the growing embryo. The bean seed has two cotyledons but the corn seed has only one. You may remember from Chapter 2 that the number of cotyledons in the seed is used to classify flowering plants. Monocots have only one cotyledon. They also have spear-shaped leaves with parallel veins. Grasses, cereals, palms, bananas and lilies are all monocots. Most other flowering plants are dicots. They have two cotyledons and leaves with network veins. The germination of a seed. When a seed starts to grow it develops into a seedling and we say it germinates. Seeds need air, water and heat to germinate. One way to observe seeds germinating is illustrated on the left. The diagrams on the right show germinating bean seeds and corn seeds.

The radicle grows downwards and develops into roots. The plumule grows upwards and develops into a shoot with green leaves. A summary of the life cycle of a flowering plant is shown in the diagram below. pollination fertilisation SEED FLOWER germination ADULT growth and development SEEDLING

1. When a seed is germinating, what part (i) lets water get inside the testa, (ii) becomes the root, (iii) becomes the first leaves, (iv) provides it with food for growing?

2. Which way does a radical grow if a seed is upside down?

3. (i) What did you predict in each of the two experiments above? (ii) If your predictions come true, what will each one tell you about what seeds need for germination?

Experiment to show that seeds need water and heat for germination.

1. Prepare two jars with seeds as illustrated above. To one jar, add enough water each day to keep the seeds slightly damp. Add no water to the other jar. Put the jars side by side in a warm place for several days. Predict what you think will happen. Look at the jars every day and check your prediction. 2. Prepare two more jars with seeds as before. Add enough water each day to keep the seeds in both jars slightly damp. Put one jar in a warm cupboard and one in a fridge. Predict what you think will happen. Look at the jars every day and check your prediction.

© D.G.Mackean

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5.14 VEGETATIVE REPRODUCTION Some flowering plants do not use sexual reproduction only. They have special leaves, stems or roots that can grow into new plants. We can also grow new plants from ordinary stems or leaves. Such ways of starting new plants are called vegetative reproduction. Underground food storage organs. At the end of the growing season some plants store food, made in their leaves, in special organs, usually underground. Later, these food storage organs grow into new plants. Some different kinds of food storage organs are illustrated and described below. As you can see, many of these food storage organs are good to eat.

Stolons. Stolons are horizontal stems that can put out roots and grow into new plants. Some plants have stolons called runners that lie above the ground. Other plants have stolons that grow underground. Most tubers, and all rhizomes, are swollen underground stolons. Some grasses spread out to cover the ground using runners or underground stolons. Bananas usually reproduce from rhizomes. Suckers grow up from the banana rhizome, near the base of the parent, and become new banana plants.

Cuttings. Many plants can be grown by planting branches, shoots or leaves cut from a living plant. If conditions are right, the cutting develops roots and grows into a new plant. Farmers and gardeners use this method to grow many plants including cassava (manioc), sugar cane or

hibiscus. Grafting is a method used to grow good fruit trees and rubber trees. A neatly cut twig, called a graft, is fixed with tape into a clean cut in a tree that is already growing. The graft soon starts to grow and becomes part of the tree. The tree and the graft must be of the same kind, or very close relatives. For example, we can graft the kind of mangoes we like best onto another kind of mango tree. When the graft is growing well, we can cut away the branches of the original tree so that only it's trunk and roots remain. All the new branches and fruit will then be the kind we prefer.

1. (i) What are sexual and asexual reproduction? (ii)

Give the names of two kinds of asexual reproduction. 2. Describe how some grasses spread using stolons.

3. Is (i) a yam, (ii) a chilli a fruit? Give your reasons. 4. Describe what you can do to grow both your

favourite kinds of mangoes on the same tree.

Asexual reproduction

Reproduction that does not involve the fusion of male and female sex cells is called asexual reproduction. Vegetative reproduction is an example of asexual reproduction. Another example is binary fission in organisms such as amoeba and bacteria.

Underground food storage organs

Bulbs are swollen leaves arranged around a short thick stem. Eg: onion, garlic, some lilies.

Tubers are swollen underground stems or roots. Eg: cassava (manioc), yam, sweet potato.

Rhizomes are underground stems too. They are horizontal, less swollen than tubers, and grow continuously. Eg: ginger, canna lily, banana.

© D.G.Mackean