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CHAPTER 3 Reproduction in plants What do you know already? ■ All living things reproduce. ■ Plants reproduce by making spores or seeds. ■ When a living thing reproduces sexually the baby plant or animal is different from the parents but it has some of the characteristics of both parents. What is this chapter about? In this chapter you will learn about: ■ the structure of the male and female parts of a flower ■ what happens when a flowering plant is pollinated ■ the difference between insect pollination and wind pollination ■ how seeds are made and how they are dispersed. Skills check Can you: ■ make accurate drawings of what you see? ■ use a hand lens to help you see small organisms and organs? 30 Classification of plants I wonder how many different kinds of plants there are in Botswana? Where do they all come from when they grow after the rains? 3.1 I don’t know how many there are in Botswana but I read somewhere that there are over a quarter of a million different kinds of plants in the world. I think there must be seeds in the ground. How do they make their seeds? New words germinate – when a seed starts growing Tshepo is correct. There are many seeds that stay in the dry ground over the winter. When the rain comes these seeds germinate and a lot of small plants called seedlings start growing. But not all plants make seeds. Remember from your work on classification of plants that many, like fungi and mosses, make spores. Spores are much smaller than seeds and they are made by plants which do not have flowers or cones. Seeds are only made by flowering plants and conifers. seedling – a young plant that grows when a seed germinates seed – what is produced after fertilisation that contains the embryo of the new plant and food to help it germinate spore – a cell, produced by a plant that reproduces asexually, which grows to form a new plant flowering plant – a plant that reproduces by making flowers Asexual reproduction in plants conifer – a plant that makes cones to reproduce, not flowers The table on the next page shows the six main classes of plants. Four classes of plants - algae, fungi, mosses and ferns - all reproduce by making spores. This method of reproduction is called asexual reproduction. Asexual means ‘not sexual’ and the organism simply asexual reproduction – a method of reproduction where a new organism grows from a single cell of the parent, and does not involve male and female sex cells 31 New words genetically identical – organisms that have the same set of genes (e.g. identical twins) gene – the part in the nucleus of a living cell that stores the information about the body made out of the cells (how the body looks, what it can do, etc.) grows a new organism from itself. The new organism has only one parent. Spores are all single cells that are made by the parent plant and they grow into new plants which are genetically identical to the parent. This means that the cells of the new plant contain the same genes in the nucleus of its cells as are in the nucleus of its parent’s cells. Spores are very small because they do not contain any food for the new plant. The new plant must find its own food where the spore falls on the Earth. Fungi Reproduce by making spores Plants without roots, stems and leaves Algae Ferns Flowering plants Conifers The main classes of plants 32 Dicotyledons Monocotyledons Reproduce by means of seeds Plants with roots, stems and leaves Mosses Sexual reproduction in plants Sexual reproduction is more complicated than asexual reproduction. Sexual reproduction requires two parents. As you learnt in primary school, humans are produced by sexual reproduction. The female makes egg cells and the male makes sperm cells. When a sperm cell and an egg cell come together, a new cell is produced. This process is called fertilisation. The new cell then grows into a new baby human. This means that the new baby contains the genes in the nucleus of its cells that come from either parent. Sexual reproduction makes a new organism that is different from both parents. But it has characteristics which are inherited from both parents. This works in the same way for plants. Flowering plants and conifers reproduce sexually. The process is called pollination. You will learn how this happens in this chapter. New words sexual reproduction – reproduction where male sex cells (sperm or pollen) join with female sex cells (eggs or ovules) to form a new cell that contains genes from both parent cells egg cell – female reproductive cell sperm cell – male sex cell in a human or animal fertilisation – when a male sex cell joins with a female sex cell to form a new cell pollination – when pollen from the anther of one flower is taken to the stigma of another flower, and a pollen tube grows to allow the nucleus of the pollen to reach the nucleus of an ovule Test yourself 1. Name two classes of plants that reproduce using spores and two classes that reproduce using seeds. 2. List two differences between a spore and a seed. 3. Explain the main difference between sexual and asexual reproduction. 33 3.2 Pollination and fertilisation The reproductive parts of a flower Before you can understand how pollination works you must know about the structure of flowers. All flowers are made up of the same parts. The next activity will help you to identify these parts. In this activity you are going to look carefully at the structure of a flower. A flower contains many different parts, which are shown in the diagram. But all flowers are different so yours will not be quite the same as the one in the diagram. In this activity you will cut up a flower and study its different parts. You should make accurate drawings of each of the parts in your exercise book. Activity 3.1 GROUP Looking at the parts of a flower You will need ■ one or more flowers from the same plant ■ a hand lens or microscope ■ a sharp knife or scalpel (take care!) ■ a clean surface to work on stigma style petal stamen { ovary anther ovule fi lament sepal A flower What to do 1. Find the sepals on the outside of the flower. Count how many there are. These are the green parts of the flower that covered it when it was in bud. Carefully cut them off. 2. Carefully remove all the petals. How many are there? These are the large colourful parts of the flower. 34 3. You should now be able to see a ring of stamens. These are the male sex organs of the flower. There are two parts to each stamen, the fi lament and the anther. The fi lament is the stalk and the anther grows on top of the fi lament. Look carefully at the anther using your magnifying glass. Can you see the pollen grains? These often look like tiny yellow dust particles. The picture shows what these look like. Carefully cut off all the stamens. 4. In the middle of the flower is the stigma. This is the female sex organ of the flower. Most flowers have just one stigma but some have more than one. Look carefully at the shape of the stigma; it is on top of a stem called the style. The style swells out at the bottom. This swollen part is called the ovary. Look carefully at the stigma. Can you see any pollen grains sticking to it? Pollen grains. Can you see the spikes on the pollen grains which help them stick to insects? 5. Carefully cut open the ovary. Look for tiny round ovules inside it. The table below lists the different parts you identified of the flower. The function of each part is shown in the second column of the table. Flower part Function Sepal The green part that covers and protects the flower when it is a bud. The sepals protect the flower when it is growing. Petal The large coloured part of the flower. They attract insects which pollinate the flower. At the bottom of each petal there is a small swelling, called a gland, which makes nectar. Nectar is a sugary substance that the insects use as food. If the flowers did not make nectar, the insects would not come to it and the flower would not be pollinated. Stamen The male sex organ of the flower. It consists of the fi lament and the anther. Anther The part of the male sex organ that makes the pollen. Pollen Pollen grains are the male sex cells of the flower. They are sticky, and stick to the insects which come to the flower. The insects then take the pollen to other flowers. Stigma The female sex organ of the flower. Pollen that is brought to the flower by an insect, sticks to the stigma. Style The stem of the stigma. The bottom of the stem is attached to the ovaries. Ovary The swollen part at the bottom of the style that contains the ovules. Ovule The female sex cell of the flower. 35 How pollination takes place Pollination by insects Insects come to flowers because they need nectar which is their food. The nectar contains sugar which gives them the energy they need. When insects come to the flower looking for nectar, some of the pollen grains stick to their bodies. These pollen grains are the male sex cells of the flower. The insect then flies to the next flower and some of these pollen grains stick to the stigma of the new flower. The pollen then makes a tube that goes down through the stigma and the style to one of the ovules. The ovules are the female sex cells of the flower. The nucleus of the male pollen cell then moves down this tube and joins with the nucleus of the female ovule. The ovule is now fertilised. The new fertilised cell, formed from the pollen and the ovule, grows into a seed. When the flower dies, the ovary grows into a seed pod. The seeds grow bigger and bigger in the pod until they are ready to disperse. New word self-pollination – pollination of an ovule by a pollen cell from the same plant Insects can take pollen from one flower to another on the same plant. In some plants this can cause fertilisation and the seed can grow. This is called self-pollination. Only a few species of plants can be fertilised like this. In most plants only pollen from another plant will pollinate the ovule. Pollination by the wind Did you know We eat a lot of fruit and seeds that have been formed through pollination of plants by insects. We therefore need pollinating insects to survive. Farmers often use insecticides to kill insects that are harmful to plants, but these insecticides can also kill the pollinating insects. It is very important that farmers take care not to kill pollinating insects. Farmers should not spray plants with insecticides when they are flowering. Some flowers are not pollinated by insects. Instead they are pollinated by the wind. Flowers like this do not have petals and do not make nectar because they do not need to attract insects. Wind pollinated flowers make a lot of pollen. Most of the pollen is wasted because it does not blow onto the stigma of other flowers. The flower of an acacia tree. It makes a lot of pollen. 36 Many trees, such as the acacia tree in the picture, are wind pollinated. Some trees that are wind pollinated produce two kinds of flowers: the male flower and the female flower. In trees like this, the female flower is often very small because it is just a stigma and an ovary. In some plants the male and female flowers are in different trees, and there are male trees and female trees. The pawpaw tree is like this. The male and female flowers of the pawpaw are shown in the picture. When you grow pawpaws you should plant a male tree at the centre of a circle of female trees. Male pawpaw flowers Did you know Some people are allergic to pollen. When trees or grasses make a lot of pollen that blows in the wind, these people get hayfever. They have a continuously running nose and eyes until the pollen season is over, but they can take medicine to help themselves. Female pawpaw flowers Test yourself 1. Describe the function of the following parts of a flower: (a) petals (b) stigma (c) style (d) pollen (e) ovule (f) anther 2. What is the purpose of the nectar that is produced by flowers? 3. Explain why wind pollinated plants usually produce much more pollen than insect pollinated plants. 4. Give a reason why most wind pollinated plants do not have petals. 37 3.3 Seed dispersal New words embryo – a group of cells that are the early stages in the growth of a baby monocotyledon – a plant with only one storage part in its seed, and leaves which have veins that come from the base of the leaf dicotyledon – a plant with two halves (storage parts) to its seed, and leaves which have veins that come sideways out of a vein in the centre dispersal – the movement of seeds away from the parent plant Did you know Spores do not contain food like seeds do. Because of this they are so small you cannot see them. They are blown in the wind until they land. A few land in places where they can get food and water and grow into new plants, but most of them die when they land. Seeds contain the baby plant, called the plant embryo. They also contain some food which the baby plant will use when it starts growing, before it can make its own food with its leaves. Activity 3.2 Looking carefully at the seed Use a large seed such as a pea or bean. Take the outer skin of the seed off very carefully. Can you see the embryo? Can you see the food used by the embryo? There are two food stores in a bean or a pea, one on each side of the embryo. Seeds need to move away from the parent plant before they can grow well. If a seed just falls to the ground and grows near the parent plant, it will not be able to get much light and water because these will be taken by the parent. Different seeds have different methods of moving away from the parent plant. We call these methods of seed dispersal. Look at the pictures in the table below. How do you think these seeds get away from the parent plant? Apple Did you know Plants like maize, mahangu and grasses make seeds that only have one food store. These kinds of plants are called monocotyledons. Plants like peas and beans that have two stores of food in their seeds are called dicotyledons. 38 PAIRS Euphorbia Blackjack Combretum Many plants make seeds which are dispersed by the wind. Sometimes these seeds have thin strands like cotton on them and sometimes they have wings. There are many plants called Euphorbia in the Kalahari, which make seeds that are dispersed by the wind, like the one in the picture. There are many different kinds of a common plant called a Combretum in Botswana, which make seed pods with four wings, like the one in the picture. Some seeds, like many grass seeds, have hooks on them. These plants use humans and animals to disperse their seeds for them, as the hooks stick to your clothes and to animal fur. 39 Some plants which produce fruit use people and animals to disperse their seeds. People will eat the sweet part of the fruit, like an apple, and then throw away the core which contains the seeds. Animals may eat the whole fruit and the seeds are then excreted. Another way of dispersing seeds. How did this seed get into the elephant dung? This is a clever seed it not only gets away from its parent, it gets itself some nice dung to make it grow well! Did you know Some seeds are dispersed when their seed pod gradually dries out and suddenly explodes open. Using ICT Look up ‘seed dispersal’ on the internet. Find pictures of different ways that seeds are dispersed. Make a webpage or use display software to show the different ways seeds are dispersed. Test yourself 1. Explain why it is important for seeds to be dispersed away from the parent plant. 2. List four ways by which seeds are dispersed. 3. Explain where a seedling gets its energy from to grow before it has produced any leaves. 40 Summary These are some of the things I have to remember about reproduction in plants. ■ Plants such as fungi, algae and mosses reproduce by making spores. These are produced asexually, and the new plant has the same genes as the parent. Plants such as conifers and flowering plants reproduce by making seeds. These are produced sexually and the new plant contains genes from both parents. ■ Most flowers contain male and female sex organs. Fertilisation takes place when the male sex cell, called pollen, combines with the female sex cell, called the ovule. This is done in two ways: by wind pollination and by insect pollination. Flowers that reproduce using insect pollination have petals and nectar to attract insects. ■ Pollen is made by the anther at the end of the stamen, which is the male sex organ of the flower. Pollen is taken by insects or the wind to the stigma, which is the female sex organ of the flower. It sticks to the end of the stigma and then produces a tube which goes down the stem of the stigma, called the style. At the bottom of the style the pollen tube reaches the ovary which contains the ovules. It reaches one ovule and fertilisation takes place when the nucleus of the pollen cell comes down the pollen tube and combines with the nucleus of the ovule. ■ The fertilised cell then grows into a seed and the ovary changes into a seed pod. ■ When the seeds are ripe they are dispersed away from the parent plant so that they can grow better and do not have to compete with the parent plant for soil and water. Different seeds have different methods of dispersal. Some seeds use the wind, some seeds stick to animals and others have fruit, which is eaten by animals, and the seed inside is excreted, undamaged. 41 Revision Exercise 1. Which of the following describes the movement of the seed away from the parent plant? 5. Many farmers who grow fruit, such as oranges, also keep bees. Give two reasons for this. A dispersal B fertilisation C germination D pollination 2. 4. Explain the importance of insects in pollination. What benefi t does the insect get from pollination? Y 6. Describe four methods of seed dispersal. Which method is used by the following plants? (a) grass (b) orange (c) the seed pod of the Camel Thorn, an acacia found in the Kalahari X What are the correct names of the two flower parts X and Y? A anther and stigma B stigma and style C stamen and ovule D anther and style 3. Draw this hibiscus flower in your exercise books. Label the sepals, petals, stamens, anthers and style. 42 (d) the seed of this plant is called Euphorbia Revision Exercise 7. Observe - explain Write a sentence explaining each of these observations. If you do not know the answer, use your scientifi c knowledge to suggest one. (a) Wind pollinated flowers produce large amounts of pollen. (b) Wind pollinated flowers have no petals. (c) Many flowers have a strong scent. (d) Pollen grains often have spikes on them. (e) The maize plant shown in the sketch is wind pollinated. The male flower is at the top. Where is the female flower? Think of reasons why the male and female flowers are in different places like this. (f) Many seeds only germinate after it rains. (g) Plants grow in the Kalahari Desert when it rains, even if they have not grown there for many years. (h) Some Kalahari Desert plants grow and flower and make seeds very quickly. (i) Most of the plants in the Kalahari Desert flower at the same time and have white flowers. Can you think of reasons for both of these? 43