8 Plant Reproduction

LOOK 1. Plants. • How many different types of plants can you see in the photograph? Describe them. • How are they different? • Which of the plants have flowers? 2. This photograph was taken in spring. Imagine the landscape in winter. Draw it. • Would you see the same plants in winter? Would there be any flowers? • What would the trees look like?

W H AT D O Y O U K N O W ? 1. Answer the questions. • Why do we plant seeds? • What part of a plant contains seeds? • Is a potato a type of fruit? What about a tomato? 2. Taking care of a plant. Bring a geranium in a flowerpot to class. Answer these questions. • What do plants need to grow? How should we take care of the geranium? Take care of the geranium for a year. Check your answers to the questions.

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1. Types of Plant Reproduction

tuber

stolons

bulb

The bulb and leaves of an onion.

The tuber and flowers of a potato plant.

The flowers and stolons of a strawberry plant.

Types of Plant Reproduction There are two kinds of plant reproduction: sexual reproduction and asexual reproduction. • The part of the plant responsible for sexual reproduction is the flower. • Asexual reproduction occurs when a new plant grows from the stem or leaves of an existing plant. All plants can reproduce by sexual reproduction. Some plants can reproduce by both methods.

Sexual Reproduction

Questions 1. How do plants reproduce? 2. How can a new plant grow from an existing plant? 3. What do flowers do?

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Flowers are responsible for sexual reproduction in plants. A male sex cell and a female sex cell unite inside a flower. The fruit and seeds develop from the flower. Later, a new plant develops from a seed. When a plant reproduces by sexual reproduction, there are differences between the parent plant and the offspring plant. For example, the color of their flowers might be different. The fruit of one might be smaller than the fruit of the other. One might be more resistant to certain illnesses than the other. Some plants, like ferns, do not have flowers. However, these plants also produce sex cells and are capable of sexual reproduction.

Compare and Analyze Cuttings

Asexual Reproduction The stem or leaves of a plant are involved in asexual reproduction. Tubers, bulbs and stolons are types of stem. • Tubers are stems in the soil. Buds sprout from the tubers. The buds produce new plants. • Bulbs are also underground stems. Bulbs have a bud inside them. The bud produces a new plant. • Stolons are stems above the ground. New stolons produce new plants. When a plant reproduces by asexual reproduction, the new plant is identical to the original plant. Plants that reproduce by asexual reproduction can also reproduce by sexual reproduction.

Cuttings allow us to grow new plants from existing adult plants. To grow a plant from a cutting, first take an existing plant. Cut off a branch with several leaves. When you plant the branch, roots will grow out of it. The roots take hold in the soil and the branch begins to grow and develop. This kind of asexual reproduction does not happen naturally. It is a technique we use to produce specific plants, like geraniums and roses.

Geranium

• Explain what you must do to grow a geranium from a cutting. • Do plants reproduce from cuttings naturally? Why or why not?

Tulips have flowers and bulbs.

Activities 1

Complete the sentences.

sexual reproduction

asexual reproduction

• Bulbs, tubers and stolons are involved in • The potato plant can reproduce by Flowers are responsible for in

2

•

involves the sex cells.

•

involves stems and leaves.

. and by

.

.

Which of the following sentences is correct? Explain. a) Some plants can reproduce sexually and asexually. b) If a plant reproduces sexually, it cannot reproduce asexually. 83

2. Flowers, Fruit and Seeds The Parts of a Flower

Fleshy Fruits

corolla pistil

stamen

stigma

anther

style

filament ovary sepal

petal calyx

The fruit of a strawberry plant.

The fruit of a blueberry bush.

Nuts

Seeds

protective casing

nut

Chickpeas

Almond nuts on an almond tree.

Chestnut

The Parts of a Flower

Questions 1. Describe flowers. 2. Describe seeds. 3. What kinds of fruit and nuts are there?

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The flower is the reproductive unit of flowering plants. The parts of a flower include the following: corolla, calyx, pistil and stamen. • The corolla is the colorful part of the plant. Petals form the corolla. • The calyx is at the bottom of the flower. Sepals form the calyx. The sepals are small green leaves that protect the flower before it opens. • The pistil has three parts: a stigma, a style and an ovary. The ovary produces the female sex cells, called ovules. The style is usually shaped like a bottle. The tip of the pistil is called the stigma. • The stamen has two parts: a filament and an anther. The anthers produce pollen. The male sex cells, or sperm, are in the pollen. Flowers usually have several stamens.

Look Seeds

The Sunflower Is Not a Flower

Seeds form inside fruit. Some fruit, like the peach, only have one seed. Other fruit, like the apple, have several. Seeds have a protective covering. The covering of some seeds, such as peas, is very thin. Other seeds are harder, like watermelon seeds. The covering of some seeds is hard and thick, like the apricot seed.

Although the sunflower looks like a flower, it is not a single flower. A sunflower is a group of many flowers. special leaves that protect the flowers

Fruits

stigma

After fertilization, part of a flower develops into a fruit. • Fleshy fruits accumulate water. Grapes, cucumbers, apples, oranges, strawberries, peppers, etc. are all fleshy fruits. • Nuts do not accumulate water. Acorns, chestnuts, almonds, peanuts, etc. are all nuts.

anthers flowers petals

A Single Flower

• Describe a sunflower. What are its flowers like? • What other plant has flowers in groups but looks like a single flower?

Activities 1

Read the key words. Look at the pictures. Put the key words in the correct box. Explain what each part of the flower does.

stigma

2

petal

anther

filament

ovary

Draw the following fruit with their seeds in your notebook. Complete the tables. peach, melon, apricot, plum, tomato, cucumber, apple, pear, green pepper Fruit with One Seed

Fruit with Several Seeds

Seeds with a Thin Covering Seeds with a Thick Covering

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3. From Flower to Fruit stigma

1

2

petal

pollen grains

stamen with pollen

stigma

ovary The pollen grains open and the male sex cells appear.

3

4 fertilization

male sex cell

female sex cells

The wind pollinates this bulrush.

The fertilized cells will turn into seeds.

Parts of the ovary will turn into fruit.

Fertilization of a Pea Plant

Pollination

Questions 1. What is pollination? 2. How are plants fertilized? 3. What do the seeds do?

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Plants with flowers reproduce sexually. This happens when pollen from the stamen reaches the pistil. Pollination is the transfer of pollen from the male part to the female part of flowers. Some plants self-pollinate. The pollen comes from the stamens of the same plant. Other plants cross-pollinate. The pollen comes from the stamen of another plant of the same species. The wind and certain animals can transfer pollen from one flower to another. • For example, the wind transports the pollen of wheat flowers. The pollen can reach the flowers of many different wheat plants. • Some animals and insects, like hummingbirds and bees, also transport pollen. When bees land on flowers to feed, pollen sticks to their bodies. They then carry the pollen from one flower to another. Bees pollinate rosemary flowers, for example. Plants that rely on animals for pollination have brightly-colored flowers or strong smells to attract the animals. Other plants have small green flowers, so animals do not notice them. The wind transports the pollen of these plants.

Look Traveling Fruit

Fertilization Most plants do not have flowers throughout the year. They flower during their reproductive season, usually in spring. A mature flower produces sex cells: male sex cells, or sperm, in the anthers and female sex cells, or ovules, in the ovary. During reproduction, a female cell unites with a male cell. The reproductive process is as follows: 1. The anther produces a lot of pollen grains. The pollen grains contain the male sex cells. 2. The pollen grains fall on the stigma. 3. The pollen grains open and the male sex cells come out. The male sex cells penetrate the pistil. 4. Fertilization occurs inside the ovary. That is where the male and female sex cells unite. After fertilization, the fertilized cells become seeds and part of the ovary turns into a fruit. The seeds fall to the ground and germinate. This produces new plants. Some seeds germinate a long way from the plants they come from. This happens because the wind or animals transport the seeds.

The fruit of a dandelion is very light. Its seed is very small. The seed is attached to a stem with a blossom at the top. When the wind blows, it carries the fruit with it. For this reason, dandelion seeds can germinate a long way from the plants they come from. a lot of fruit

a single fruit blossom seed

stem

• Could the fruit be transported by the wind without its blossom? • Could it be transported by the wind if the seeds were big and heavy?

Activities 1

Complete the sentences. a) Pollination is b) The

. and

are involved in pollination.

c) In self-pollination, the pollen from a flower's stamen falls on the plant.

of the

d) In cross-pollination, the pollen from a flower's stamen falls on the of a plant. e) During fertilization, a f) Fertilization takes place in the 2

sex cell unite.

of a flower.

Explain the differences between: a) pollination and fertilization. b) female sex cells and male sex cells.

3

sex cell and a

c) pollen and male sex cells. d) seeds and female sex cells.

Which parts of a flower are involved in fertilization? 87

4. Germination and Growth Bean Germination

1. The root comes out.

2. The root and the stem grow.

3. The leaves form.

4. The root and the stem continue growing. The existing leaves grow. New leaves come out.

Germination A seed produces a new plant through the process of germination. A seed must fall to the ground and have water to germinate. A bean germinates in various stages. First, the bean becomes buried in the soil. Later, a small root grows downward. Then the stem starts to grow. Finally, the leaves grow. Some seeds need special conditions to germinate. For example, some need to spend the cold months of winter in the soil to be able to germinate in spring.

Growth

Questions 1. What is germination? 2. How does germination happen? 3. How long do plants live? 4. How do trees change during the year?

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A plant grows gradually after germinating. It keeps growing until it becomes an adult plant. Some plants, like trees, keep growing throughout their lives. The life span of plants varies a lot. In general, trees can live for many years. Other plants, like poppies, live for only one year. The seasons of the year affect the life of plants. Plants flower in spring. Their fruit and seeds grow and mature in summer. Some plants lose their leaves in autumn. Their branches are bare during winter.

Look and Compare How Can We Measure the Age of a Tree?

Trees throughout the Year Trees change throughout the year. For example, trees flower in spring. Reproduction also takes place in spring. The leaves on trees also change. Evergreen trees have leaves all year long. For example, pine trees are evergreen. Deciduous trees lose their leaves in autumn. For example, apple trees are deciduous.

We can calculate how old a tree is by counting the rings on a cross section of its trunk. Each ring represents one year of growth. The rings generally vary in color and thickness because tree growth varies from year to year. The longest-living plant is believed to be a species of pine tree called the bristlecone pine. Some bristlecone pines are 4,850 years old. This means that they were alive when the Egyptians were building the pyramids.

A lot of plants have lost their leaves by winter.

Growth Rings of a Tree

• Why does the number of rings on a trunk tell us how old the tree is? • Which live longest, plants or animals? Discuss.

These plants grow new leaves in spring.

Activities 1

Complete the sentences. Stages of Germination

1. A 2. The 2

is buried in the soil. grows

3. Then, the .

4. Finally, the

grows.

grow.

Explain how plants change throughout the year. Give examples. 89

SOCIETY

Greenhouses What Are Greenhouses? Greenhouses are enclosed areas where we grow plants. We grow plants for food in many greenhouses. We also grow plants to decorate the streets and our homes. A lot of the peppers, green beans, tomatoes, roses and tulips that we see in markets grow in greenhouses. We grow decorative plants like geraniums in greenhouses.

Cultivating Plants In greenhouses, we can control everything that affects plant reproduction and growth. We can keep the temperature and humidity at optimum levels. We can give each plant the water and fertilizer it needs. We can also give the plants substances to keep them healthy. Consequently, the plants grow quickly. They produce fruit several times a year. Monitoring plants in a greenhouse.

The Advantages Growing plants in greenhouses gives us a lot of fruit all year long. Not long ago, each fruit had its season. For example, you could only buy cucumbers in summer. Now you can buy cucumbers at any time of the year. This is because we grow them in greenhouses. We also grow fruit in greenhouses, like these peppers.

1

Describe the greenhouses in the photographs. • How big are they? • What materials are they made of? • What plants do we grow in them?

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2

What are the advantages of growing plants in greenhouses?

3

Find out about the greenhouses in your community. What plants grow in them?

INVESTIGATE

Animal Pollination 1

Look at the pictures. 1

2

2

3

Put the sentences in the correct order. Write them in the table. The pollen from the stamen sticks to the bat’s face. The bat drinks the nectar of the cactus flower. The pollen on the bat’s face comes into contact with the stigma of another cactus flower. The bat flies away from the flower with a lot of pollen on its face.

HEALTH EDUCATION

4

Pollination of a Cactus Flower

1. 2. 3. 4.

Wash Fruit before You Eat It You can eat some fruit, like apples, with the skin still on. The skin contains nutrients too and is good for your teeth. Before you eat fruit, wash it well to remove dirt and other substances. These substances can get on the skin of the fruit while the plant is growing. It can also get on the skin when the fruit is transported or sold.

Grapevine

• Why should we wash fruit before eating it? • Make a list of fruits that we can eat with the skin still on. How do you normally eat fruit?

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Test Your Knowledge 1

Copy and complete the diagram. Plant Reproduction

involves

involves leaves these produce

for example

sex cells

bulbs

2

female

Complete and answer. a) Copy the pictures. Label the parts. b) Write these sentences in the correct boxes: • The pollen grains in the stamen fall on the stigma of the pistil. • The female sex cells and the male sex cells unite inside the pistil. • The pollen grains open. The male sex cells appear and move to the inside of the pistil. c) What is this process called? What is the result? 1

3

2

Put these stages in the correct order. Complete the diagram. • Formation of the fruit and seeds. • Fertilization.

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3

• Development of the plant. • Appearance of the flowers.

1.

2. Pollination.

3.

6.

5. Germination.

4.