Download Reproduction in plants

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