This lesson will cover:
- Sexual reproduction in
- Flower structure
- Asexual reproduction in
By the end of this lesson you
will be able to:
- Understand the difference
between sexual and asexual reproduction in plants.
- Understand the functions
of the parts of a flower.
- Understand how pollen is
transferred from one flower to another.
- Understand how fertilisation
takes place in a flower.
- Understand how and why seeds
- Understand the conditions
necessary for germination.
- Sexual reproduction
takes place in nearly all organisms. Two different sexes, male and female,
are involved in sexual reproduction. The male and female produce specialised
sex cells called gametes. In humans the male gametes or sperm are produced
in the testes. Female gametes or ova are produced in the ovaries of
the female. Flowers also have specialised organs that produce gametes.
The sex organs of a plant are located in the flower. The pollen contains
the male gametes. The ovary contains the female gametes. In humans,
fertilisation takes place when the male gamete fuses or joins with the
female gamete to form a single cell called a zygote. The zygote grows
into a child. Fertilisation also takes place in plants when the male
gamete fuses with the female gamete to form a zygote. The zygote grows
into a seed. The zygote divides into many cells, which will eventually
grow into an adult organism.
- Flower structure
Male flower structure
The sex organs are contained inside the flower. The sepals are there
to protect the flower while it is in bud. The petals which are brightly
coloured are there to attract insects by their colour and their scent.
The stamens are the male organs and are made of two parts, the anther
and the filament. Pollen grains are produced inside the anthers. The
filament is there to hold up the anther.
Female flower structure
The carpel is the female organ
and produces the female gametes. The carpel is composed of three parts
- the stigma, the style and the ovary. The pollen lands on the stigma.
The style connects the stigma to the ovary. The ovary contains the ovules.
The ovule contains the female gamete. The nectary secretes sugar to
attract insects. Different flowers have different sizes and numbers
of sepals, petals, carpels and stamens but all of them have the same
For fertilisation to take place, the male gamete inside the pollen grain
must reach the female gamete inside the ovule. The transfer of pollen
from the anthers to the stigma is called pollination. Insects can carry
pollen. Insects such as bees are attracted to a flower by the scent
and colour of the petals. The bee lands on the flower and feeds on the
nectar. Nectar is a sweet sugary substance that the flower provides
for the insects. Pollen from the anther is transferred to the bee's
back. The bee flies to another flower of the same type, again attracted
by the colour and the scent. While feeding on the second flower the
stigma collects the pollen from the bee's back. Daisies, dandelions
and roses are examples of plants that depend on insects for pollination.
Moths and butterflies also pollinate flowers.
Pollen can also be transferred from the anther to the stigma by the
wind. In a wind pollinated flower the stamen releases light grains of
pollen that are carried aloft by the wind. The feathery stigmas of the
flowers trap the pollen grains as they float by. Only pollen from the
same type of flower can grow on the stigma. Grasses are an example of
plants that depend on the wind for pollination. In grass, flowers are
grouped together in twos and threes forming spikelets. The flowers are
small but they have the same basic structure as an insect pollinated
flower. They have ovaries, stigmas, stamens and petals.
Differences between wind
and insect pollinated flowers
Flowers that depend on insect pollination are different from wind pollinated
flowers. Insect pollinated flowers have coloured scented petals. The
stamens are located inside the flower. The stigma is also located inside
the flower. They produce nectar for visiting insects. Wind pollinated
flowers are smaller and have green petals. The stamens hang outside
the flower and produce small light pollen grains. The feathery stigmas
are also on the outside of the flower to catch the pollen as it floats
Fertilisation takes place after pollination. The male nucleus inside
the tube fuses with the female nucleus in the ovule to form a zygote.
A tube grows out of the pollen grain on the stigma and down through
the style to the ovary. The male gamete is contained inside the pollen
tube. As it penetrates the wall of the ovary the tube grows towards
the ovule. The tube enters the ovule through a tiny hole called the
microphyle. The male nucleus inside the tube fuses with the female nucleus
in the ovule to form a zygote. This is fertilisation and results in
the formation of a zygote.
After fertilisation the fertilised
ovule grows into a seed. As the petals and stamens wither and die the
ovary wall forms the fruit. Plants produce two different types of fruit,
fleshy and dry. Tomatoes, apples and pears are examples of fleshy fruits.
Hazel nuts, peas and dandelion seeds are examples of dry fruits.
Reasons for dispersal
The seeds of a plant must be dispersed or scattered as far from the
parent plant as possible. Otherwise too many plants grow close together
in a small area and the parents will compete with the seedlings for
space, light and minerals. To prevent competition between the parent
plant and its seedlings, the seedlings are scattered or dispersed away
from the parent. This gives them a better chance of survival. Seeds
and fruits are scattered by different methods.
Mechanisms of seed dispersal
Dandelions and thistle seeds are scattered by the wind. Animals disperse
seeds by eating the fleshy fruits and passing the seeds out later. Self-dispersal
is when plants use their own methods to scatter seeds. Peas are scattered
by exploding pea-pods. Plants that live in water, such as the water
lily, produce seeds that float and are then carried downstream by the
current. When the seed lands and conditions are suitable, it will grow
Conditions for germination
Many seeds are dispersed in the autumn and lie dormant in the ground
during the winter. In the spring, the extra light and heat cause the
seed to germinate. Seeds need warmth, oxygen and water to grow. This
can be demonstrated by setting up the following experiment. Collect
four test tubes and label them as A, B, C and D.
- Test tube A contains
cress seeds on moist cotton wool. These seeds have warmth, moisture
- Test tube B contains
cress seeds on dry cotton wool. These seeds have warmth and oxygen
but no water.
- Test tube C contains
cress seeds in cold boiled water. Water that has been boiled has
no oxygen. The layer of oil on the top of the water prevents oxygen
in the air from dissolving. These seeds have warmth and moisture
but no oxygen.
- Test tube D contains
cress seeds on moist cotton wool and is be placed in a fridge. These
seeds have oxygen and water but no heat.
The tubes are left for a few days and are then examined. It is observed
that only the seeds in Test tube A will have germinated. Thus we may
conclude that heat, water and oxygen are needed for germination.
Germination of a pea
If we soak some dried peas
overnight and then place them on moist cotton wool, we can observe
the stages of germination. As the pea germinates, the seed coat or
testa splits and the root or radicle grows out. As the root lengthens
the shoot or plumule then emerges. The root develops and the shoot
pushes upwards, forming leaves. The seedling grows into a full adult
plant which will produce its own flowers and seeds.
Life cycle of a plant
The life cycle of a plant describes the process of a seed growing into
a plant, pollination and fertilisation in the plant, the dispersal of
seeds and the germination of those seeds. Consider the life cycle of
a sweet pea. In the spring, when conditions are right, the seeds germinate
and grow. In the summer, the sweet pea produces flowers, which are pollinated
and fertilised. Fruits and seeds form. In the autumn, the seeds are
dispersed and the parent plant dies. The seeds lie dormant in the ground
during the winter until the spring arrives, when the cycle starts over
- Asexual reproduction
Reproduction can take place
without gametes and without fertilisation. This is called asexual reproduction.
When we take cuttings from a plant we are helping the plant to reproduce
asexually. Cuttings when planted will grow into a new plant identical
to the parent plant. Strawberries propagate themselves by runners.
This lesson has covered:
1. In sexual reproduction
the male and female produce specialised sex cells called gametes.
2. Fertilisation takes place
when the male gamete fuses or joins with the female gamete to form a
single cell called a zygote.
3. The sex organs of a plant
are contained inside the flower and consist of the sepals, petals, stamens
(produce the pollen with the male gametes) and carpels (which produce
the ovule with the female gamete).
4. The transfer of pollen
from the anthers to the stigma is called pollination.
5. Pollen can be transferred
by wind or by insects.
6. Insect pollinated flowers
have colourful petals and nectar to attract insects. The stamens and
carpels are inside the flower.
7. Wind pollinated flowers
have feathery stigmas to collect pollen and stamens that hang outside
8. After pollination a tube
grows from the pollen grain to the ovule carrying the male nucleus to
the female nucleus and fertilisation takes place.
9. The fertilised ovule
grows into a seed and the ovary forms the fruit.
10. Fruits and seeds are
dispersed by different methods: wind dispersal (for dandelions), animal
dispersal (for berries), self-dispersal (for pea-pods) and water dispersal
11. The conditions needed
for germination are water, heat and oxygen.
12. The stages in the germination
of a pea are firstly the testa or seed coat splits and the root or radicle
emerges. Next the shoot or plumule emerges and grows above ground. Finally
the root develops and leaves form on the shoot.
13. The life cycle of a
plant describes the process of a seed growing into a plant and the plant
forming seeds, which are dispersed to start the cycle again.
14. Asexual reproduction
can take place without gametes and without fertilisation.