Reproduction in Plants

Reproduction is a characteristic of all living organisms. It is the process by

which organisms make more of their kind. There are two types of reproduction,
1. Asexual reproduction and
2. Sexual reproduction.
Sexual Reproduction
Sexual reproduction is the process involving the fusion of male sex cells and
female sex cells to produce a new offspring.

The structure of a flower

1 Stigma  Receives pollen grains during the process of
pollination
 Produces a sticky substance to ensure pollen
grains get stuck
2 Style  Supports the stigma
 Provides pathway for pollen tube to the
ovules
3 Ovary  Produces ovules
 Later develops into fruit after fertilisation
4 Ovule  Are the female sex cells
 Develops into a seed after the process of
fertilisation
5 Petals  Attract insects for pollination by their bright
colour and nice scent
 Their shape promotes pollination as they
direct insects towards the nectaries
6 Sepals  Protects the flower bud
7 Anther  Produces pollen grains

8 Pollen grains  These are the male sex cells

In a plant, the organs which are responsible for sexual reproduction are the
flowers. The diagram below shows a typical flower.

Structure of a flower - Diagram

Notes compiled by M. MLILO “TO GOD BE THE GLORY” Page 65

Sex cells are called gametes. The male gametes are produced by the anthers
and are inside the pollen grains. The female gametes are produced by the ovary
and are inside the ovules. Some flowers can produce both male and female
gametes. They are called hermaphrodite.

Pollination:

Pollination is the transfer of pollen grains from the anthers of a flower to the
stigma of a flower.

Notes compiled by M. MLILO “TO GOD BE THE GLORY” Page 66

Methods of Pollination
There are two methods of pollination,

1. insect pollination and

2. wind pollination.
Some flowers pollinate by insects while others pollinate by wind and wind and
insects can also be called the agents of pollination.

Insect pollination
Insect pollinating flowers have special attractive features like brightly colored
petals, attractive scents and sugary nectar. These features’ aim is to attract
insects like bees to come and collect their nectar. While an insect is collecting
the nectar, its body will touch the anthers. The pollen grains of insect
pollinating flowers have hooks and spikes all over them in order to stick to the
bodies of the insect that touch it. When this happens, the pollen grain sticks to
the body of the insect. When the insect moves on to another flower to collect
its nectar, the pollen grain falls off the insect onto the stigma of the flower,
thus insect pollination took place.

Wind Pollination
Wind pollinating flowers however look very different from insect pollinating
ones. This is because they do not need attractive features such as bright colors
and scents and nectar to attract insects. Instead, their petals are green or
brown with their anthers and stigmas hanging outside the flower to be exposed
to the wind. When wind is strong enough, it will blow the pollen grains off the
anther and carry it along. At some point the wind will get weaker and the pollen
grain will land, if it lands on the stigma of a flower of the same species then the
pollination was successful. The surface of pollen grains of wind pollinated plants
is smooth because it does not need spikes to catch on insects.

Comparing wind pollinated and insect pollinated flowers

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 Insect pollinated Flowers Wind Pollinated Flowers

Petals Petals are bright and Dull coloured, usually green or

produce a nice scent to brown and produce no scent as
attract insects there is no need to attract insects

Size of pollen Produce large sticky pollen Produce small smooth pollen grains
grains grains which are light enough to be
carried by the wind
Quantity of Relatively small number of Produces a large quantity of pollen
pollen grains pollen grains as chances of grains to increase the chances of
produced pollination are high pollination

Position of Inside the flower Are hanging outside the flower

stigma and
anthers
Nectaries They have nectarines No need for nectaries
which produce nectar to
attract insects

Types of Pollination
There are two type of pollination
1. Self Pollination and
2. Cross Pollination:
Self pollination - The transfer of a pollen grain from the anther to the stigma
of the same flower or the stigma of a flower of the same plant.

Advantages of self pollination

1. Good characteristics can be maintained in the offspring because there is

no variation.
2. Chances of success for self pollination are much greater especially were
plant population is small.
Cross Pollination - The transfer of a pollen grain from the anthers of one plant
to the stigma of a flower on another plant of the same species.

Self pollination is considered sexual reproduction because it involves the fusion

of two gametes together even though they might come from the same flower or
plant.

Notes compiled by M. MLILO “TO GOD BE THE GLORY” Page 68

Why cross pollination is considered desirable
1. It causes variation in plants which allows plants to adapt to changing
environments. This prevents the extinction of plant species.
2. It allows the breeding of better varieties by combining plants with desired
characteristics.

How are plants adapted to promote cross pollination

1. Unisex plants – some plants have female plants and male plants and no plant can
pollinate itself e.g. paw paw plant
2. Stigmas and anthers of plants in certain plants mature at different times. The
stigma matures earlier than the anthers and can only receive pollen from
another plant.
3. Anthers which are lower than stigmas. This ensures that insects touch the
stigma on their way in with pollen from another plant. On their way out, they
crawl out using the petals carrying pollen from this flower to another plant.
4. Self sterilisation – some plants produce chemicals on the stigma which do not
allow pollen from the same plant to develop.

Disadvantages of Cross pollination

Refer to advantages of self pollination.

Fertilization
 This is the fusion of the male and female sex cells to produce a zygote. It
occurs after a pollen grain has landed on the stigma of a flower of the
same species.
 The stigma produces chemicals that cause the pollen grain to develop a
pollen tube.
 The pollen tube then grows down the style until it reaches the ovule in
the ovary.
 The nucleus of a pollen grain moves down the tube until it gets inside the
ovule where it fuses with the nucleus the ovule.
 The fusion produces a zygote which develops first into an embryo and
latter the whole ovule develops into a seed.

Notes compiled by M. MLILO “TO GOD BE THE GLORY” Page 69

 Diagram

Results of fertilization
1. The fused nuclei develop into a zygote further develops into an embryo
(made up of the plumule and the radical). This develops into a new plant
during germination.
2. The ovule becomes the seed.
3. The ovary develops into a fruit.

Seeds and Fruits:

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During the early stages of fertilization, some parts of the flower become
useless, they die and fall off. These parts include the sepals, the petals and the
stamens.

The fertilized ovule becomes a seed. Inside the seed is the zygote which
develops into an embryo plant. An embryo plant has a shoot called plumule and a
root called a radicle. The plant makes food for the seed and brings it to the
seed where it is stored in a structure called cotyledon inside the seed. The
outside layer of the ovule becomes thicker and harder and is now called the
testa. In the seed there is a hole called the micropyle, this is the hole through
which the male gamete entered the ovule. When the seed is fully grown, it
becomes dormant. That means it loses its water and metabolic reactions inside
it stop. These reactions are reactivated when the seed is planted in favourable
conditions including water and air; this is when the seed grows into a new plant.
This enables the seed to survive hostile conditions until it is put in somewhere
where it can grow into a plant.

The ovary of a flower contains many ovules. The ones that are fertilized turn
into seeds and the ovary its self turns into a fruit in which the seeds are. The
function of the fruit is to protect the seeds and to disperse them from the
parent plant to colonize new area.

SEED DISPERSAL

 This is the scattering of seeds away from the parent plant and is
important for two reasons,
o It helps the plant to spread to new areas.
o It prevents the overcrowding of plants around the parent plant
where competition for water, mineral salts and sunlight will prevent
healthy growth.
Methods of seed dispersal

1. Wind dispersal occurs in plant with seeds which have wing like structures
which allow the wind to carry them to new areas.
2. Some plants have small horns or thorns which enable them to stick to
animal skins and are carried to new areas.

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 3. Some plants produce fruits which are sweet and edible. Animals eat them
and drop the seeds in their dung some distance away from the parent
plant.
4. Some fruit split with great force throwing the seeds some distance away
from the parent plant
Seed Germination

Germination is the process by which a seed develops into a new plant. A seed remains
dormant until it is put in suitable conditions to start growing. These conditions
are:

 Water(moisture)
 Air (oxygen)
 Suitable temperature(Warmth)

If the seed is put in an area of these conditions it will start germinating.

Germinating includes the following steps:

1. The water activates enzymes stored in the seed,

2. Enzymes start to digest the starch stored in the seed into sugars,
3. Suitable temperature is required for the optimum operation of these
enzymes and oxygen is required for respiration.
4. The sugars produced are taken to the embryo and are used as an energy
source for growth,
5. The radicle (root) grows first, fixing the seedling into the soil where it
can absorb more water,
6. The seed is now called a seedling,
7. Then the shoot grows vertically,
8. Once the shoot grows the first green leaf the seedling becomes fully
independent (before that it was depending on food stored inside the
seed)
9. The plant is now able to photosynthesise and make its own food.

Types of Seeds

There are two types of seeds;

1. Dicotyledonous seeds and

2. Monocotyledonous seeds
Structure of a dicotyledonous seed

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 Diagram

Structure of the monocotyledonous seeds

Diagram

Functions of Parts on a seed

1.
Radicle - develops into the root system.
2.
Plumule - develops to form the shoot, stem and leaves
3.
Cotyledon - a store of food which is used during germination.
4.
Endosperm -stores food which is used during germination.
5.
Testa/Coat - protects the seed.
6.
Micropyle - allows water and oxygen to enter the seed for germination
to occur and also allows carbon dioxide produced during respiration to
leave the seed.
7. Hilum - shows the place where the seed was attached to the ovary.
Differences between a maize seed and a bean seed.

Bean Seed Maize Seed

1. Has two cotyledons Has one cotyledon

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2. Does not have an endosperm Has an endosperm
3. Has a micropyle Does not have a micropyle

Experiment

Aim: To investigate the conditions necessary for germination.

Materials

Maize/bean seeds, 4 transparent containers, cotton wool, filter paper, pyrogallic

acid and water.

Method

1. Take four batches of ten seeds per batch.

2. Set them up in four different conditions as shown in the diagram above;
A. Water + oxygen + low temperature (put in fridge or in ice blocks)
B. Oxygen + suitable temperature + no water
C. Water + suitable temperature + no oxygen
D. Water + suitable temperature + oxygen (this is the control)
3. Leave the apparatus for seven days and observe what happens.
4. Record the results and calculate percentage germination.
Observations

A. All seeds in A did not germinate because there was no suitable

temperature.
B. All seeds in B did not germinate because dry seeds do not have enough
moisture to germinate.
C. All seeds in C did not germinate because the pyrogallic acid removed all
the oxygen from the air. Seeds cannot germinate without oxygen.

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 D. Some seeds in D germinated because this set up had all the conditions
necessary for germination to occur. (oxygen, moisture and suitable
temperature)
NB: a seed has germinated when a young root or radicle appears through the
tough testa. The shoot will appear later.

Calculating percentage germination

% Germination = No of seeds which germinated X 100

No of seeds planted

In setup D above: ten seeds where planted and eight seeds germinated. The %
germination is calculated as follows;

% Germination = 8 X 100
10

= 80 %

Vegetative reproduction / Asexual reproduction

 Occurs when a piece of the parent plant is taken and used to grow a new plant.
 No sex cells are used during this type of reproduction.
 Plants which use this type develop modified parts such as roots, stems and
leaves which can develop into new plants.
Two types of vegetative reproduction

a) Natural vegetative reproduction:- new plants develop naturally from the parts of
the parent plant.
b) Artificial vegetative reproduction:- new plants develop only with the
intervention of humans from parts of parent plant.
Natural reproduction

1. Stem Tubers e.g. potatoes

o These are swollen parts at the end of an underground stem and they
store food during photosynthesis.
o They have buds (eyes) which can develop into new plants

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 Diagram

2. Root tubers e.g. sweet potatoes

 These are swollen parts of roots which store food during
photosynthesis. They also have buds from which will develop new
plants if the right conditions exist.

3. Rhizome e.g. strawberry plants, couch grass

 Is an underground stem that grows horizontally underground and have
nodes from which new stems will develop towards the surface to
produce new plants.

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 Diagram

Artificial vegetative reproduction

i. Cuttings e.g. sweet potatoes, sugar cane, cassava

 Small parts of the plant are used to produce other plants. These
portions of the parent plant are put into the ground in wet soil and
will eventually grow into a new plant.

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 Diagram of a cutting

ii. Layering
 a branch or shoot is pegged underground while still attached to the
parent plant.
 Roots will develop on the buried part and it is cut off from the parent
plant.

Advantages of vegetative reproduction

1. New plants have the good features of the parent plant as there is no variation.
2. There is a higher chance of survival for the new plant as it has a larger store of
food.
3. Plants mature much more quickly as there is no time wasted during germination.
4. No need to find a mating partner and therefore it is quicker. The methods do
not need pollination, fertilisation, and seed dispersal.
5. Asexual reproduction is beneficial in an unchanging environment where the
parent organism is well adapted to survive
Disadvantages of vegetative reproduction

1. No genetic variation and therefore no improvement to meet changes in the

environment or diseases. This can lead to extinction of species.
2. Overcrowding leads to competition for nutrients, water, and light. This affects
the growth of plants.
3. Pests and disease spread quickly because of overcrowding.
4. Not possible to obtain large number of plants in a short time compared with
seeds

Notes compiled by M. MLILO “TO GOD BE THE GLORY” Page 78