Download 3.6.1 Reproduction of the Flowering Plant 2.3.7 Functions of Meiosis

3.6 REPRODUCTION AND GROWTH
3.6.1 Reproduction of the Flowering Plant
Sexual Reproduction
Sexual reproduction involves two parents. Each parent makes sex cells called gametes.
Gametes are haploid cells capable of fusion.
Two gametes join (one from each parent) to form a diploid zygote. The zygote develops into an embryo
which goes on to become the new individual
Sexual reproduction involves the production and transfer of gametes, fertilisation, and development
of an embryo
Remember
2.3.7 Functions of Meiosis in multicellular organisms (the role of meiosis)
Meiosis is important in multicellular organisms for :
(a) keeping the parental chromosome number the same by
forming haploid cells (gametes) in sexual reproduction
Meiosis halves the chromosome number when gametes are
formed. This means that the normal chromosome number is
restored again at fertilisation.
(b) introducing variation in the species by exchange of genetic
material between homologous chromosomes
The daughter cells produced in meiosis are not identical due to
crossing over or the exchange of genetic material that takes place when the haploid cells or gametes are
formed. This results in variations or differences in organisms made as a result of sexual reproduction.
As variations are the basis for evolution, sexual reproduction is more beneficial to a species than
asexual reproduction
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co. Cork
3.6 REPRODUCTION AND GROWTH
Flower structure
The function of the flower is reproduction
Structure and function of the floral parts
Structure
Function
Sepal
Protect the flower when it is a bud
Petal
Large and brightly coloured to attract insects.
(Fragrance and nectar might also be present)
Stamen
(male part)
Carpel
(female part)
Anther
Produces pollen grains (microscopes).
Filament
Supports the anther
Stigma
Traps pollen grains
Style
Holds stigma in position to trap pollen
Ovary
Site of fertilisation and becomes the fruit
Receptacle
Supports the flower
Flowers usually have a number of carpels and stamens.
Monocotyledonous plants (monocots) have flower parts in multiples of 3
e.g. 3, 6 or 9 petals, stamens etc
Dicotyledonous plants (dicots) have flower parts in multiples of 4 or 5
e.g. 4, 8, 12 petals etc or 5, 10, 15 petals etc
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co. Cork
3.6 REPRODUCTION AND GROWTH
Stamens
The stamen is the male part of the flower.
The anther makes the pollen grains (microspores) by
meiosis and releases them when it breaks open.
(Pollen grains causes hay fever in some people)
The Pollen grain produces male gametes
The filament holds the anther so that the pollen grains can be transferred away from the flower. It
contains a vascular bundle to bring food and water up to the anther
Carpals
The carpel is the female part of the flower.
The stigma is where the pollen grain lands
and is stimulated to germinate.
The style connects the stigma to the ovary.
The ovary contains one or more ovules.
Each ovule makes an embryo sac (megaspore)
by meiosis.
The embryo sac produces an egg cell and polar nuclei.
After fertilisation the ovule becomes the seed and the ovary becomes the fruit.
Petals
Petals are large and colourful to attract insects. They may have a fragrance and a nectary containing
sugary nectar to further attract the insects.
In some flowers the petals may be missing or small and green and the pollen is carried away on the
wind
Wind Pollinated Flower e.g. grass
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co. Cork
3.6 REPRODUCTION AND GROWTH
Pollination
Pollination is the transfer of pollen from an anther to the stigma of a flower of the same species
Self- pollination
Self-pollination is the transfer of pollen from an anther of one flower to stigma of a flower on the same
plant
This leads to self fertilisation or inbreeding and weak offspring
Cross-pollination
Cross-pollination is the transfer of pollen from the anther of one flower to the stigma of a flower on a
different plant of the same species
This leads to cross fertilisation and stronger healthier offspring with variation from the parent plants.
Methods of Pollination
Pollen is transferred either by insects and animals or by the wind
Insect Pollinated
Wind Pollinated
The structure of the flower helps pollination and it is adapted to one or other type of pollination
Insect/Animal pollinated flowers
Wind pollinated flowers
Petals
Large & colourful
Small & green
Scent
Scented
No Scent
Nectar
Nectaries to produce nectar
No nectaries or nectar
Pollen
Small amounts, large & sticky
Large amounts, small & smooth
Stigmas
Sticky & inside petals
Feathery & outside petals
Anthers
Short & inside petals
Long & outside petals
Examples Daisies, Dandelions, Buttercups
Ms. B. Fennessy
Grasses, conifers
Loreto Secondary School
Fermoy
Co. Cork
3.6 REPRODUCTION AND GROWTH
Fertilisation
Fertilisation is the joining of a haploid female gamete with a haploid male gamete to form a diploid
zygote which develops into an embryo






The pollen grain lands on the stigma and germinates to form a pollen tube
The pollen tube grows down through the style to the opening of the embryo sac.
The generative nucleus (originally in the pollen grain) divides by mitosis in the tube to form two
male gametes.
One of the gametes fertilises the egg cell or female gamete to form a diploid zygote
The diploid zygote develops into an embryo
A second fertilisation between the other male gamete and the polar nuclei results in the
formation of the endosperm (3n)
Double Fertilisation
On sperm nucleus or gamete (n) joins with the egg cell (n) to form a diploid zygote (2n) which develops
into an embryo
The second sperm nucleus (n) joins with the two polar nuclei (both n) for form the triploid (3n)
endosperm nucleus.
The pollen tube means that the male gametes of flowering plants can move towards the egg without the
need for external water. This is a major adaptation towards life on land
Aside:





When the pollen grain lands on the stigma it is stimulated to grow by sugars made by the stigma
The growth of the pollen tube is controlled by the tube nucleus which dies when the pollen tube
reaches the opening of the embryo sac (micropyle)
The pollen tube grows towards chemicals released by the ovule – chemotropism
The haploid generative nucleus divides by mitosis as it moves down the pollen tube to form two
haploid sperm nuclei or male gametes
When the pollen tube reaches the micropyle the male gametes go into the embryo sac
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co. Cork
3.6 REPRODUCTION AND GROWTH
Seed structure
The entire ovule becomes the seed.
The outer wall of the ovule (integuments) gets thicker and hardens into the seed coat or testa.
The zygote becomes an embryo plant which has a plumule
(shoot), a radicle (root) and one or two cotyledons (seed leaves).
The cotyledon(s) store food.
The epicotyl is the part of the plumule between the cotyledons
and the first true leaf.
The hypocotyl connects the radicle with the cotyledons.
The triploid endosperm nucleus becomes the endosperm. This is a triploid food storage tissue which
nourishes the developing embryo. Sometimes the cotyledons absorb all of the endosperm and hence
non-endospermic seeds are formed e.g. broad bean Other times the cotyledons will absorb only some of
the endosperm and hence endospermic seeds are formed e.g.maize.
Eventually the embryo stops growing and becomes dormant.
So as the ovule develops, a seed is formed which consists of an embryo and a food supply. The food
supply is contained either in an endosperm or in seed leaves (the cotyledons).
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co. Cork
3.6 REPRODUCTION AND GROWTH
Monocotyledon, dicotyledon classification
Plants are classified based upon the number of seed leaves (cotyledons) in the seed.
Monocotyledons (Monocots) contain one seed leaf (cotyledon) and dicotyledons (Dicots) contain two
seed leaves (cotyledons)
The cotyledons are food reserves for the young plant after it germinates in the soil. It uses these food
reserves until it is able to make its own food.
In monocots, the cotyledon rarely stores food; rather it absorbs food molecules from the endosperm
and passes them on to the embryo.
Monocot seeds are endospermic because they still use the endosperm to store their food.
In dicots, the cotyledons usually store the food that the embryo uses.
Dicot seeds are non-endospermic because they no longer have endosperm. It has been absorbed by the
cotyledons e.g. broad bean seeds.
Remember
Feature
Monocotyledons
Dicotyledons
Cotyledons
(seed leaves)
One cotyledon
Two cotyledons
Flower parts
Usually in multiples of 3
Usually in multiples of 4 or 5
Venation
(Veins in leaves)
Usually parallel
Usually reticulate (netted)
Stems
Herbaceous (non woody)
Woody or herbaceous
Vascular bundle
arrangement
Scattered
In a ring
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co. Cork
3.6 REPRODUCTION AND GROWTH
Fruit Formation
After pollination, the ovule develops into a seed and the ovary develops into a fruit.
The developing seeds produce growth regulators (auxins) to stimulate growth of the fruit tissues
A fruit is a mature ovary or sometimes a modified floral part (e.g. the receptacle). The seeds are
protected by a seed coat and may be contained within the fruit.
The function of the fruit is to
 Protect the seeds
 Help with seed dispersal
Once the fruit has formed the rest of the flower
(e.g. the petals) dies and falls away.
©BF
Seedless fruit production
Seedless fruit production is caused by :
 The genetic variety of plants
 Growth regulators.
The genetic variety of plants
Seedless fruits can be produced either naturally or by special breeding programmes
In nature there are some plants where the fruit develops without pollination or fertilisation (i.e.
pathenocarpy). Growing such plants allows the production of seedless fruits like grapes, bananas etc
Seedless fruit can also be produced by carrying out special breeding programmes. This involves
changing the chromosome number of gametes so they cannot produce a functional seed but good fruit is
still produced e.g. water melons
Growth regulators
Developing seeds produce growth regulators (auxins) to stimulate growth of the fruit tissues
Therefore seedless fruit can be produced by spraying flowers with growth regulators or auxins. The
growth regulators stimulate the ovary to swell with food and become a fruit before fertilisation even
occurs e.g. seedless tomatoes, grapes
Genetics and growth regulators also play a role in the production of bigger and larger fruit and
vegetables.
Growth regulators increase the cluster, number and sizes of berries and grapes
Ethene is used to ripen bananas, melons, tomatoes, and to degreen oranges, lemons, grapefruit.
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co. Cork
3.6 REPRODUCTION AND GROWTH
Fruit and seed dispersal
Dispersal is when a plant spreads its seeds away from the parent plant and scatters them over a wide
area
Need for dispersal
Seeds are dispersed to ensure a better chance of survival.
Dispersal helps
 Avoid overcrowding and poor growth
 Minimises competition for light, water, minerals and space
 Allows colonisation of new areas
Examples of wind, water, animal and self-dispersal
Plants have a wide variety of seed dispersal techniques. The fruit helps with seed dispersal
Wind dispersal
Small, light seeds easily blown by wind.
Fruits are often winged e.g. sycamore parachutes or have feathery parachutes e.g. dandelion and they
fly to a new place
Animal dispersal
Fruits attach to animal using hooks e.g. goose grass, burdock and are carried away to a new place.
Animals eat edible fruits. The seeds are not digested but pass through the birds system and land in a
new place e.g. blackberries, strawberries
Water dispersal
Fruits are buoyant and float on water to a new place e.g. coconut
Self dispersal
Seeds are formed in pods. The pods shrivel and burst, scattering the seeds away from the parent plant
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co. Cork
3.6 REPRODUCTION AND GROWTH
Dormancy
Dormancy is a resting period for a seed where it does not germinate or grow even though environmental
conditions are suitable
During dormancy there is very little cell activity or metabolism. The seeds usually do not germinate
until the following spring.
Advantages of dormancy





Allows time for seed dispersal
Allows time for embryo to develop properly
Allows seedling to avoid harsh winter weather
Provides a bank of seeds in the soil for the future
The seeds germinate in the spring which gives them the maximum growing season
Dormancy in agricultural and horticultural practices
Dormancy can be a problem for farmers and gardeners because they often want seeds to germinate at
times that suit them and not leave it as nature intended. Therefore, they often treat seeds before planting
in order to break dormancy, maximise germination and get the seeds to germinate at the same time.
Agricultural and horticultural practices to break dormancy:




Soaking the seeds in water to soften the seed coat (testa) which allows water and oxygen to enter
Exposing the seeds to cold temperatures for some time e.g. putting the seeds into a fridge
Scraping the seed surface with sandpaper to break the tough testa
Spraying the seeds with growth regulator (e.g. giberellin) to encourage germination
Aside:
Causes of dormancy



There might be plant growth inhibitors e.g. abscisic acid in the testa which stop the embryo from
growing. These might need to be washed away by the rain or broken down by the cold before the
seed can germinate
The testa might be too tough to let water and oxygen into the seed and it needs time to break down
Some seeds need to experience a long period of cold before they will germinate e.g. apple seeds
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co. Cork
3.6 REPRODUCTION AND GROWTH
Germination
Germination is the restart of growth by the plant embryo when environmental conditions are suitable
Factors necessary for germination
Factors necessary for germination are:
 Water
 Oxygen
 Suitable temperature.
The effect of water on germination





It helps to swell the seed and break the seed coat (testa).
It allows the dry seed to start metabolism again.
It acts as a medium or a place for the chemical reactions to occur.
It activates the enzymes which break down the food reserves (lipids, carbohydrates and proteins) in
the endosperm or cotyledons.
It is a transport medium for digested products
The effect of oxygen on germination

Oxygen is needed for metabolism. It is used in aerobic respiration – so it helps release energy from
the food stores.
The oxygen is an atmospheric gas found in the soil air spaces. If the seed is buried too deep or if the
soil is waterlogged, the seed will be oxygen starved so germination or growth will not take place
Suitable temperature


A suitable temperature is needed to allow enzyme reactions to take place.
The rate of enzyme action is affected by temperature, therefore temperature affects metabolism and
growth rates in the germinating seed. It affects chemical reactions like respiration, digestion and
protein synthesis.
Seeds often have a temperature range (usually between 5oC and 30oC) within which they will
germinate, and they will not germinate above or below this range.
I’ve soaked too much!
I am fit to burst!
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co. Cork
3.6 REPRODUCTION AND GROWTH
Outline the role of digestion and respiration (in germination)
As a result of taking in water the seed is hydrated. This activates the enzymes and starts metabolism.
The food storage centres (cotyledons and/or endosperm) and the embryo are the main areas of activity.
Role of Digestion
Germination starts with the digestion of the food stored in the cotyledons and/or endosperm
The enzymes digest the food stores to provide the materials for the growth of the embryo. Digestion is
a catabolic reaction and needs water. The carbohydrate starch is converted to glucose, lipids (oils) are
converted to fatty acids and glyercol and proteins are converted to amino acids. These soluble products
are translocated (moved) to the growing regions of the embryo. This means that the dry weight (mass)*
of the cotyledons and/or endosperm falls
Some of the glucose is used to make cell walls. The amino acids are used to make enzymes and protein
and so the embryo grows. This means that the dry weight of the embryo increases.
Respiration
When the food products have been moved from the cotyledons and/or endosperm to the embryo,
respiration takes place.
The lipids (oils) and glucose release energy when they are broken down in respiration (catabolic
reaction). This energy is used by the embryo to carry out the anabolic reactions of making protein and
growing. This means that the overall dry mass of the seed falls due to energy losses in respiration
Once the seedling forms the first true leaves, photosynthesis then becomes the main source of glucose.
Some of this glucose is stored as starch and the embryo continues to grow. This means that the dry
weight of the seedling increases.
* Dry weight is the weight without water. Dry weight is used for biological tissues to eliminate the variable of water.
Digestion of Food
Reserves
The digested food stores
are used to make cell walls
and proteins
Cotyledon dry
weight decreases
Glucose
Starch
Embryo dry weight
increases
Amino
Acids
Protein
Fatty acids & glycerol
Lipids
Ms. B. Fennessy
Loreto Secondary School
Respiration of lipids
& glucose
Overall dry mass
falls due to energy
loss
Fermoy
Co. Cork
3.6 REPRODUCTION AND GROWTH
Stages of seedling growth
A seedling is a young plant that is grown from a seed
There are two types of seedling growth
 Cotyledons stay below the soil e.g. broad bean seed
 Cotyledons move above the soil e.g. sunflower
The Cotyledons stay below the soil







The seed absorbs water and swells and the enzymes become active
The radical emerges and grow down due to gravity
The plumule emerges and the epicotyl begins to grow
The epicotyl pushes the plumule up through the soil. It is curved over for protection
The cotyledons shrivel and wither as food is translocated out of them and sent to the embryo
The radicle becomes a tap root (primary root) with many side roots
Once above the ground, the plumule straightens up and the first true foliage leaves are formed
which start to photosynthesise.
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co. Cork
3.6 REPRODUCTION AND GROWTH
The cotyledons move above the soil







The seed absorbs water and swells and the enzymes become active
The radical emerges and grow down due to gravity
The hypocotyl begins to elongates
The hypocotyl grows upwards carrying the cotyledons with it
The cotyledons come above the ground
Once above the ground, the pericarp falls off, the hypocotyl straightens and the cotyledons turn
green (they look like leaves, store food, help protect the plumule and carry out photosynthesis)
The plumule comes out from between the cotyledons and the first true foliage leaves are formed
which start to photosynthesise
Seed
An embryo (embryonic plant)
A food supply (cotyledon or endosperm)
A covering (seed coat/testa
Ms. B. Fennessy
Loreto Secondary School
Germination
Seedling
Plant
Fermoy
Co. Cork
3.6 REPRODUCTION AND GROWTH
Vegetative propagation: asexual reproduction in plants
Vegetative propagation (vegetative reproduction) is a type of asexual reproduction for plants.
It is a process where new growth occurs without the need for seeds. It involves only one parent and the
new plants are genetically identical to each other and to the parent plant. They are clones which means
that there is no variation. However, the new plants are produced much faster than with sexual
reproduction. They are produced naturally from modified stems, roots, leaves and buds
Examples of vegetative propagation
Modified stem e.g. Strawberry runners
©BF
Runners are stems that run over the surface of the soil from plants like strawberries. When the runner is
far enough from the parent plant, the terminal bud produces a daughter shoot and roots. When the new
plant is established the runner between parent and daughter plant degenerates
Modified root e.g. Dahlia root tubers
A root tuber is a fibrous root that swells
up with stored food. It survives under
ground over the winter when the aerial
shoots die away. In the following
spring, the root-tubers produce new shoots
from buds at the base of the old stem.
The tubers can also break off and the bud
near the top grows into a new separate
plant.
Ms. B. Fennessy
Loreto Secondary School
©BF
Fermoy
Co. Cork
3.6 REPRODUCTION AND GROWTH
Modified leaf e.g. Mother of Thousands plantlets
Small plantlets grow along the edge
of the leaf. The plantlets fall off when
they are fully developed. The roots
grow and take hold in the soil and a
new plant is formed
©BF
Modified bud e.g. Onion bulb
An onion bulb has a reduced stem. Roots
extend from the underside of the stem into the
soil. There are a number of leaves attached to
the stem. These are swollen with food.
It survives underground over the winter when
the aerial shoots die away. In the following
spring, the apical bud produce new shoots
and a new plant is formed.
©BF
Comparison of reproduction by seed and by vegetative propagation.
Advantages
Disadvantages
Reproduction by seed
Reproduction by vegetative
propagation
Variation in offspring (allows
evolution)
Simple reliable process only
dependent on mitosis
Less competition due to seed
dispersal
Rapid growth as offspring attached
to parent
Seeds stay dormant forming a seed
bank in soil
No external agents needed e.g.
insects not needed
Complex process dependent on both
meiosis and mitosis
No variation (evolution is slower)
Depends on external agents for
pollination & dispersal
Overcrowding & competition for
water, space & light
Wasteful of flower parts e.g. pollen,
seeds & fruit
No seeds formed so there is no
seed bank in the soil
Slow growth of young plants e.g.
long time for adult plant
Identical offspring (clones) so
susceptible to same disease
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co. Cork
3.6 REPRODUCTION AND GROWTH
Artificial propagation in flowering plants.
Asexual methods are used in agriculture and horticulture to grow new plants that are genetically
identical to the parent plants and each other. These methods are used to grow plants with desirable
characteristics that do not come true from seed.
Exemplify any four methods used by horticulturalists to artificially propagate plants
1. Cuttings e.g. Germanium
A cutting is taking part of a shoot and getting
it to grow into a new plant.





The shoot is cut at an angle just below a node.
The cut end is dipped in rooting hormone
powder to encourage new roots to develop.
It is planted in moist soil in a pot.
New roots grow from the base of the
cutting .
When the plant is established, it can be
planted out.
(The cutting can be put directly into sand to
prevent waterlogging the new roots. The shoot
can be covered with a plastic bag to
prevent transpiration)
2. Grafting e.g. Apple tree
Grafting is where the shoot system (scion)
of one plant is joined to the root system
(stock) of another.




The main shoot of a crab apple tree is cut
off at an angle. The root system is used as
the stock.
A branch of an eating apple tree is cut at
similar angle (scion)
The cut surfaces of the scion and stock are
lined up and bound together
The process is successful if the
meristematic tissue merges
Ms. B. Fennessy
Loreto Secondary School
©BF
Fermoy
Co. Cork
3.6 REPRODUCTION AND GROWTH
3. Layering e.g. Carnations
Layering involves bending over a long
stem so that it touches the ground




A stem of the parent plant is bent
down and pegged into the soil.
A cut is made at the bend to
encourage roots to grow.
The piece on the ground is
covered in soil
When a root system has developed
the daughter plant can be
separated from the parent plant
©BF
4. Tissue culturing (micro-propagation) e.g. Orchids
Micropropagation is the growth of a large number of plants from very small plant pieces or cells.





A plant is cut into many small pieces
The cells are grown in-vitro on a suitable medium
A clump of similar cells called a callus is formed
The growing conditions are changed so that the callus
grows into a young plant
When the plants are large enough they can be planted
out like normal plants
This section
reappears in
the course
©BF
In this way large numbers of identical plants can be grown. The plants are all genetically identical to
each other and to the parent plant and are clones.
The advantage of micropropagation is that a large number of plants can be produced in a short time
however because there is no variation, when a disease strikes, all the plants are equally susceptible and
they could all die.
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co. Cork