sexual reproduction of plants

7/28/2019 Sexual Reproduction of Plants

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Sexual Reproduction of the

Flowering Plant


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Review the plant life cycle

1

4

2

pollen is

transferred

3

After fertilization

flower withers

seeds disperse

and germinate

into new plant

seeds develop

in ovary

4


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Structure of the flower


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Carpel

Petal

Anther

Filament

Stamen

Stigma

Style

Ovary

OvuleSepal



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Function of floral parts

Sepal : To protect the

flower (and to prevent it

from drying out

Petals : To attractinsects to the flower for

pollination


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Stamen : To produce

the pollen grains in the

anthers. (Each pollen

grain produces twomale gametes, one of

which can fertilise an

egg cell)


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Function of floral parts -Stamen

Anther

Produces pollen

Filament

Holds the anther in place


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Carpel : To produce

the ovules (Each ovule

contains an egg cell

inside an embryo sac)


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Function of floral parts - Carpel

Stigma

Where pollen landsafter pollination

Style

Pollen travels down this

Ovary

Contains ovules


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Pollination


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Pollination

Transfer of pollen from the anther to the stigma of

a flower of the same species


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Pollination

Self pollination

Transfer of pollen from

an anther to a stigma

of the same plant

Cross pollination

Transfer of pollen from

the anther to the stigma

of a different plant ofthe same species


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Methods of pollination

Animal Pollination

Wind Pollination


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Petals brightly

coloured, scented with

nectaries

Small amounts of stickypollen

Anthers inside petals

Stigmas sticky, inside

petals

Adaptations for animal (insect) pollination


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Petals small, not

coloured brightly

Anthers outside petals

Stigmas large, feathery

and outside petals

Pollen Large numbers,

light, dry and small

Adaptations for wind pollination


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Adaptations for wind pollination


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Fertilisation


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Fertilisation

Fertilisation is the fusion of the male (n) and

female (n) gametes to produce a zygote (2n)

The pollen grain produces the male gametes

Embryo sac produces an egg cell and polar

nuclei


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The pollen grain

produces the male

gametes

Embryo sacproduces polar

nuclei and an egg

cell

Polar nuclei

Embryo sac

Egg cell


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Stigma

Style

Ovary


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Embryo Sac

Polar nuclei

Egg Cell


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Pollen Grain


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Pollen Tube


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Tube Nucleus

Generative Nucleus


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Tube nucleus

disintegrates

Mitotic division

of generativenucleus to form

2 male gametes


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1 Male gamete fuseswith the 2 polar nuclei

to form the triploid

endosperm nucleus1 male gamete

fuses with the egg

nucleus to form the

diploid zygote


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3N endosperm

nucleus

2N Zygote

Double

fertilisation


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Seed formation

Endospermic & Non-Endospermic

Monocots & Dicots


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Seed Formation

The zygote grows

repeatedly by mitosis

to form an embryo

An embryo consists ofa plumule (future

shoot), a radical (future

root) and cotyledons

(food stores needed forgermination)

3N

endosperm

nucleus

2N Zygote


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Seed Formation

The endosperm

nucleus (3N) divides

repeatedly to form the

endosperm in

endospermic seeds.

This endosperm acts as

a food store for the

developing seed

e.g. maize

3N

endosperm

nucleus

2N Zygote


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Seed Formation

In non-endospermic

seeds the endosperm is

used up in the early

stages of seed

development so the

food is stored in the

cotyledons

e.g. bean

3N

endosperm

nucleus

2N Zygote


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Endosperm

Food storefor

developing

embryo

Embryo

Plumule,radicle,

cotyledons

Integuments, becomes the seed coat

Seed Formation


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If all the

endosperm is

absorbed by

the developing

embryo theseed is a non

endospermic

seed e.g. broad

bean

Seed Formation


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If all the

endosperm isnot absorbed

by the

developing

embryo theseed is an

endospermic

seed e.g.

Maize

Seed Formation


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Seed types and structure

Seed

embryo

Plumule (immature shoot)

Radicle (immature root)

Cotyledon (food supply

or seed leaf)

endosperm Food store

All seeds

In some seeds


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Endosperm

Seed coat

(testa)

Cotyledon

Plumule will

develop into a

new shoot

Radicle will develop

into a new root

Endospermic Seed e.g. Maize


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Seed coat

(testa)

Cotyledon

Plumule

Radicle

Non-Endospermic seed e.g. Broad Bean


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e.g. Broad Bean e.g. Maize

Plumule

Radicle

Cotyledon

Endosperm

Nonendospermic and Endospermic seed


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Classification of seeds

Classified according to two features:1. Number of cotyledons (Seed leaves)

Monocotyledon one cotyledon

E.g. Maize

Dicotyledon - Two cotyledons

E.g. Broad bean

2. Presence of endosperm Present Endospermic e.g. maize

Absent Non-endospermic e.g. broad bean


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Broad BeanNon-Endospermic Dicot

Testa2 Cotyledons


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Fruit

Fruit formation

Seedless fruitsFruit and seed dispersal


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Fruit Formation

The ovule becomes the seed

The ovary becomes the fruit


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Fruit Formation

A fruit isa mature

ovary that may contain

seeds

The process of fruit

formation is stimulated

by growth regulators

produced by the seeds


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Seedless Fruits

Can be formed in two ways

1. Genetically

Either naturally or by

special breedingprogrammes

e.g. seedless oranges


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Seedless Fruits

2. Growth regulatorse.g. auxins

If large amounts ofgrowth regulatorsare sprayed onflowers fruits mayform withoutfertilisation

e.g. seedlessgrapes


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Fruit and seed dispersal

Need for dispersal

Minimises competition

for light, water etc.

Avoids overcrowding Colonises new areas

Increases chances of

survival


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1.

Wind2. Water

3. Animal

4. Self

Types of dispersal


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Methods of dispersal

1. Wind

Sycamore and ash

produce fruit with wings

Dandelions and thistlesproduce fruit with

parachute devices

Both help the dispersethe seeds more widely

using wind

h d f d l


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2. Water

Light, air filled fruits

that float away on

water E.g. coconuts, water

lilies

h d f di l


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3. Animal

Edible fruit

Animals attracted

to bright colours,smells and food

Seed passesthrough digestive

system unharmed E.g. strawberries,

blackberries, nuts

M h d f di l


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3. Animal

Sticky fruit

Fruits with hooks

that can cling to thehair of an animaland be carriedaway

E.g. burdock,goose grass

M h d f di l


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4. Self

Some fruits explode open when they dry out

and flick the seed away

E.g. peas and beans


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Dormancy and germination

D (d fi i i )


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Dormancy (definition)

A resting period when seeds undergo no

growth and have reduced cell activity or

metabolism

D ( d )


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Dormancy (advantages)

Plant avoids harsh

winter conditions

Gives the embryo

time to develop Provides time for

dispersal

A li i i i l d h i l


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Application in agriculture and horticulture

Some seeds need a period of cold before

they germinate

It may be necessary to break dormancy in

some seeds before they are planted foragricultural or horticultural purposes

This can be done by placing them in the

fridge before they are planted

G i i


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Germination

The re-growth of the

embryo after a period

of dormancy, if the

environmentalconditions are

suitable

G i i F


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GerminationFactors necessary

Water

Oxygen

Suitable temperature

Dormancy must be complete

G i i F


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Water

Activates the

enzymes

Medium forgermination

reactions e.g.

digestion

Transport medium

for digested

products

G i ti F t


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Oxygen

Needed for aerobic

respiration

Suitable temperature

Allows maximum

enzyme activity

E t i G i ti


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Events in Germination

Digestion

Of stored food in endosperm and cotyledon

Respiration

To produce ATP to drive cell division

Events in germination cease when the plantsleaves have developed and the plant has

started to photosynthesise

E t i G i ti (d t il)


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Events in Germination (detail)

Water is absorbed Food reserves are digested

Digested food is moved to the embryo

New cells are produced using amino acids Glucose is turned into ATP to drive cell division

Radicle breaks through the testa

Plumule emerges above ground

New leaves begin to photosynthesise

E t i G r i ti


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Plumule

Radicle

Cotyledon

E nts in G rmin ti n


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Plumule

Radicle

Changes in dry weight of seeds during germination


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Mass drops initially due to respiration of stored food, but then

begins to increase due to photosynthesis

Drymassofseed(g)

Time (days)




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Food reserves in endosperm are transferred to the growing embryo

Drymassofseed(g)

Time (days)

Embryo

Endosperm



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Germination of broad bean (hypogeal)


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Germination of broad bean (hypogeal)

Germination of broad bean


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Seed water is absorbed

through the micropyle

Ground




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The testa splits

Radicle emerges




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Radicle continues to grow

Plumule emerges




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The plumule is

hooked to protect

the leaves at the tip

Epicotyl




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The plumule

grows above

the surface of

the soil

Lateral rootsdevelop




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Plumule

straightens and

the leaves open

out

Throughout Hypogeal

germination the

cotyledons remain

below the ground


Germination of sunflower (Epigael)


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Seed

water isabsorbed through the

micropyle

Germination of sunflower (Epigael)

Germination of sunflower


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Radicle

emerges




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Hypocotyl Hook


Seed coat

discarded Germination of sunflower


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Radicle grows

downwards

discarded

Cotyledons




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Leaves emerge

Cotyledons wither


In Epigeal germinationthe cotyledons rise

above the ground

Learning Check


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Learning Check

Outline the main stages of sexualreproduction in plants

Review the plant life cycle


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1

4

2

pollen is

transferred

3

After fertilization

flower withers

seeds disperse

and germinate

into new plant

seeds develop

in ovary

4


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Asexual Reproductionin Plants

Vegetative Propagation

Definition


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Definition

Asexual reproduction does not involve the manufacture or union of sex

cells or gametes e.g. binary fission,

fragmentation, spore formation and budding It involves only one parent and offspring are

genetically identical (have the same genetic

content) to the parent



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A form of asexual reproduction in plants

Does not involve gametes, flowers, seeds or

fruits

Offspring are produced by a single plant(genetically identical to parent)

Can happen naturally or it can be done

artificially


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Natural

e.g. runners, tubers, plantlets, bulbs

What happens?


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What happens?

Part of the plant becomes separated from theparent plant and divides by mitosis to grow

into a new plant

As a result the offspring are geneticallyidentical to the parent


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Parts of the parent plant may be speciallymodified for this purpose:

1. Stem

2. Root

3. Leaf

4. Bud

1 Modified Stems


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1. Modified Stems

Runners

horizontal, running over

the soil surface

terminal bud of therunner sends up new

shoots

e.g. strawberry,

creeping buttercup.


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Creeping buttercup


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Modified Stem (continued)


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( )

Stem Tubers

swollen underground

stem tips

buds (eyes) produce

new shoots

e.g. potato


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2 Modified Roots


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2. Modified Roots

Root Tuber

swollen fibrous roots

the tuber stores food,

but the new plant

develops from a side

bud at the base of the

old stem

e.g. dahlia, lessercelandine

Note:


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Note:

Tap Roots e.g. carrotand turnip, are swollen

roots for food storage in

biennial plants they

are not reproductiveorgans


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4. Modified Buds


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Bulbs

A bulb contains an

underground stem,

reduced in size Leaves are swollen

with stored food

e.g. onion, daffodil,tulip

4. Modified Buds


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Bulbs

The main bud

(apical bud) will

grow into a newshoot)

The side buds

(lateral buds) will

also grow into new

shoots


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Comparison of reproduction byseed (sexual) and by vegetative

propagation (asexual)

Advantage to seed formation


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Sexual (seed) Asexual (vegetative)

Cross pollination ensures

variation (allows evolution)

No variations can be

advantage in commercial

horticulture

More resistant to disease All plants are of same speciessusceptible to disease

Dispersal reduces competition Overcrowding and competition

Seeds can remain dormant and

survive unfavourable conditions

No seeds formed no

dormancy

Advantage to vegetative propagation


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Sexual (seed) Asexual (vegetative)

Complex process Simple process

Depends on outside

agents for seed dispersal

No outside agents

needed

Slow growth of young

plants to maturity

Rapid growth

Wasteful e.g. petals,pollen, fruit

No waste


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Vegetative propagation

Artificialused by gardeners to propagate plants

e.g. cuttings, layering, grafting and budding

Cuttings


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Cutt gs

Parts of a plant (usuallyshoots) removed from

plant allowed to form

new roots and leaves

rooted in water, well-watered compost, or

rooting powder

e.g. busy lizzie,

geranium


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Grafting


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Grafting

Part of one plant (scion)is removed and

attached to a healthy,

rooted part of a second

plant (stock) Useful qualities from

both plants combined

into one e.g. rose

flower and thorn-lessstem

e.g. apple trees


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Layering


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A branch of a plant is bent over and pinned tothe earth at a node

When roots develop the branch is separated

from the parent plant. Useful for the propagation of woody plants

e.g. blackberry, gooseberry.


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Micropropagation (Tissue Culture) (1/3)


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Cells removed fromplant and grown as a

tissue culture in a

special medium

Growth regulators andnutrients added so that

growing cells form a

group of similar cells

called a callus



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Different growth regulators are then added so thatthis tissue develops into a plantlet

Plantlet can be divided up again to produce many

identical plants

Entire plant can be grown from a small piece ofstem, leaf or root tissue

Used in mass production of house plants and crops

such as bananas and strawberries



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Provides a largernumber of plants more

quickly than cuttings.

Can be used to check

cells for a particularfeature e.g. resistance

to chemicals or a

particular disease


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Cloning


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All offspring genetically identical - produced

asexually

Clones are produced by mitosis All the offspring from the various methods of

vegetative reproduction (both natural and

artificial) mentioned are examples of clones


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END

sexual reproduction of plants

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