Download Sexual Reproduction in Plants

Sexual Reproduction
in Plants
Contents
Definitions
Flower Structure
Flower parts & functions
Two important facts
Pollen Grain Development
Embryo Sac Development
Pollination
Agents of Pollination
Insect pollinated flowers –
adaptations
Wind pollinated flowers –
adaptations
Hay fever
Fertilisation
Plant Breeding Techniques
Tissue Culture
Seed Types & Structure
Classification of seeds
Fruits
Fruit & Seed Dispersal
Agents of dispersal
Dormancy
Germination
Enzymes found in seeds
Germination of Broad Bean
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Germination of a sunflower
Definitions
sexual reproduction: reproduction involving
the union of sex cells or gametes, which
fuse at fertilisation to form a zygote.
zygote: a diploid cell resulting from the union
of two haploid gametes - a fertilised egg –
will grow by cell division into a new
individual.
Two parents needed – male and female.
New individual will possess traits of both
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parents.
Flower Structure
flower: reproductive shoot of plant,
containing four whorls – i.e. sepals, petals,
stamens and carpels.
Are formed from buds.
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The structure of a typical
flower
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Another typical flower
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Flower parts and functions
Part
Description
(1/3)
Function(s)
Forms the base of the
Receptacle Tip of stem
flower
Leaf-like, ususlly
Protects flower before
Sepals
green
it blooms
Attract pollinators e.g.
Petals
Usually coloured
insects
Male part made up of Produces the male
Stamens
anther and filament
gametes
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Flower parts and functions
Anther
Filament
Carpel
Stigma
(2/3)
Produces pollen
Sac-like structure
grains that produce
the male gamete
Supports the anther
Stalk-like structure and supplies food and
water
Female part made
Produces the female
up of stigma, style
gamete: the egg
and ovary
Place where pollen
Top of the carpel
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lands
Flower parts and functions
Style
Ovary
Ovule
Nectary
Neck of the carpel
Swollen base of
carpel
Bears an embryo
sac
Sugar sac
(3/3)
Connects stigma to
ovary
Contains ovules
Produces the egg cell
and polar nuclei
Provides energy-rich
food for pollinators
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Lilium, anther. T.S. L.P.
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Two important facts

Pollen grain produces male gametes

Embryo sac produces an egg cell and polar
nuclei
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Pollen Grain Development
(1/2)
Pollen grains form in the anther – in pollen
sacs
 Pollen sacs contain microspore mother cells
(2n) – lined by tapetum – nutrition
 Divide by meiosis to produce four haploid
microspores (n)
 These become the pollen grains – enclosed
by a thick wall

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Pollen Grain Development
(2/2)

Each microspore nucleus divides by mitosis
to produce two nuclei – both haploid
 A tube nucleus (n) and a generative nucleus
(n)
 After pollination the generative nucleus
divides again by mitosis to produce two
male gametes – both haploid (n)
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Pollen grain development
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A mature pollen grain
Consists of a tube nucleus and two haploid
male gamete nuclei.
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Lilium, mature pollen grains
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Embryo Sac Development
(1/2)

The carpel contains one or more ovules
 Each ovule contains nutritive tissue –
nucellus
 Nucellus contains embryo sac mother cell =
megaspore mother cell (2n)
 Divides by meiosis to produce four haploid
cells
 Three of these degenerate
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Embryo Sac Development
(2/2)

Remaining nucleus divides three times by
mitosis to produce
 Eight haploid nuclei
 Arrange themselves as in diagram on next
slide
 Note position of egg cell and polar nuclei
 Remaining cells play no part in
reproduction
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Embryo sac development 1
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Embryo sac development 2
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A mature embryo sac
Consists of eight nuclei one of which is the
haploid egg and two others are the polar
nuclei.
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Pollination
pollination: is the transfer of pollen from the
anther of the stamen of one flower to the
stigma of the carpel
of (a) the same flower or another flower on
the same plant (self-pollination)
or (b) another flower on a different plant of
the same species (cross- pollination).
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Agents of Pollination

Insect

Wind

Self
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A bee pollinating a flower
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Insect pollinated flowers –
adaptations
(1/2)

Flowers large, conspicuous
 Brightly coloured
 Small quantity of pollen produced
 Hooked, sticky pollen
 Nectaries produce nectar
 Scented
 Anthers and stamens inside flower
 Large pollen
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Insect pollinated flowers –
adaptations
(2/2)

Heavy pollen
 Small anthers
 Small stigma
 Stigma rounded
 Stigma inside flower
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A wind-pollinated flower –
rye grass
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Wind pollinated flowers –
adaptations
(1/2)

Small, inconspicuous
 Green
 Large quantity of pollen produced
 Smooth pollen
 No nectaries
 Not scented
 Anthers and stamens outside flower
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Wind pollinated flowers –
adaptations
(2/2)

Smaller pollen
 Lighter pollen
 Larger anthers
 Larger stigma
 Stigma feathery
 Stigma outside flower
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Pollen grains, mixed wind
and insect distributed
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Clouds of pollen from hazel
catkins
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Hay fever

A seasonal allergy
 The symptoms include sneezing fits, a
blocked or runny nose, a tickle in the roof
of the mouth and itchy watery eyes.
 It is caused by the pollen of certain windpollinated plants e.g. grass
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Fertilisation
fertilisation: the union of a haploid (n) male
gamete with a haploid (n) female gamete
resulting in the formation of a diploid (2n)
zygote.
How does this occur?
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After pollination
(1/3)

Pollen grain has landed on stigma.
 It germinates and a pollen tube grows down
through the stigma and style to the embryo
sac.
 The two male gamete nuclei are inside the
pollen tube.
 Pollen tube enters micropyle –
 tip of tube bursts open –
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After pollination
(2/3)
tube nucleus disintegrates –
 allows free passage for male gamete nuclei
into the embryo sac.
 FERTILISATION - is the union of one of
the male gamete nuclei (n) with the nucleus
of the egg cell (n) to produce a diploid
zygote (2n).

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After pollination

(3/3)
The second male gamete nucleus (n) fuses
with the two polar nuclei to produce the
triploid endosperm nucleus (3n). This is a
double fertilisation.
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A mature ovule
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What becomes of the
various parts
The ZYGOTE develops into the embryo plant.
The ENDOSPERM forms a food supply for the
developing embryo.
The OVULE becomes the seed.
The OVARY becomes the fruit.
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Plant Breeding Techniques
(1/3)
How can we produce wheat that will grow
quickly, be resistant to pests and diseases,
and produce a large quantity of good quality
grain?
Two methods.
The first method
 Choose two plants that have all the desired
traits between them – grow them.
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Plant Breeding Techniques
(2/3)
Artificially pollinate – remove anthers from
one plant – this prevents self-pollination
 When carpels are ripe dust them with pollen
taken from the other plant
 Cover flower with bag – this prevents crosspollination from other plants.
 Seeds produced will hopefully have the
required traits.

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Plant Breeding Techniques
(3/3)
The second method
 Tissue culturing
 Cells removed from required plant
 Cells grown on sterile nutritive medium
 One of these cells taken and grows into an
embryo on another medium
 Embryo plant grows into new plant
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Tissue culture
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Seed Types & Structure
(1/2)
Plumule (immature shoot)
embryo
All seeds
Seed
Radicle (immature root)
Cotyledon (food supply
or seed leaf)
In some seeds
endosperm
Food store
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Seed Types & Structure
(2/2)

When seeds are mature they lose water and
become hard and dry.
 They can survive adverse conditions – e.g.
cold and drought.
 They can be dispersed away from the parent
plant to reduce competition.
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L.S. of cereal grain
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Classification of seeds
Based on two features
1. The number of cotyledons – if one then is a
monocotyledon e.g. maize, grass, oats; if two
then is a dicotyledon e.g. broad bean,
snapdragon, buttercup.
2. The presence or absence of endosperm (food
store) – if present seed is endospermic (e.g.
usually monocotyledons); if absent nonendospermic (e.g. most dicotyledons). 46
Endospermic seed
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A maize seed - endospermic
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Broad bean seed– nonendospermic
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Fruits

A fruit is a ripened ovary
 Or sometimes a modified part of the flower
(e.g. receptacle) that may contain seeds e.g.
strawberry – red part = receptacle & pips on
surface = fruits.
 Fruit
- protects the seeds
- enables seeds to be dispersed
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Dry fruits
How do seedless fruits form?
1.
2.
3.
4.
May be a genetic variety and occur naturally e.g.
some bananas & cucumbers.
Spray flowers with auxin or gibberellin (growth
regulators) and fruits form without fertilisation
taking place.
Gibberellin may also cause bigger fruit to form.
Ethene is used to ripen fruit and de-green skins
of oranges, lemons and grapefruit.
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Fleshy fruits
Fruit & Seed Dispersal
Necessary to:
 Prevent overcrowding
 Prevent competition for water and light
 Allows colinisation of new areas
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Agents of dispersal
Wind – fruit / seeds light and winged or hairy
e.g. dandelion, sycamore.
Water – buoyant e.g. water lilies.
Animal – (a) eat fruits – seeds indigestible –
pass through gut and deposited elsewhere
(b) seeds have hooks – attach to animal’s
coat e.g. cleavers, burdock.
Self-dispersal – seeds in pods – pods dry out
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– burst open – eject seeds from pod.
Dandelion fruits
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A pea pod and seed
Dormancy
(1/2)
dormancy: a period of rest before growth,
during which the rate of metabolism is
reduced.
Seed will not germinate during this time even
if given ideal conditions, because other
requirements may be necessary before
germination can occur.
These include: 58
Dormancy

(2/2)
Seed coat too hard - must wait for it to be
softened by nature.
 Cold conditions necessary - ensures
springtime germination.
 Desert plants - chemical inhibitors in seed
must be washed out by heavy rain - ensures
water for further growth.
 Embryo not mature - seed not 'ripe' - must
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wait.
Advantages of Dormancy
Allows time for dispersal before germination.
Allows seed to avoid unfavourable growing
conditions of winter.
Maximises growing conditions in springtime.
Provides a long growing season the first year.60
Germination
germination: is the beginning of growth of a
seed into a new plant.
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Factors necessary for
Germination

Water – enters through the micropyle hydrates cell cytoplasm, activates enzymes,
dissolves food reserves, and bursts testa.

Oxygen - for respiration - ATP production.

Temperature - allows enzymes to work.
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Enzymes found in seeds
Enzyme
Substrate
Product
Use of Products
Amylase
Starch
Maltose
Respired for energy
Maltase
Maltose
Glucose
Cellulose cell walls
Amino
Acids
Fatty Acids
and glycerol
For making enzymes
and cell membranes
Proteinase Proteins
Lipase
Lipids
For energy and making
cell membranes
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Germination of Broad Bean
Seeds
(1/2)

Seeds absorb water and swell
 Radicle bursts through testa and grows
downwards into soil
 Hooked plumule emerges and grows
upwards out of soil
 Above the soil the plumule straightens out
and leaves emerge
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A germinating seed
Germination of Broad Bean
Seeds
(2/2)

Radicle develops primary and lateral roots
 Cotyledons remain below ground – act as
food supply for embryo plant – eventually
shrivels up and withers when true leaves
begin to photosynthesise.
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Germination of a broad bean
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Germination of a sunflower
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END
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