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UNIT15: REPR()DUCTION IN PTANTS
Learning Objectives:g Define asexual reprocluction and sexual reproductiong ldentify, draw and state the sepals, petals, sramens and carpels ofan insect-pollinated,
dicotyledonous fiowerA ldentify arul describe the stamens and stigmas of an insect-pollinatecl, dicotyledonous
flower9 Outline the process of pollination and distinguish self-pollination and cross-pollinationA Compare insect-pollinated andwincl-pollinated.flowersg Describe the growth of the pollen tube and its entty into the ovule after fertilisation
TYPES OF REPRODUCTION
' Reproduction refers to the process by which an organism produces others of its same kind. The purpose of reproduction is to make sure the continuity of a species.. There are two types of reproduction-asexual and sexual.
SEXUAL REPRODUCTION !N FLOWERING PIANTS
Structure of a flower. Flower is the reproductive organ for flowering plants.
stamen(male reproductive system)
filament antherpetal
stigma
style
ovary
ovule
carpel or pistil(female reproductive system)
Asexual Sexual
A process whereby one parentproduces genetically identicaloffspring
lnvolves mitosis:
A cell divides to produce twoidentical daughter cells with thesame number of chromosomes.
As a result, the daughtercells haveidentical hereditary materials asthe parent cell.
A process whereby two sex cells (gametes), eggand sperm, fuse together to form an entirely neworganism
Sexual reproduction results in offspring that aregenetically different from the parents.
lnvolves meiosis process that a cell divides toproduce the daughter cells with haploid (n) ofthe number of chromosomes .
During fertilisation, the combination of twogametes restores the diploid (2n) number ofchromosomes.
For humans, the haploid number (n) is 23 andthe diploid number (2n) is 46.
a
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receptaclesepal
nectary
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Part Description and function
Petal
Usually brightly coloured and sometimes scented to attract insects tobring about pollination for insect-pollinated flowers
Wind-pollinated flowers have dull, unscented petals
Arranged in a circle or cylinder (collective name: Corolla)
Flowers of grasses have small leaf-like structures (bracts), whichenclose the reproductive organs
Stamen
Male reproductive organ of a flower (collective name: Androecium)
Each stamen has a stalk called the filaments, with an anther at theend.
The characteristics of filament and anther:
Filament
A long stem-like structure
Position the anther so that pollen grains can bepollinated by agents
Anther
Meiosis takes place to produce male gametes inpollen
Made up of two or four pollen lobes
Each lobe contains two pollen sacs where thepollen grains are formed
Each pollen grain has two nuclei in its cytoplasm-generative nucleus and pollen tube nucleus
Pollen grains contain male gametes produced by
meiosis (haploid cells which will fuse with haploidfemale ovum during fertilisation)
Anther also has a vascular bundle-Phloem andxylem./ Phloem transports sugar as energy needed for
cell division and amino acids for synthesisingproteins and new protoplasm
,/ Xylem transports water and mineral salts
a
Receptacle
Expanded end of a flower stalk to which all flower structures are
attached
Sometimes after fertilisation, it becomes fleshy and edible. For
example, apple, pear and strawberry
Sepal
a Outside the ring of petals, green in colour and much smaller(col lective name: Calyx)
Protect the flower when it is in the bud
NectaryFound in the insect-pollinated flowers only
Produce sugary nectar which attracts insects
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Carpel
Female reproductive organ (Collective name: Gynaecium)
Each carpel has an ovary, with the style and stigma.
lnside the ovary are one or numerous ovules, which will becomeseeds after fertilisation.
The characteristics of structures in the carpel:
Stigma
Has sticky or feather-like surface for pollen grainsto attach to
Secretes sugary fluid to stimulate the germinationof pollen grains after pollination
Style
Joins the stigma to the ovary
Made up of soft tissues that allows the pollen tubeto grow down towards the ovule
Ovary
Attached to the receptacle of the flower
Contains and protects the ovules
Meiosis takes place to produce egg cells in ovule
Ovule
Attached to the placenta through funicle in theovary
Contains female gametes called ova (singular:ovum) produced by meiosis (also haploid cells)andthe definitive nucleus
o REVTST0N 1
2
1,. Definereproduction.
There are two types of reproduction-asexual and sexual. Describe these two types ofreproduction.
(a) Asexualreproduction
(b) Sexualreproduction
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3. The diagram below shows the structure of a flower.
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(a) Label the parts and one function of each part.
4
5
11
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1
6
2
3
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Label Name Function
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3
4
5
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7
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(b) What is the collective name for the part that makes up of(i) Structures 2 and 3
(ii) Structures 4, 5 and 6
4. Explain the following terms.
(a) Corolla
(b) Androecium (plural:androecia)
(c) Gynoecium (plural: gynoecia)
POLLINATION
. Pollination is the transfer of pollen grains from the anther to the stigma.
' When pollen grains are transferred to the stigma of the same flower or a ditferent ftower of thesame plant, it is called self-pollination.
' When pollen grains are transferred to the stigma of a flower on another plant of the same kind,it is called cross-pollination.
Advantages and disadvantages of types of pollination
Type ofpollination Advantage Disadvantage
Self-pollination
It is advantageous where externalagents, for example, wind orinsects, are not reliable.Less energy is needed to attractand reward pollinators.lf a given trait is well-adapted tothe environment, it helps keep thisfavourable trait in the species.Allows a particular species tocolonise a certain area.
a
It is similar to asexual reproductionbecause it produces offspring thatare genetically identical to theparent. Therefore, there will be a
loss of genetic diversity.Due to constant changingenvironmental conditions, thefavourable trait may not be able toadapt in the future. Complete andcontinuous self-poll ination meanscertain extinction.
Cross-pollination
lncreases genetic variability sothat there is a higher chance forthe species to survive in changingenvi ronmental conditionsOffspring can obtain favourabletraits from both parents to ensureits viability.
a Can destroy well-adapted traits, sooffspring are not guaranteed to beviableRel ies on effective cross-pollinationstrategies to ensure that the pollenis actually transferred to a differentplant of the same species.
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Differences in the features of cross-pollinated flowers
. For plants that carry out cross-pollination, there are differences in theirflowers.
. They can be classified as wind-pollinated flowers and insect pollinated flowers
. The diagrams below show the wind- and insect-pollinated flowers.
featherystigma
non-floweringpetal
stigma
anther style
filamentanther
lodiculesovary
wind-pollinated flower insect-pollinated flower
petal
nectarguide
ovule
ovary
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Feature Wind-pollinated Insect-pollinated Significance
Petals
I nconspicuous,smallflowers
Petals are green
usually
Relatively largeand conspicuousflowers
Petals brightlycoloured
lnsects respond to thestimulus of colour and scentand are attracted to thei nsect-pollinated f lowers.
Scent No scent Scented
Nectaryguide
Absent a Present
I n insect-pollinated f lowers,the nectar guides lead theinsects to the nectaries.When they collect or feedon the nectar, they brushagainst the anthers todislodge pollen grains in theprocess.
Pollen
Large quantities
Small, dry andsmooth pollengrains
Smallerquantities
Larger, stickyand heavypollen withrough surface
ln wind-pollination, onlya very small portion ofpollen grains is likely toland on a ripe stigma. lflarge quantities of pollengrains are not produced,
the chances of successfulpollination are very low.Smooth, light grains areeasily carried by wind and donot stick together.
ln insect-pollination, fewerpollen grains will be wasted.The rough surfaces of thegrains are more likely to stickto the body of the insect.
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Anthers
Large, looselyattached tofilament
Stamens hangoutof the flower andare exposed to thewind
Small, firmlyattached tofilament
lnside the flowerwhere insectsare likely tobrush against
The wind is more likely todislodge pollen grains fromexposed, dangling anthersthan from those enclosed inpetals.
Stigma
Large andfeathery
Projects outsidethe flower
Sticky
lnside the flower
The feathery stigmas act asa 'net' with a large surfacearea to trap pollen grains inthe air.
FERTILISATION
. When a pollen grain from an anther lands on a stigma, pollination takes place.
. After pollination, fertilisation will follow.
. Fertilisation is a process that involves the fusion of the male and female gametes in the ovule insidethe ovary of a flower.
Process of fertilisation
. After pollination, the pollen grain absorbs sugary liquid from the stigma and germinates
mature pollen grainpollen tube
nucleus
generativenucleus
stigma
pollen tube
ovule
definitivenucleus
ovary wall
ovum micropyle
malegametes
style
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A microscopic pollen tube grows out of the grains. lts growth is controlled by the pollen tubenucleus.
The tube secretes enzymes to break down the surrounding tissue of the stigma and style so thatit grows down the style and into the ovary where it enters the micropyle in an ovule.
The pollen tube nucleus eventually disintegrates. However, the generative nucleus divides to
form two male gametes.
The male gametes within the pollen grain enters the ovule when the tip of the tube bursts by
absorbing the sap.
One of the male gametes combines with the nucleus of the ovum.
./ First male gamete * Ovum -+ Zygote
./ Second male gamete + Definitive nucleus -+ Endosperm nucleus
The zygote undergoes cell division to form embryo in the seed.
The endosperm nucleus develops into endosperm which is the food storage tissue.
After fertilisation,./ ovary develops into the fruit./ ovule develops into the seed
2
L. Define pollination.
The diagram below shows the pollination process between the flowers of the same plant and of
different plants of same species.2
B
Plant 1 Plant 2
(a) (D Name the type of pollination A, B and C shown in the diagram
B
tA
A
C
Hence, explain the difference between the types of pollination
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(b) Using your biological knowledge, state two common features that most likely be found in flowersthat carry out pollination through(i) process A
(ii) process C
(c) (i) State three advantages of process A in a plant.
(ii) State two disadvantages of process A in a plant.
(d) (i) State three advantages of process C in plants
(ii) State two disadvantages of process C in plants.
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3. The diagram below shows two types of flowers, A and B
stigma
anther
anther
nectarystigma
Flower A Flower B
(a) (i) Name the agent of pollination for flowers A and B.
(ii) State three reasons for your answer in (a)(i) based on the structures of these two flowersshown in the diagram.
(b) Complete the table below to show the other characteristics of these flowers.
C
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Characteristics Flower A Flower B
Petals
Pollen grains
Odour
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4. The diagram below shows parts of a stamen
(a) ldentify parts P to S
P
R
(b) The diagram below shows the component found in one of the structure Q. X secretes anenzyme to digest part of the tissues of the female reproductive organ of a flower.
(i) Name the components X and Y
(ii) What is the function of Y?
(c) (i) Name the structure that contains structure R.
P
a
R
S
a
S
X
Y
(iD Explain the importance of this structure R
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5. The diagram below shows the fertilisation in a flower,
germination ofpollen grain
ova ry
stigma
style
definitive nucleus
ovum
micropyle
generative nucleus
pollen tube nucleus
pollen tube
ovu le
male gametes
(a) Describe in details how this process takes place in the flower,
(b) What would happen to the ovule and ovary after fertilisation?
Section A: Multiple-Choice Questions
L. Which of the following plant structures contain haploid nuclei?
A pollen grains B root tip cells C sieve tubes D xylem vessels
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