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.

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receptaclesepal

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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

6

7

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9

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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

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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.

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Characteristics Flower A Flower B

Petals

Pollen grains

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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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