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Biology – Form 3 Page 32 Ms. R. Buttigieg

3 Reproduction – Increasing in Numbers

See GCSE Biology Chapters:

� 8 – Reproduction in flowering plants pg. 67 - 78

3.1 Sexual & Asexual reproduction Reproduction is the production of a new organism or organisms by an existing member or members of members of the same species. Reproduction is essential as it ensures that the species does not become extinct, since no living organism lives forever. � Reproduction takes place in two ways, asexual and sexual.

� Some organisms may use both methods while others use only one.

� In some organisms, sexual reproduction is used in difficult times, e.g. drought (no rain), absence

of food, while asexual reproduction is used when conditions are good for growth.

Asexual reproduction (means without sex) gives rise to individuals,

which are genetically identical to each other and to their parent.

The genetically identical individuals produced are called clones.

In asexual reproduction

• Only one parent gives rise to new individuals.

• The offspring are identical to the parent.

Sexual Reproduction - This involves not one, but two, parent organisms. The

parents produce special cells for reproduction, which are called gametes.

• Most species produce two types of gametes – male and female.

• For a new organism to develop, the male and female gametes must meet so

that their nuclei can join together (fertilization) and form a cell which has a

complete nucleus called a zygote.

• Each offspring inherits half the parents’ genes (found in chromosomes)

Both types of reproduction are widespread and have their own particular advantages and disadvantages.

( From www.bioclix.org )


3.2 Main methods of Asexual reproduction

a. Binary fission – Occurs in single celled organisms like Amoeba. The organism reproduces by splitting into two as shown in the diagram. This also happens in bacteria.

b. Budding – In this method a small outgrowth arises from the organism and gets larger till it breaks off. This budded part eventually grows into a new organism. Yeast is a fungus consisting of single cells that reproduces by budding. Hydra shown in the diagram also reproduces by budding. (see GCSE Biology pg. 303)

c. Spore formation – This occurs in mosses, ferns, and fungi such as pin mould or Mucor (shown in the diagram). In mosses the capsule is formed at the tip of the stalk. In ferns the capsules are found underneath the leaves. In pin mould a network of hyphae gives rise to sporangia full of spores. When they mature they break open releasing the spores. If the spores find a suitable material on which to grow they

give rise to a new fungus. d. Vegetative Propagation - In summer plants form special underground organs filled with food. These remain dormant in winter and the following summer give rise to new plants. This enables plants to grow from year to year. These are called perennating organs e.g. bulbs (onion), tubers (potato) and tap roots (carrot). A single potato produces many tubers and each can give rise to a new plant. So perennating organs

serve both as a way of surviving the winter and a means of reproduction (vegetative reproduction)

Biology – Form 3 4 Ms. R. Buttigieg

Other methods of vegetative reproduction include: Runners A side branch from the main stem

grows and creeps along the surface of the ground. At intervals it sends down roots and eventually these connections wither away resulting in new plants E.g. strawberries.

E.g. spider plants

Cuttings A healthy branch is cut and planted.

Roots develop and it gives rise to a new plant.

Grafting A twig is cut from the tree you want to

propagate. This is grafted (placed) into the cut stem of another tree (stock). In this way you place the cut stems of the 2 plants in contact with each other and the tissues join. The stock is chosen for it good roots and its resistance to disease. The twig or branch is chosen for its good flowers or fruits.

Advantages and Disadvantages of Vegetation Propagation

Advantages Disadvantages � Does not depend on pollination or seed

dispersal so always gives rise to new plants

� There is no need for a dormant period � A new plant exactly like the parent

develops

� The quality of the plants can never be improved as there is no genetic variation.

� Disease and weaknesses are passed on to the offspring

� In runners etc. plants tend to get overcrowded as they remain close.

Biology – Form 3 Page 35 Ms. R. Buttigieg (adapted from notes by Michelle Grima)

Comparing Sexual and Asexual Reproduction Sexual Reproduction Asexual Reproduction Takes longer Very fast Time and energy is wasted in finding a mate No mate is needed Smaller number of offspring which are genetically diverse

Large number of offspring genetically identical to the parent (harmful genes can be passed on)

Promotes adaptation to changing environments Clones are created so no adaptation is possible Can be important when a useful characteristic needs to be passed on.

Answer the following on your A4 notebook:

1. Give the meaning of the word reproduction.

2. Distinguish between sexual and asexual reproduction

3. Name 3 methods of asexual reproduction and for each one name an organism that carries it

out.

4. The diagram below shows a particular type of plant reproduction.

(i) Is this type of reproduction sexual or asexual? ________________. (1)

(ii) What name is given to the population of offspring produced as shown in the diagram above?

________________. (1)

(iii) Give one commercial advantage or disadvantage of this type of reproduction.

_____________________________________________________________ (1)


3.3 Sexual Reproduction in flowering Plants The structure of an insect-pollinated flower

(see GCSE Biology pg. 67-9)

� The flower is the sex organ of flowering plants. � Its job is to produce the male and female gametes and bring them together so that fertilisation

can take place. � This results in the production of seeds from which the young plants will grow. � This is a form of sexual reproduction.

The main flower parts

• Carpel: female part of the flower. o Stigma: collects the pollen from the pollinating agent, insect or wind, and

chemically stimulates pollen germination. o Style: positions the stigma for effective pollen collection. o Ovary: site of fertilisation, protects the developing seeds, aids in seed dispersal.

• Stamen: male part of flower. o Anther : pollen formation and release. o Filament: positions the anther for effective pickup of pollen by the pollinating agent.

• Petal: Attracts insect pollinators by colour, food reward, and fragrance. • Sepal: Protects the flower bud and support the petals of the open flower. • The receptacle is the swollen end of the stem from which the modified leaves of the flower

arise.


Basic parts of a flower

Stigma The tip of the carpel. Male sex cells are deposited here

Style Thin stalk connecting the stigma to the ovary.

Ovary Swollen bottom part of the carpel. Produces and contains ovules

Ovule Unripe seed containing the unfertilised female egg cell.

Anther Male sex cells called pollen grains are made here

Filament Stalk which supports the anther

Receptacle End part of the stem to which the other parts of the flower are attached

Sepals Usually green and leaf like. They cover the petals to protect them when the

flower is closed

Petals Usually white or coloured. At the base is a nectary which makes a sugary

liquid called nectar on which the insects feed

Differences between insect-pollinated and wind-pollinated flowers. Adaptations of Insect Pollinated Flowers

• Attract Pollinators (like insects, birds): brightly coloured petals, petal shape, food reward

– pollen and/or nectar, fragrance – volatile chemicals released into the air.

• Pollen Collection by Insect: sticky pollen – stays in contact with anther until insect arrives.

• Pollen Capture By Flower: sticky stigmas so pollen from insect will pass to them.

Adaptations of Wind Pollinated Flowers

• Pollen Collection by Wind: smooth – easily removed from

anther, small pollen easily transported by the wind.

• Pollen Capture by Flower: stigmas large and feathery –

greater surface area to capture pollen, stigmas outside the

flower

• A lof of Pollen Production: makes up for the excessive

losses.

• No petals, hard bracts protect the anther and stigma

• No scent and No nectar

• E.g. grasses


INSECT POLLINATED WIND POLLINATED

large, brightly coloured petals - to attract insects small petals, often brown or dull green - no need to attract insects

often sweetly scented - to attract insects no scent - no need to attract insects

usually contain nectar - to attract insects no nectar - no need to attract insects

moderate quantity of pollen - less wastage than with wind pollination

pollen produced in great quantities - because most does not reach another flower

pollen often sticky or spiky - to stick to insects pollen very light and smooth - so it can be blown in the wind

anthers firm and inside flower - to brush against insects anthers loosely attached and dangle out - to release pollen into the wind

stigma inside the flower - so that the insect brushes against it

stigma hangs outside the flower - to catch the drifting pollen

stigma has sticky coating - pollen sticks to it stigma feathery or net like - to catch the drifting pollen

o Advantages of insect pollination: � The insects’ way of life makes it very likely that the pollen will reach a suitable plant.

� Less pollen needs to be made.

o Disadvantages of insect pollination: � Petals have to be grown which uses energy and resources. � Nectar also has to be made which uses energy and resources.

o Advantages of wind pollination: � There is no need to make petals or nectar. � This saves a lot of energy.

o Disadvantages of wind pollination: � It is less likely that the pollen will meet a suitable stigma.

� Pollen has to be made in huge numbers

Answer the following questions on your A4 notebook

1. In what ways do flowers attract insects?

2. Why does the insect reach down into the first flower?

3. How does the insect carry pollen to the second flower?

4. Where does the insect leave the pollen on the second flower?

5. Why do anthers hang outside the flower in wind-pollinated flowers?

6. Why are stigmas so large and feathery?

7. Why is so much pollen made?

8. Why is the pollen so light?

9. In a table give five (5) differences between wind and insect pollinated flowers.


Pollination and Fertilization Pollen Structure

• haploid – half the number of chromosomes • single cell • double wall — made of outer layer called exine and inner layer called intine • binucleate — tube nucleus (n) and generative nucleus (n)

Pollination is the transfer of ‘male gametes’ in pollen from the anther to the stigma.

• Cross-pollination: pollen is transferred to the stigma of another plant. Increases genetic variation, population more resistant to environmental change.

• Self-pollination: pollen transferred to the stigma of the same flower or a flower of the same plant. Guarantees reproduction if pollinating agent is absent or not efficient.

Here's how it works:

1. An insect or the wind carries pollen grains from the anther of another flower. 2. The pollen grains land on the stigma and a pollen tube grows down through the style to the

ovary. As it grows it gets food from the tissues in the style. 3. The nucleus of the pollen grain passes down the tube and enters from the micropyle. It

FERTILISES the egg cell inside the ovule. 4. The fertilised egg cell develops into an embryo. The ovary becomes the fruit and the ovule

becomes a seed - from which (once dispersed) the offspring plant will grow.


Seed Formation

After fertilization the petals fall off. They have done their job. The fertilized egg grows into the embryo. The embryo will eventually grow into the new plant.

Each ovule in the ovary grows to form a seed. Each seed is made up of:

� the embryo � a food store (one or two cotyledons) � a seed coat or testa

Fruit - The ovary becomes a fruit. The fruit protects the developing seeds and plays an important role in seed dispersal. Seed Dispersal - the scattering of offspring away from each other and from the parent plant. Advantages of Dispersal

• Improved chance of success by reducing competition and overcrowding. • Enables colonisation of new suitable habitats — increased chance of species survival.

Adaptations of Seeds as Dispersal Agents

• Can survive a long period. • Large food reserve — improved chance of successful growth on germination. • Early growth accomplished in parent plant before dispersal also improves the chance of

successful seedling establishment on germination.

Seed with thin testa

Stone Flesh fruit skin

Shoes and socks after walking through a field of cockleburs. These are known as hitchhikers


Methods of Seed Dispersal Method Characteristics Example of plant Wind � light weight seeds

� high air resistance � a parachute of fine hairs so they

can be blown by the wind � some seeds like the one below

have wings

� Orchid � ‘parachute’ of dandelion, ‘wings’

of sycamore (see left photo)

Water � floating (buoyant) fruit � floating seed

� sedge � water lily

Animal � passive – have hooks so stick to animal (sometimes called hitchhikers)

� active - the animal seeks the fruit as a food source

� bright colours to attract animals

� burdock

� tomato

Mechanical � the drying pod ‘flicks’ out the seeds.

This is also known as self dispersal

� Pea

Germination and the conditions controlling germination.

Seed Germination - the restart of growth by the plant embryo using the food stored in the seed.

The seed of a dicotyledonous plant has three main parts:

� the seed coat - a tough protective outer covering;

� the embryo - consisting of the young root (radicle) and shoot

(plumule) which will develop into the adult plant;

� the food store - a store of food for the young plant to use

until it is large enough to make its own food.


Conditions for germination Germination is the start of growth in the seed. Three factors are required for successful germination;

1. water - allows the seed to swell up and the embryo to start growing 2. oxygen - so that energy can be released for germination 3. a suitable temperature - germination improves as temperature rises (up to a maximum)

Water - Plant cells are 90% water.

• Needed so the seed can swell up and burst (softens the testa). • Needed to activate enzymes in the seed • Convert starch into sugars • Transport sugar to growing regions • Expand and maintain the shape of the growing root and shoot

Oxygen

• Oxygen is needed for respiration to provide energy for the growing embryo

Suitable Temperature (Warm) – Many seeds don’t grow till spring or summer.

• Required for optimum enzyme activity and so for optimum growth. • Some of the enzymes are involved in the digestion of the complex food reserve so they need

an optimum temperature.

(See the experiments on pages 76-78 on GCSE Biology Chapter 8 – very important) There are 2 main types of germination –hypogeal and epigeal Hypogeal germination of the Broad Bean – The epicotyl (the part of the plumule above the cotyledons) starts to elongate, leaving the cotyledons in the soil. Common in peas and beans.


Epigeal germination – In this type, the part of the embryo’s stem, just above the radicle, the

hypocotyls, starts to elongate and pulls the plumule and cotyledons up with it. Once above the soil

the hypocotyls straightens, the leaves of the plumule open up and the cotyledons will fall off. The

French bean shows this type of germination.

Stages of Seedling Growth

• Conditions for germination are suitable and dormancy is over.

• Water and oxygen enter the seed.

• Metabolism speeds up.

• Enzymes digest the complex food reserve (starch, protein, lipid).

• The digested food is transported to the growing areas of the embryo.

• The food is used for growth.

• Most of the food is used in aerobic respiration resulting in loss of dry weight.

• The radicle grows out forming a rooting system.

• The plumule emerges forming a shoot system.

• Photosynthesis begins.

• Dry weight increases and seedling is independent when photosynthesis is greater than

respiration.


Answer the following questions 1. Look at the picture of a grass flower

(a) Say whether this flower is insect or wind pollinated.

(b) Name a structure which is absent or not labeled on this flower which would be significant in

flowers which use the other method of fertilization.

(c) Choose one feature of the stamen, and one feature of the stigma, which are shown on the

diagram and explain how each feature helps with the plant's method of pollination.

(d) Name a plant which reproduces asexually by means of runners and give a brief description of

how this is achieved.

2. Describe in detail with the help of diagrams, the progress of fertilization in flowering plants 3. Define the following: a. fruit b. seed c. micropyle d. germination 4. Give three conditions needed for germination. 5 a. Draw a large labelled diagram to show the structure of a named seed. (JL 2000)

b. Describe an experiment to illustrate that oxygen is needed for germination to occur. 6 a. Draw and label a large diagram to show the structure of a named insect pollinated

flower you have studied.

b. State the functions of 2 parts of the flower that you have labelled.

c. Suggest two (2) ways by which an insect-pollinated flower might differ from a wind-

pollinated flower. (JL2001)

7. Pods are classified as ‘fruit’.

(i) What is a fruit?

(ii) What is the main function of fruit?

(iii) Name a plant you have studied that produces pods.


(JL 2002)

8.

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