12 3 reproduction in flowering plants
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12.3 Reproduction in Flowering Plants Vegetative structures : stems, roots and leaves Reproductive structures : flowers Vegetative Propagation : reproduction of vegetative structures Sexual reproduction : introduce variation through meiosis by

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12.3 Reproduction in Flowering Plants

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12.3 Reproduction in Flowering Plants


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  • Vegetative structures:

    stems, roots and leaves

  • Reproductive structures:

    flowers

  • Vegetative Propagation:

    reproduction of vegetative structures


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  • Sexual reproduction:

    introduce variation through meiosis by

  • 1. Independent assortment chromosomes

    during metaphase

  • 2. Recombination of genes by

    crossing over between homologous

    chromosomes at prophase I


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12.3.1 Floral Structure

  • Both self-fertilization and cross-fertilization

    introduce variation.

  • External agents, e.g. insects , help in transfer

    of genetic material when they assist in pollination

    of flowers.

    This, however, exposes the vulnerable gametes to

    desiccation. Spermatophytes thus have evolved

    their male gamete within a spore, the microspore

    or pollen grain.


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Structures of a typical flower:

Angiosperms reproduce sexually by producing flowers


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stomium

liberated pollen grain


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  • The floweris the organ of sexual

    reproduction in flowering plants

  • Flowers are usually hermaphrodite or

  • bisexual but sometimes unisexual

sepals: outermost whorl; form the calyx; green in colour; Function -

1 Protect the flower bud

2 Carry out photosynthesis


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  • Petals: form the corolla; often brightly coloured & attractive;

    with insect guides which guide insects to the base of the petals for nectar secreted by

    the nectary;

  • In some flowers, the calyx & corolla are undistinguishable and are collectively called theperianth


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  • Stamens: male reproductive parts or

  • androecium;

  • Each stamen consists of an antherand a long filament;

  • Each anther contains four pollen sacs with pollen grains inside;

  • The pollen grains contain the male gametes


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stomium

liberated pollen grain


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Carpels: female reproductive parts or

gynoecium (or pistil);

Each carpel consists of an ovary which

contains ovules withova inside;

At the top of the ovary is the style, with a

stigma at the tip to receive pollen grains for

fertilization


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  • Actinophmorphic (regular): flowers with

    petals & sepals of similar size and shape

    exhibiting radial symmetry

  • Zygomorphic (irregular): flowers with

    unequal sepals & petals of different shapes

    and arranged in bilateral symmetry



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  • 12.3.2 Pollination

  • - meiosis occurs inside the pollen sacs of

    anther;

  • When mature, pollen sacs split open

    expose their pollen grains

  • - female gametes (egg nuclei) are inside ovules

  • funicle: ovule stalk

  • integuments: protective layer

  • micropyle: a small opening through

    which pollen tube grows into ovule


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  • Development of the pollen grain(for reference):

  • pollen mother cells (2n)

  • meiosis

  •  tetrad (4) haploid (n) cells

  •  microspores (pollen grains)

  • mitosis

  •  generative nucleus & tube nucleus

  • mitosis

  • 2 male nuclei & tube nucleus


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  • Development of the ovule (for reference):

  • nucellus

  •  megaspore mother cell (2n)

  • meiosis

  •  4 megaspores (n)

  •  3 degenerated, 1

  •  embryo sac mitosis 3 times

  •  8 nuclei (3 antipodal cells,

  • 2 polar nuclei, 2 synergids with 1 egg cell)


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  • Pollination: the transfer of pollen grains from anthers to the stigmas;

  • external agents are needed –

  • (1) by insect

  • (2) by wind


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12.4.1 Fertilization &

Development in Flowering Plants

  • pollen grains send out pollen tubes which grows

    down the style & ovary, towards the micropyle

  • pollen grains are attracted by sugars in stigma

    and secrete enzymes to digest a pathway through

    the style


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Double Fertilization:

  • male gamete goes into the ovule and fertilizes with the egg cell

  • the other male gamete fuses with the polar nuclei to form the triploid endosperm



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12.4.2 Methods of Preventing Inbreeding

Self-pollination: the transfer of pollen from the

anther to the stigma of the same flower,

or of another flower of the same plant

Cross-pollination: the transfer of pollen to a flower

on a different plant of the same species

*If pollen lands on the stigma of a plant of a different species, it usually dies.

  • Since cross-pollination results in a great

    variability of more adaptable offspring,

    many plants prefer cross-pollination






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12.4.3 Development of Fruits and Seeds

Fate of floral parts after fertilization


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12.4.3 Development of Fruits and Seeds

Fate of floral parts after fertilization


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12.4.3 Development of Fruits and Seeds

Fate of floral parts after fertilization


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12.4.3 Development of Fruits and Seeds

Fate of floral parts after fertilization


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12.4.3 Development of Fruits and Seeds

Fate of floral parts after fertilization


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12.4.3 Development of Fruits and Seeds

Fate of floral parts after fertilization


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12.4.3 Development of Fruits and Seeds

Fate of floral parts after fertilization


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12.4.3 Development of Fruits and Seeds

Fate of floral parts after fertilization


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12.4.3 Development of Fruits and Seeds

Fate of floral parts after fertilization


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12.4.3 Development of Fruits and Seeds

Fate of floral parts after fertilization


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12.4.3 Development of Fruits and Seeds

Fate of floral parts after fertilization


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12.4.3 Development of Fruits and Seeds

Fate of floral parts after fertilization


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  • The most common food stores in seeds is

    carbohydrates (in the form of starch).

  • Many young seeds store sugar but this

    changes to starch as they mature.

  • Lipid are often stored in the cotyledons and

    may form a high percentage of the dry

    weight,

    e.g. peanuts. Proteins are found to a lesser

    extent in seeds but wheat has an aleurone layer

    and protein is stored in the cotyledons of

    legumes and nuts.


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  • A fruit is an ovary after fertilization and it contains seeds

  • Functions:

  • Fruits:protect & disperse the embryo

  • Seeds:protect the embryo

  • Provide food for the embryo

  • Help in dispersal of the plant


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Fruits and Seeds Dispersal

  • Dispersal of seeds by fruits –

  • (1) by wind,

  • (2) by animals,

  • (3) by water,

  • (4) by mechanical force,

  • (5) Censor mechanism, and

  • (6) Casual mechanism.

Significance of fruits & seeds dispersal

  • It prevents overcrowding & competition for

    limited resources.

  • It allows the plants to have their seeds to

    land on suitable places for germination, thus

    increases the chance for the plants to

    colonize new areas.


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Acer劍葉槭

Clematis甘草藤


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With hooks: Burdock牛旁

By animal: juice fruits

With hooks stick onto our clothes

With hooks: Bur-marigold鬼針草


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1. Wind Dispersal

- fruits are light and have a large surface area to catch wind

  • examples: sycamore (pericarp extended to form a wing), dandelion (hairy parachute)

    2. Animal Dispersal

  • fruits possess hooks or spines to cling to animals

    bodies

  • fruits as food for animals: 'stoned seeds' remain

    intact & pass out with the animals' faeces,

    e.g. apple, cherry


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3. Water Dispersal

- fruits like coconut contain air space which makes the fruit buoyant

- seed is covered in a spongy or fibrous layer

4. Mechanical Dispersal

- pods with a leathery skin which splits open to throw seeds away over a fairly wide area, e.g. pea


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5. Censer Mechanisms

  • fruits are borne at the ends of long stalks with holes through which seeds are shaken

  • as they blow in the wind; pores are closed in wet conditions, e.g. poppy

    6. Casual Mechanisms

  • taking opportunities & using any available

    means of dispersal,

    e.g. acorns (from oak tree) may be rolled

    along the ground by wind or carried

    about by squirrels or float downstream

    in rivers



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12.4.4 Dormancy

The water content of seeds is very low (5-10%

weight) and is the major factor in preventing

germination.

Periods of dormancy may last for a number of years.

Some seeds fail to germinate for one reason or another:

1. Light is necessary for germination of certain

seeds

2. A sustained period of cold is needed to make

some seeds of temperate climates germinate to

ensure their seeds do not germinate in dry

autumn & cold winter

3. Some seeds need the heat of a flash-fire for

germination


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4. Time is needed for maturity / internal chemical

changes to complete before germination

5. Seed coat impermeable to water &/or gases;

time for decay of seed coat;

some seeds need physical abrasion or partial digestion in intestine to break dormancy

6. Presence of natural chemical inhibitors


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12.4.5 Germination & Early Growth

in Flowering Plants

Conditions necessary for seed germination

1. Water - taken up through the micropyle;

softens seed coat;

cotyledons swell up & burst through seed coat;

enzymes are activated to break down starch to

simple sugars and proteins to amino acids;

fats are converted into fatty acids & glycerol;

these dissolve in water & are transported to the

growing points of the embryo


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  • Glucose, fatty acids & glycerol provide

    respiratory substrates from energy for

    growth is released;

  • Glucose is also used in the formation of

    cellulose cell walls;

  • Amino acids are used to form new

    enzymes and structural proteins within new cells.


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  • 2. Temperature

  • warmer temperatures enable enzymes

  • to work

  • 3. Oxygen - enables seeds to respire

  • aerobically to supply energy for growth


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Epigeal: growth of the cotyledon above soil

Hypogeal: growth of the cotyledon below soil


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