Sexual Reproduction in Flowering Plants : Structure of Flower , Pollination and Double Fertilization

20/08/2020 Vinod 0 Comments

Structure of Flower :-

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Fig. 1. Bisexual Flower.

A Flower is the Sexual Reproductive Organ in Plants. Flowers are often the most attractive structures of plant.

  • The Flower Anatomy can be divided into following components –
  1. Calyx.
  2. Corolla.
  3. Androecium.
  4. Gynoecium.

Calyx :-

These are green Petal like structure that are found in right above the base of the flower called Receptacles.

  • The Calyx is formed of sepals.
  • Their main function is to protect the flower while it is still in the bud stage.

Corolla :-

It is the collective name given to the petals which are colourful parts of the flower. The petals of different flowers are found in different colours and in addition, most of the times they are fragment as well.

  • It is the petals that attract the pollinating agents to the flower.
  • The Corolla is located above the Calyx layer.

Androecium :-

It is the male Reproductive part of a flower. It is considered the third whorl in a flower.

  • The Androecium is the name given to a group of Stamens.
  • It Stamen is made up of Connective, Filament and Anther.

Connective :- It is basal part of the Stamen which connect with Receptacle of the Flower.

Filament :- It is middle part of the Stamen and filament like structure connect with Anther.

Anther :-

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Fig. 2. Diagram of Anther .

The anther are lobed structure present at the top of Stamen. They produce Pollen which contain the male gamete ( pollen grain ).

  • Each anther is bilobed, each lobe contain two spherical structures called microsporangia.
  • Each microsporangium is bounded by four players –

Outermost – Epidermis.

Innermost – Tapetum ( Endodermis ) and

Middle layers are present in between the Epidermis and Tapetum.

Microsporogenesis :-

The formation of microspores inside the microsporangium or Pollen sacs of seed plants.

  • A diploid cell in the microsporangium called microsporemother cell, Undergoes meiosis and gives rise to four haploid microspores.
  • Each microspore then develops into a pollen grain ( The microgametophyte).
  • Each Pollen grain is generally oval and small. It is double layered , The outermost wall is exine and made up of sporopollenin, Inner wall is intine and made up of pectocellulose.

Microgametogenesis :-

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Fig. 3. Stages of Microsporogenesis and Microgametigenesis .

The pollen grain or microspore is the first cell of the male gametophyte,

which contains only one haploid nucleus. During early stage of development, it remains within the microsporangium.

The cell undergoes unequal division and forms a small generative cell and large vegetative cell.

  • Later on generative cell divides and give rise to two spherical cells then the male gamete becomes 3 celled stage (one vegetative and two generative cells ), the second division may take place either in Pollen grain or in Pollen tube.

Gynoecium :-

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Fig. 4. Diagram of Gynoecium .

It is the female reproductive organ in a Flower.

It is composed of the stigma , style and ovary .

  • The style is a filamentous on top of which lies the stigma which functions to hold transferred pollen grains .

Once the pollen lands on the stigma , it is passed downwards through the style to the ovary.

  • The Ovary is lobed and consists of ovules , ovules are formed by Placenta present within the ovarian cavity.
  • Ovule contain the female gamete ( Ovum ) .

Ovule ( Megasporangium ) :-

ovule 2
Fig. 5. Diagram of Ovule .

The Ovule is made up of the nucellus , the integuments that form the outermost layer and the female gametophyte ( Embryo sac in Flowering Plant ) .

Nucellus :- It is the largest part of the Ovule . It houses the Embryo Sac as well as nutritive tissue and actually remains present in some Flowering Plants after fertilization as a source of nutrients for the Embryo .

Integuments :- It is the tough outer protective layer of the Ovule.

  • Gymnosperms such as Pine trees and Sqruce trees , usually have one integument in an Ovule , so we call them unitegmic .
  • Angiosperms like Maples and Daisies, typically have two integuments and we call them bitegmic .
  • The integument encloses the nucellus except for a small gap , which is called the micropyle.

Types of Ovule :-

Ovules have been separated in six categories based on their shapes –

  1. Atropous :- This is where the body of these Ovule is straight do that the Chalaza, where the nucellus and integuments merge , the funicle , which attaches the Ovule to the Placenta and the micropyle are all aligned .
  2. Anatropous :- In this case, the Ovules become completely inverted during developments so that the micropyle lies close to the hilum .
  3. Hemi – anatropous :- The body of these Ovules becomes at a right angle in relation to the funicle , so looks like the Ovule is lying on its side.
  4. Campylotropous :- The body of this type is bent and the alignment between the Chalaza and micropyle is lost . The Embryo sac is only slightly curved .
  5. Amphitropous :- The body of the Ovule is very much curved that the Embryo sac and the Ovule itself take the shape of a horseshoe .
  6. Circinotropous :- The funicle in this case is especially long that it creates a nearly full circle around the Ovule whose micropyle is ultimately pointing upwards.

Embryo Sac :-

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Fig . 6 . Diagram of Embryo Sac .

Embryo Sac is the female gametophyte ( an oval structure in the nucellus of the Ovule ) of flowering plants , formed by the division of haploid megaspore nucleus .

  • It is the site of fertilization of the egg and development of the embryo .
  • It consists of six haploid cells without cell walls ( two synergids , three antipodals and one egg cell ) and two haploid polar nuclei . Sometimes the two haploid polar nuclei fuse together to form a single endosperm .
  • At fertilization one male gamete ( pollen grain ) fuses with the egg nucleus to form a single zygote which develop into the embryo .
  • The second male nucleus ( pollen grain ) fuses with the primary endosperm nucleus to form the endosperm nucleus .
  • Three nuclei ( cells ) of the micropylar end form the egg apparatus and rest three chalazal end are called antipodal cells .
  • In the egg apparatus the middle one is the largest , called egg ( ovum ) and rest two are the synergids or helping cells .

Megasporogenesis :-

The development of megaspores inside the megasporangium is called megasporogenesis .

  • The megaspore mother cell is diploid ( 2n ) which undergoes meiosis and forms four haploid ( n ) megaspores . The first division of megaspore mother cell is transverse forming two cells . Both the cells again divide transversely and the four Megaspores are formed .
  • The megaspores are then arranged in an axial row , called linear tetrad . Out of four megaspores , only one which remains towards the Chalaza end behaves as functional megaspore and other three which remain towards the micropyle end , gradually degenerate .
  • The functional megaspore forms the female gametophyte ( embryo sac ) .

Megagametogenesis :-

megasporogenesis1
Fig . 7 . stages of Megagametogenesis .

The formation of female gametophyte ( embryo sac ) is called Megagametophyte .

  • Megaspore is the first cell of the female gametophyte . The functional megaspore becomes enlarged at the expense the tapetum and the nucellus form the female gametophyte .
  • Initially , the embryo sac is uninucleate and with further growth its nucleus divides by three successive divisions and forms eight nuclei . Out if eight nuclei , initially four remain towards the micropyle end and other four towards the Chalazal end
  • One nucleus form each pole then moves towards the centre and forms a pair of polar nuclei . these nuclei fuse together and form diploid ( 2n ) nucleus .

Also read :- sexual reproduction and its stages

Also read :- Asexual reproduction and their types

Pollination :-

Self pollination
Fig . 8 . Pollination in Flowers .

Pollination is the transfer of pollen grain from a male part ( anther ) of plant to a female part ( stigma ) of a plant .

Types of Pollination :-

There are two types of Pollination –

  1. Self Pollination .
  2. Cross Pollination .

Self Pollination :-

The transfer of pollen grain from the anther to the stigma if the same Flower or another Flower on the same Plant is called Self Pollination .

  • Self Pollination generally occurs in bisexual Flowers .
  • There are two types of Self Pollination –

( a ) Autogamy :- In autogamy , pollen grain is transferred to the stigma of same flower .

( b ) Geitonogamy :- In geitonogamy pollen grain is transferred from anther of one Flower to the stigma of another Flower on the same flowering Plant .

  • Some Plants have mechanisms that ensure autogamy such as Flowers that do not open ( Cleistogamy ) .

Some Plants such as Viola , Oxalis and Commelina produce two types of flowed –

Chasmogamous :-

Flowers which are similar to Flowers of other species with exposed anthers and stigma .

Cleitogamous :-

Flowers which do not open at all . In such Flowers the anthers and stigma lie close to each other .

Advantages of Self Pollination :-

  • If a given genotype is well suited for an environment , self pollination helps to keep this trait stable in the species .
  • Not being development on pollinating agents like Wind , Water , Insect and other animals .
  • During self pollination , the pollen grains are not transmitted from one Flower to another , as a result the wastage of pollen grain is less .
  • The Plant that develops in that way of Pollination can’t make changes in their characters .

Disadvantages of Self Pollination :-

  • The disadvantages of Self pollination come from a lack of variation that allow no adaptation to the changing environment .
  • To the reduced health of the species due to the breeding of related specimens .
  • Genetic defects of self pollinating plans can’t be eliminated by genetic recombination .

Cross Pollination :-

The transfer of pollen grain from the anthers of one Flower to the stigma of another Flower by the action of pollinating agents such as wind ,insect etc is called Cross Pollination .

  • Cross Pollination generally occurs in unisexual Flowers of the Plants having different genetic materials .

Xenogamy :-

Transfer of pollen grains from anther to the stigma of different Plant . This is the only type of Pollination which during Pollination brings genetically different types of pollen grains to the stigma .

Types of Cross Pollination :-

( a ) Entomophily :- Cross Pollination by Insects .

( b ) Ornithophily :- Cross Pollination by Birds .

( c ) Zoophily :- Cross Pollination by Animals .

( d ) Hydrophily :- Cross Pollination by Water .

( e ) Anemophily :- Cross Pollination by Air .

( f ) Cheiropterophily :- Cross Pollination by Bats .

Adaptation in Insect Pollinated Flowers :-

  • Large and coloured petals to attract Insects .
  • Nectar to attract Insects .
  • Pollen grains are sticky so that they can stick easily to Insect’s body parts .
  • Scent to attract Insects .

Examples – Cestrum nocturnum , Nyctanthes abortristis , Jasminum auriculatum , Banana , Lotus etc are the Insect Pollinated Flowers .

Adaptations in Wind Pollinated Flowers :-

  • Large production of pollen grains .
  • Anther is well exposed .
  • Flowers are not attractive and scent emitting .
  • Feathery and sticky stigma .
  • The Pollen grains are light and non sticky so that they can be attracted in wind current .
  • Flowers don’t possess nectars .
  • They have a single Ovule in each Ovary and numerous flowers packed into an Inflorescence .

Example – Grasses , Pine , Wheat , Maize , Bamboo , Rice , Sugarcane etc.are the wind Pollinated Flowers .

Adaptations in Water Pollinated Flowers :-

  • They produce a large number of pollens because most of them get lost by the flow of water .
  • The stigmas are large and feathery to catch the pollens .
  • The male Flowers after maturation get detached from the plant and float above the water surface .
  • The stalks the pollinated Flowers are usually very long to project their stigmas above the water surface .
  • No animal transfers are needed for Pollination .
  • They are not very colourful and do not produce nectar .

Examples – Vallisneria , Hydrilla , Water lily , Hyacinth are water pollinated Plants .

Adaptations in birds pollinated Flowers :-

  • The Plants typically have colourful , often red , yellow ,orange Flowers .
  • Flowers with long tubular structures holding ample nectar.
  • Orientation of the Stamen and stigma that ensure contact with the pollinator.

Example – Passiflora ,Eucalyptus ,Campsis radicans etc .

Advantages of Cross Pollination :-

  • Progeny shows enhanced vigor .
  • Offsprings are more viable and resistant .
  • There is a possibility to get new desirable characters .
  • It helps in evolution .
  • Undesirable characters of the Plant can be eliminated .

Disadvantages of Cross Pollination :-

  • Pollination many fail due to distance barrier .
  • Flowers have to totally depends on the external agencies for Pollination .
  • More wastage of pollen .
  • It may introduce some undesirable characters .

Double Fertilization in Flowering Plants :-

doublefertilization
Fig . 9 . Female Reproductive parts of a Flowering Plant .

It is a complex fertilization mechanism of FloweringPlants ( Angiosperm ) .

When the pollen tube releases the two male gametes into the synergids , one of the male gamete moves towards the egg cell and fuses with it’s nucleus .This results in formation of a diploid cell ( zygote ).

The other male gamete moves towards the polar nuclei located in the central cell and fuses with them to produce triploid Primary Endosperm . As this involves the fusion of three haploid nuclei it is termed triple fusion .

Since two types of fusions Syngamy and Triple fusion take place in an Embryo Sac the phenomenon is termed Double Fertilization .

Significance of Double Fertilization :-

  • Fertilization of Endosperm ( 3n ) for providing proper nutrition to the developing Embryo .
  • Formation of Embryo ( 2n ) .

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