Biology Short Notes: Sexual Reproduction in Flowering Plants

FLOWER –

it is the fascinating reproductive organ of angiosperms.


It consists of :-

1. Calyx
2. Corolla
3. Androecium
4. Gynoecium

PRE- FERTILISATION – STRUCTURES AND EVENTS

• —  Hormonal and structural changes are initiated leading to differentiation and development of floral primordium.
• —  Inflorescences are formed which bear floral buds and then flowers.
• —  Androecium and Gynaecium differentiate and develop.

STAMEN 

—  Stamen consist of

1. Anther – terminal bilobed structure
2. Filament – long slender stalk. Proximal end of the filament is attached to the thalamus or

petal.

STRUCTURE OF ANTHER

• —  Anther has two lobes (bilobed). Each lobe consists of two theca. Hence it is dithecous.
• —  Anther is a tetragonal structure which consist of four microsporangia located two in each lobe.
• —  Microsporangium develops into pollen sacs.
• —  Pollen sacs run longitudinally and contain pollen grains.

STRUCTURE OF MICROSPOANGIUM


—  A typical microsporangium appears circular in outline.

It is surrounded by 4 walls.

1. Epidermis          –  protects and help in dehiscence of anther.
2. Endothecium
3. Middle layers
4. Tapetum             – nourishes the developing pollen grain

SPOROGENOUS TISSUE


It is compactly arranged homogenous cells which are present at centre of each microsporangium when the anther is young.

MICROSPOROGENESIS

• —Process of formation of microspores from pollen mother cell through meiosis.
• —The cells of the sporogenous tissue/microspore mother cell (2n) meiotically divide to form microspores which are arranged in a cluster of 4 cells called MICROSPORE TETRAD.
• —When the anther matures and dehydrates, the microspore dissociate from each other and develop into pollen grains
• —Thousands of pollen grains formed inside a microsporangium- released with dehiscence of anther.

POLLEN GRAINS

Pollen grains are male gametophyte – spherical in shape.

STRUCTURE OF POLLEN GRAIN


Pollen grains are made of 2 layered Wall,

1. Exine :- Made of sporopollenin- most resistant organic matter known,
2. Intine :-

-Thin and continuous layer


– Made of cellulose and pectin


3. Germ pores


– apertures on exine where sporopollenin is absent


– forms pollen tube.


4. A plasma membrane surrounds cytoplasm of pollen grain.

MATURE POLLEN


—  A mature pollen consist of 2 cells with nucleus (Vegetative and Generative)

VEGETATIVE CELL

• Bigger
• Abundant food reserve
• Large irregular nucleus
• Responsible for the development of pollen grain

GENERATIVE CELL

• Small
• Involves in syngamy (fuse with an egg)
• Dense cytoplasm and nucleus.

Effect of Pollen on Human:

• Pollen grains cause allergy and bronchial afflictions
• Leading to chronic respiratory disorders like asthma, bronchitis Eg :- Parthenium (carrot grass)

POLLEN PRODUCTS:

• Rich in nutrient.
• Pollen tablets and syrup

– food supplements


– claims to increase performance of athletes and race  horse.

Period of viability

• Once shed the pollen grains have to land on the stigma before they lose viability if they have to bring about fertilization.
• Period of viability depends on temperature and humidity. Example:- cereals takes 20 minutes and members of rosaceae, leguminoseae, solanaceae  take months.

• Pollen grains stored by CRYOPRESERVATION.
• Used in crop breeding programmes.

PISTIL, MEGASPORANGIUM AND EMBRYO SAC :

GYNAECIUM – female reproductive part of flower

• —  Gynaecium with 1 pistil – Monocarpellary
• —  Gynaecium with more than 1 pistil – Multicarpellary
• —  Fused pistil – Syncarpous
• —  Free pistil – Apocarpous.

PISTIL


Pistil consist of

1. Stigma
2. Style
3. ovary

• ovarian cavity
• placenta

Ovules arise from placenta.

• single ovule – wheat, paddy
• Many ovules – papaya, water melons, etc.

MEGASPORANGIUM (OVULE)

• —  Ovule is a small structure attached to placenta.
• —  Funicle – stalk by which ovule is attached to placenta
• —  Hilum – junction between ovule and funicle
• —  Integuments – protective envelops
• —  Micropyle – small opening at the tip of ovule into where pollen tube enters
• —  Chalaza – basal part of ovule
• —  Nucellus (2n)-mass of cells enclosed in integuments. Has abundant food reserve.

MEGASPOROGENESIS

• —  Process of formation of megaspores from megaspore mother cells is called MEGASPOROGENESIS.
• —  Megaspore mother cells divide meiotically to form 4 megaspore (haploid)
• —   out of 4, only 1 megaspore is functional and forms gametophyte and the rest degenerate.

 FEMALE GAMETOPHYTE (EMBRYO SAC)

• —  The embryo sac develops from the functional megaspore (n).
• —  MONOSPORIC DEVELOPMENT:- formation of embryo sac from a single megaspore.

FORMATION OF EMRYO SAC

• —  Nucleus of functional megaspore divides mitotically to form 2 nuclei which move to opposite poles forming 2-nucleate embryo sac.
• —  Two more mitotic nuclear division results in 4-nucleate and later 8- nucleate stages of embryo sac.
• —  Then cell wall is laid down leading to organization of female embryo sac.

STRUCTURE OF EMBRYO SAC

• —  Egg apparatus – present at the micropylar end and consist of 2 synergids and 1 egg cell

Synergids have cellular thickenings at micropylar tip called FILIFORM APPARATUS – guides the pollen tube into the synergid

• —  Antipodal – 3 cells present at chalaza end
• —  Polar Nuclei – Large central cell.

POLLINATION

• —  The transfer of pollen grains from anther to stigma of a pistil is called pollination.
• —  Based on the source of pollen, pollination is of  3 types:-

– AUTOGAMY


– GEITONOGAMY


– XENOGAMY

1. AUTOGAMY

• Transfer of pollen grains from anther to stigma of the SAME flower.

REQUIREMENT:-

1. Synchrony in pollen release and stigma receptivity.
2. Closeness of stigma and anther

• Chasmogamous flowers- flowers with exposed anthers and stigma
• Cleistogamous flowers-flowers which do not open at all
• Cleistogamy is disadvantageous because there is no chance of variation.
• Ex:- oxalis ,viola

2. GEITONOGAMY

• —  Transfer of pollen grains from anther to stigma of another flower of the same plant.
• —  Genetically similar
•   Ex:- cucurbits

3. XENOGAMY

• Transfer of pollen grains from anther to stigma of another flower of different plant
• Genetically different pollen grains are brought to the stigma.

Agents of Pollination:


1)      Abiotic agents:


a)      Wind


b)      Water


2)      Biotic agents:


a)      Insects


b)      Birds


c)       Bats


d)      Reptiles


e)      Mammals

Adaptations in flowers for Pollination

I. Wind Pollination

• pollen  grains :– light, non- sticky, winged
• anther :- well exposed
• stigma :- large and feathery
• flower :- one ovule, arranged as inflorescence

         Ex : corn cob, cotton, date palm

II. Water Pollination

                   – Bryophytes, Pteridophytes, Algae 

• pollen grains : protected by mucilaginous covering

Ex : Fresh water plants- Vallisneria, Hydrilla


Sea grass- Zostera

Main features of wind and water pollinated plants

         – produce pollen grains in large no.

         – do not produce nectar

III. Insect Pollination

       – Flowers : large, colourful, fragrant, rich in nectar

       – Pollen grains : sticky

       – Stigma : sticky

Certain rewards to pollinators:

• nectar and (edible) pollen grains as foods
•  provide safe place for laying eggs

Ex : Amorphophallus, Yucca

Outbreeding Devices

Continued self – Pollination – Inbreeding depression

Ways to avoid Self-pollination : 


(i) Pollen release & stigma receptivity – not synchronised


(ii) Stigma and anther – placed at different positions


(iii) Self-incompatibility


(iv) Production of unisexual flowers

Eg: castor, maize (prevents autogamy)


papaya (prevents autogamy & geitonogamy)

Pollen – Pistil Interaction


All events – from deposition of pollen on stigma till the pollen tube enters the ovuleis called     Pollen-pistil interaction.


•        Recognition of compatible pollen


•        Germination of pollen grains


•        Development of Male Gametophyte

Artificial Hybridization

• Crossing diff varieties of species- hybrid individual- with desirable characters of the parent plants
• desired pollen grains for pollination- stigma protected from contamination
• Emasculation : removal of anther
• Bagging : flower covered- bag made up of butter-prevent contamination of stigma from unwanted pollen

Bagged flower- attains receptivity – mature pollen grains- dusted on the stigma – rebagged – fruits allowed to develop

• Double Fertilisation
• Syngamy

– pollen tube releases male gametes into synergids


– fusion of 1 of male gametes and egg cell


– fusion of 2nd male gamete and polar nuclei =Triploid endosperm nucleus- PEN (Triple Fusion)


– PEN – now called Primary Endosperm Nucleus – Endosperm

Post- fertilization Events        


All events that occur in a flower, after double fertilization is called Post- fertilization events

Major events are : 


(i)            Development of endosperm


(ii)             Development of embryo


(iii)             Maturation of ovule into seed


(iv)             Maturation of ovary into fruit

Endosperm

†     Two types of endosperm development :


(i) Free nuclear type (common method)


(ii) Cellular type


†      Cells of endosperm– store food materials- used for developing embryo


†      Non – Albuminous / Non-Endospermic seeds- endosperm completely utilized – before maturation of seeds. Ex: pea


†      Albuminous / Endospermic seeds- a portion of endosperm remain in mature seeds. Ex: castor

Embryo


†     Embryogeny – early stages of embryo development


†      Zygote   à     Proembryo    à   Mature embyo (heart-shaped)

Embryo consists of:


– embryonal axis


– cotyledons


– plumule


– radicle

Monocotyledonous Seed


–          Scutellem = Cotyledon


–           Coleorrhiza: undifferentiated sheath covering radical & root cap


–           Coleoptile: sheath covering plumule

Seed


–          Fertilized and mature ovule develops into seed.

Seed consists of:


–          cotyledon(s)


–          embryonal axis


–          Seed coat – double layered – formed by integuments

• Testa (outer coat)
• Tegmen (inner coat)

–          Micropyle: – small o        pening on seed coat, it facilitates entry of H2O & O2 into seeds (for germination)


–          Hilum:- scar on seed coat


–          Seed     – Albuminous / Non-Albuminous


–          Perisperm : remnants of nucellus that is persistent. Ex: Black pepper


–          Dormancy:   state of inactivity

Advantages of Seeds

• To plants

(i) Seeds – reserve food materials- nourish seedling


(ii) Seed coat- protection to young embryo


(iii) Seeds of large no of species –live for several years


(iv) Seeds – better adaptive strategies- dispersal to new habitats- better survival

• To mankind

(i) used as food – throughout the year


(ii) seed – basis of agriculture

Fruit


–          True fruit : – Fruit formed from the ovary


–          Parthenogenesis:  If ovary transform to fruit without fertilization. Ex : Banana


–          Parthenocarpy – induced with gibberellins & auxins without fertilization.


–          False fruit: any part other than ovary- forms the fruit. Ex: Apple

Apomixis & Polyembryony 


Other modes of reproduction

Apomixis


–          Form of asexual reproduction- mimics sexual reproduction- seed formed without fertilisation


–          Formation of apomictic seeds :


·         diploid cell (formed without meiosis) – develop into embryo without fertilization


·         cells of nucellus (2n) surrounding embryo sac- protrude into embryo sac – develop into embryos. Ex. Citrus and Mango.

Polyembryony 


–          Occurrence of more than one embryo in a seed


–          Often associated with apomixes.  Ex: Citrus, groundnut

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