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 cobcotton, 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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