Chapter 2

CHAPTER 2 - SEXUAL REPRODUCTION IN FLOWERING PLANTS


FLOWER -  itis the fascinating reproductive organ of angiosperms.
It consist 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