CHAPTER 2 - SEXUAL REPRODUCTION IN FLOWERING PLANTS
FLOWER - itis the fascinating reproductive organ of angiosperms.
It consist of :-
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 consist of
- Anther – terminal bilobed structure
- Filament – long slender stalk. Proximal end of the filament is attached to the thalamus or
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.
- Epidermis - protects and help in dehiscence of anther.
- Middle layers
- Tapetum - nourishes the developing pollen grain
It is compactly arranged homogenous cells which are present at centre of each microsporangium when the anther is young.
- —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 are male gametophyte - spherical in shape.
STRUCTURE OF POLLEN GRAIN
Pollen grains are made of 2 layered Wall,
- Exine :- Made of sporopollenin- most resistant organic matter known,
- Intine :-
- 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.
— A mature pollen consist of 2 cells with nucleus (Vegetative and Generative)
- Abundant food reserve
- Large irregular nucleus
- Responsible for the development of pollen grain
- 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)
- Rich in nutrient.
- Pollen tablets and syrup
- 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 consist of
- ovarian cavity
- single ovule – wheat, paddy
- Many ovules - papaya, water melons, etc.
- — 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.
- — 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
- — Antipodal - 3 cells present at chalaza end
- — Polar Nuclei - Large central cell.
- — 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:-
- Transfer of pollen grains from anther to stigma of the SAME flower.
- Synchrony in pollen release and stigma receptivity.
- 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
- — Transfer of pollen grains from anther to stigma of another flower of the same plant.
- — Genetically similar
- Ex:- cucurbits
- 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:
2) Biotic agents:
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
II. Water Pollination
- Bryophytes, Pteridophytes, Algae
- pollen grains : protected by mucilaginous covering
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
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
(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
- 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
- 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
† 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
† Embryogeny – early stages of embryo development
† Zygote à Proembryo à Mature embyo (heart-shaped)
Embryo consists of:
- embryonal axis
- Scutellem = Cotyledon
- Coleorrhiza: undifferentiated sheath covering radical & root cap
- Coleoptile: sheath covering plumule
- Fertilized and mature ovule develops into seed.
Seed consists of:
- embryonal axis
- Seed coat - double layered - formed by integuments
- Testa (outer coat)
- Tegmen (inner coat)
- 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
(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
(ii) seed - basis of agriculture
- 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
- 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.
- Occurrence of more than one embryo in a seed
- Often associated with apomixes. Ex: Citrus, groundnut