SE-UAL REPRODUCTION IN FLOWERING PLANTS – Biology CBSE
FLOWER :-
it is the fascinating reproductive organ of angiosperms.
It consist of :-
- Calyx
- Corolla
- Androecium
- 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
- Anther – terminal bilobed structure
- 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.
- Epidermis – protects and help in dehiscence of anther.
- Endothecium
- Middle layers
- 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,
- Exine :- Made of sporopollenin- most resistant organic matter known,
- 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
- Stigma
- Style
- 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:-
- 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
2. GEITONOGAMY :-
-
- — Transfer of pollen grains from anther to stigma of another flower of the same plant.
- — Genetically similar
Example:- 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
Example : 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
Example : 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 unise-ual 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 a se-ual reproduction- mimics se-ual 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