CHAPTER 7 - SEXUAL REPRODUCTION IN FLOWERING PLANTS
FLOWER - itis the fascinating reproductive organ of angiosperms.
It consist of :-
PRE- FERTILISATION – STRUCTURES AND EVENTS
— Stamen consist of
STRUCTURE OF ANTHER
STRUCTURE OF MICROSPOANGIUM
— A typical microsporangium appears circular in outline.
It is surrounded by 4 walls.
It is compactly arranged homogenous cells which are present at centre of each microsporangium when the anther is young.
Pollen grains are male gametophyte - spherical in shape.
STRUCTURE OF POLLEN GRAIN
Pollen grains are made of 2 layered Wall,
-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.
— A mature pollen consist of 2 cells with nucleus (Vegetative and Generative)
Effect of Pollen on Human:
- food supplements
- claims to increase performance of athletes and race horse.
Period of viability
PISTIL, MEGASPORANGIUM AND EMBRYO SAC :
GYNAECIUM - female reproductive part of flower
Pistil consist of
Ovules arise from placenta.
FEMALE GAMETOPHYTE (EMBRYO SAC)
FORMATION OF EMRYO SAC
STRUCTURE OF EMBRYO SAC
Synergids have cellular thickenings at micropylar tip called FILIFORM APPARATUS – guides the pollen tube into the synergid
Agents of Pollination:
1) Abiotic agents:
2) Biotic agents:
Adaptations in flowers for Pollination
I. Wind Pollination
Ex : corn cob, cotton, date palm
II. Water Pollination
- Bryophytes, Pteridophytes, Algae
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:
Ex : Amorphophallus, Yucca
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
Bagged flower- attains receptivity - mature pollen grains- dusted on the stigma – rebagged - fruits allowed to develop
- 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
- 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
(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
(i) used as food - throughout the year
(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