These notes for CBSE Class 12 Biology, Chapter 2, cover Sexual Reproduction in Flowering Plants. They detail the structure of a flower, including its four whorls: calyx, corolla, androecium (male reproductive organ), and gynoecium (female reproductive organ). The process of microsporogenesis, the formation of pollen grains from microspore mother cells via meiosis, is explained, including the role of the tapetum and the structure of pollen grains with their exine and intine walls. Megasporogenesis, the formation of the embryo sac from the megaspore mother cell, is also described, highlighting the development of the 8-nucleated embryo sac. The notes explain pollination, the transfer of pollen, and the crucial process of double fertilization, involving syngamy and triple fusion. Post-fertilization changes, leading to the development of the embryo, endosperm, fruit, and seed, are discussed. Special reproductive mechanisms like apomixis and polyembryony, along with outbreeding devices and artificial hybridization techniques, are also covered. These notes are designed to aid students in revising the key concepts for their exams.
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FLOWERS? Site of sexual Reproduction ? Male and female reproductive organs are bome on flower
•Four whorls – calyx (sepals), corolla (petals), androecium (Male reproductive organ), gynoecium (Female reproductive organs) •Function of calyx:protects the bud. •Function of corolla :attracts insects by its colour
? Androecium consists of Stamens.
? Stamen consists of Stamens.
? Ather bilobed 4 Microsporangle.
Refer fig. 2.1 of NCERT (L.S. OF A FLOWER WITH DIFFERENT PARTS)
MICROSPOROGENESIS |
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| The process of formation of micro spores from pollen mother cell through meiosis is called microsprogenesis. Tapetum :Inner most layer of wall of microsporangium .Cells have dense cytoplasm. Generally have more than one nucleus .Nourishes the developing pollen grain | ||
? Microspore (n) ? Pollen grains (n) |
have two outer walls;
i) Exine
ii) Intine
? Exine is mase of spropollenin..(Hardest natural substance). Intine is made of cellulose
and pectin.
? Mture llen grains have two cells – large vegetative cell & small generative cell. ? Generative cell froms to male gameetes by mitotic division.
? Pollen grains (Refer fig 2.7 of text book)shed in 2-celled /
3 celled stage)
See Fig 2.5 a and b page 23
Gynoecium / carpel (the female reproduuctive organ)
(Structure of anatropous ovule)Megasporangium
? Each Carpel consists of ovary ,style & stigma.
? Ovules are attached to ovary by plaxenta.
? Funicle – stalk of ovule .
? Hilum, a region where funicie is attached.
? Micropyle
a pore for entry of pollen tube & to imbibe
water.
Anatropous ovule
Megasporogenesis |
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Megaspore mother cell (2n) Meiosis |
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4 Megaspores (n) (3 megaspores degenerate, 1 remains functional) |
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Funtional Megaspore (n) (Divides 3times by mitosis) |
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8 Nucleated Embryo Sac formed |
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| 2 polar nuclei at center(n each ) | 3 cells at chalazal end called antipodals(n) | 3 cells group at micropylar end -the egg cell(n) & 2 synergids(n) | ||
POLLINATION– transfer of pollen from anther to stigma. Agents of pollination –air, water,
insect.bat,bird,man
? Pollen grains geminate on stigma & pollen tube grows through style.
? Pllen tube reaches micropyle
& releases two male gametes into embryo-sac Fertilisation is the process of fusion of male& female gametes (n+n) to form a diploid (2n) zygote.
Syngamy :Fusion of one male gamete(n) with egg (n)Zygote(2n)Produced
First Fusion Fusion of two Polar Nuclei(n+n=2n) Second fusion Male Gamete(n) Fuses with the fusion product of the two polar nuclei(3n)
Third Fusion fusion of male gamete with egg cell.
? Double fertilzation
i)Fusion of male gamete with egg – First fertilization ,SYNGAMY
ii)Fusion of fusion product of polar nuclei with male gamete – Second fertilization .TRIPLE FUSION
Refer fig 2.13 in NCERT
Post fertilisation changes:
Stages of embryo development after fertilization:
1. Zygote divides by mitosis into suspensor & embryo cells
2. Suspensor cell forms a globular basal cell which remains
embedded in the endosperm & a
multicellular suspensor bearing the embryo
3.Globular embryo becomes heart-shaped & then mature
embryo with radicle, plumule &Cotyledons
? Primary endosperm nucleus – divides repeatedly to form
endosperm, food for the embryo.
? Mature ovary becomes fruit. ? Mature ovule becomes seed. ? True Fruit develops only from the ovary, e.g. mango, tomato
False Fruit develops from parts of the flower other than the
ovary e.g. apple, peach etc.
i) Albuminous (with Endosperm) ii) Non albuminous(without Endosperm)
I) Apomixis- Production of seeds without fertilisation e.g.species of Astereceae and grasses. ii) Polyembryony- Occurrence of more than one embryo in a seed e.g.Orange.
Outbreeding devices:Continued self-pollination result in breeding depression. Flowering plants have developed many
devices to discourage self-pollination & encourage cross-pollination such as
Bearing unisexual flowers Unisexuality Anther & stigma mature at different times Dichogamy Anther & stigma placed at different positions Heterostyly Pollen grains of a flower do not erminate on the stigma of the
same flower Self incompatibility Artificial Hybridisation Heterostyly
Types of cross-pollination performed by man for crop improvement. Achieved by
i) Emasculation i.e. removal of anthers from the flower bud of a bisexual flower before the anther
dehisces using a pair of forceps and
ii) Bagging i.e. covering the emasculated flowers with a bag of suitable size to protect them from
contamination with unwanted pollen
If flower is unisexual, emasculation is not needed. Flower bud bagged & when the stigma becomes
receptive, pollination is done using desired pollen & the flower is rebagged
Pollen –pistil InteractionPollen –pistil Interaction
i) All the events from pollen deposition on the stigma until the entry of the pollen
tube into the ovule are together called pollen-pistil interaction.
ii) It is a dynamic process involving pollen recognition by stigma/pistil for
compatible pollen by accepting them and if incompatible rejecting them.
The flower is the site of sexual reproduction in flowering plants, containing both male and female reproductive organs.
The four whorls of a flower are the calyx (sepals), corolla (petals), androecium (male reproductive organ), and gynoecium (female reproductive organ).
Microsporogenesis is the process of formation of microspores (pollen grains) from a microspore mother cell through meiosis.
Double fertilization is a process unique to flowering plants where one male gamete fuses with the egg cell (syngamy) and another male gamete fuses with the polar nuclei (triple fusion).
After fertilization, the zygote develops into an embryo, the primary endosperm nucleus forms the endosperm (food for the embryo), the ovary matures into a fruit, and the ovule matures into a seed.
Apomixis is the production of seeds without fertilization, a special mechanism of reproduction found in some species.
Pollen-pistil interaction includes all events from pollen deposition on the stigma to the entry of the pollen tube into the ovule, involving recognition and acceptance or rejection of pollen.
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