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Sexual Reproduction In flowering Plants Class 12 Biology Revision Notes
Class 12 Biology students should refer to the following concepts and notes for Sexual Reproduction In flowering Plants in standard 12. These exam notes for Grade 12 Biology will be very useful for upcoming class tests and examinations and help you to score good marks
Sexual Reproduction In flowering Plants Notes Class 12 Biology
♦ Site of sexual Reproduction.
♦ Male and female reproductive organs are borne on flowers.
PARTS OF A 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
Male Reproductive Organ
♦ Androecium consists of Stamens.
♦ Stamen consists of anther, filament & connective (when anther is bilobed)
♦ Anther bilobedhas 4 Microsporangia.
Refer fig. 2.1 of NCERT (L.S. OF A FLOWER WITH DIFFERENT PARTS)
MICROSPOROGENESIS:The process of formation of micro spores from pollen mother cell through
meiosis iscalled 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 mother cell (2n)
Pollen grains (n)
LAYERS OF ANTHER WALL
Pollen grains have two outer walls; i) Exine ii) Intine
♦ Exine is made of sporopollenin.(Hardest natural substance). Intine is made of cellulose and pectin.
♦ Mature pollen grains have two cells – largevegetative cell & small generative cell.
♦ Generative cell forms two male gametes bymitotic division.
♦Pollen grains (Refer fig 2.7 of text book)shed in 2-celled /3celled stage
Gynoecium / carpel (the female reproduuctive organ)
(Structure of anatropous ovule) Megasporangium
♦Each Carpel consists of ovary, style &stigma.
♦ Ovules are attached to ovary by placenta.
♦ Funicle – stalk of ovule
♦ Hilum, a region where funicle is attached
♦ Integuments –cover embryo sac.
♦ Micropyle – a pore for entry of pollen tube &to imbibe water.
Megaspore mother cell (2n)
4 Megaspores (n)
(3 megaspores degenerate, 1 remains functional)
Funtional Megaspore (n) (Divides 3times by mitosis)
8 Nucleated Embryo Sac formed
3 cells group at micropylar end -the egg cell(n) & 2 synergids(n)
3 cells at chalazal end called antipodals(n)
2 polar nuclei at center(n each )
Ref fig 2.7 and 2.8 of text book
POLLINATION– transfer of pollen from anther to stigma. Agents of pollination –air, water, insect.bat,bird,man.
¤ Pollen grains germinate on stigma & pollen tube grows through style.
¤ Pollen 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 fertilization: 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 theembryo.
• 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 flowerother than the ovary e.g. apple, peach etc.
Seeds two types: i) Albuminous (with Endosperm)
ii) Non albuminous(without Endosperm)
Special mechanism of reproduction:-
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.
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 donot germinate on the stigma of the same flowerSelf incompatibilityartificial HybridisationHeterostyly
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