12th Biology Study Matetrial-Chapter2(sexual reproduction inflowering plants flowers) - Tamilyoungsters
CHAPTER 2 – SEXUAL REPRODUCTION INFLOWERING PLANTS FLOWERS
Site of sexual Reproduction.
Male and female reproductive organs are borne on lowers.
PARTS OF A FLOWER: Four WHORLS – CALYX (sepals), COROLLA (petals), ANDROECIUM (Male reproductive organ), GYNOECIUM (Female reproductive organs). Male Reproductive Organ
Androecium consists of Stamens.
Stamen consists of anther, filament & connective (when anther is bilobed)
Anther bilobed has 4 Microsporangia. Refer fig. 2.1 of NCERT (L.S. OF A FLOWER WITH DIFFERENT PARTS)
MICROSPOROGENESIS: Microspore mother cell (2n)
Meiosis Microspore (n) Mitosis Pollen grains (n) Pollen grains have two outer walls; i) Exine ii) Intine
Exine is made of sporopollenin (HARDEST NATURAL SUBSTANCE)
Mature pollen grains have two cells – big vegetative cell & small generative cell.
Generative cell forms two male gametes by mitotic division.
Pollen grains shed in 2-celled/3celled stage2 GYNOECIUM / CARPEL (THE FEMALE REPRODUUCTIVE ORGAN)
Refer fig 2.7 of text book (Structure of anatropous ovule)
Each Carpel consists of ovary, style & stigma.
Ovules are attached to ovary by placenta.
The 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
MEGASPOROGENESIS
Megaspore mother cell (2n) Meiosis 4 Megaspores (n) (3 megaspores degenerate, 1 remains functional) Funtional Megaspore (n) (Divides 3 times 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.Transfer of pollen to stigma Self pollination (Autogamy) Auto gamy (Same flower)
Double fertilization
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 fertilisation,
ii)Fusion of fusion product of polar nuclei with male gamete – Second fertilisation.
Xenogamy (different flowers on different plants of the same species) Cross pollination (Hetrogamy) Geitenogamy Different flowers same plant Refer fig 2.13 in NCERT POST FERTILISATION CHANGES: STAGES OF EMBRYO DEVELOPMENT AFTER FERTILISATION
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.
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.
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 Anther & stigma mature at different times Anther & stigma placed at different positions Self-incompatibility where pollen grains of a flower donot germinate on the stigma of the same flower
ARTIFICIAL HYBRIDISATION: 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.
Questions Short Answer Type Questions (3 marks.)
1. Explain the phenomenon of double fertilization in angiospermic plants.
2. Trace the development of the mature ovule from a megaspore mother cell.
3. Enumerate the steps in artificial hybridisation.
4. Differentiate between monoecious and dioecious plants, with an example for each.
5. How is pollination effected in Vallisneria?
Long Answer Type Questions (5 marks).
1. Represent diagrammatically the formation of an embryo sac from a megaspore mother cell.
2. Draw a well-labelled diagram of the L.S of embryo of grasses. How does it differ from that of bean.
