CLASS 12TH BIOLOGY CHAPTER- 2 SEXUAL REPRODUCTION IN FLOWERING PLANTS NCERT QUICK REVISION NOTES FOR NEET AND CBSE EXAM
CHAPTER 2: SEXUAL REPRODUCTION IN FLOWERING PLANTS
Autogamy: When pollen grains of a flower are transferred from anther to stigma of the same flower.
Microsporogenesis: Process of formation of microspores from a pollen mother cells.
Sporopollenin is one of the most resistant organic substance. It is not affected by high temperature, strong acids or alkali. No enzyme can degrade it. Pollen Products: Pollen grains are rich in carbohydrates, proteins and unsaturated fats. Their consumption is believed to increase performance of athlete and horses. They are used in the form of tablets and syrups.
MEGASPORANGIUM (OVULE):
Female gametophyte (embryo sac): In a majority of flowering plant out of four megaspores one of the megaspore is functional while other three degenerate,(monosporic development)
Agents of Pollination: Biotic and Abiotic agents help in pollination.
Dicot Embryo: A typical dicot embryo consist of an embryonal axis and two cotyledons. The portion of embryonal axis above the level of cotyledons is
Non albuminous seed: Those seeds in which no residual endosperm is found because it is completely consumed during development of the embryo.
Flower structure; development of male and female gametophytes; pollination types, agencies and examples; outbreeding devices; pollen-pistil interaction; double fertilization; post fertilization events development of endosperm and embryo, development of seed and formation of fruit; special modes apomixes, parthenocarpy. polyembryony; significance of seed dispersal and fruit formation.
Coleorhiza: A protective sheath of radicle in monocot seed.
Coleoptile: A protective sheath of plumule in monocot seed.
Perisperm: It is diploid persistent nucellus e.g. Blackpipper, beet.
Nucellus: Multicellular tissue in the centre of ovule in which embryo sac is present.
Viability of Seed: Ability of seed to retain the power of germination.
Sporogenous tissue→Microspore mother cell-MMC → Microspore tetrad → 4 Microspores → 4 Pollen grains
Pollen Viability: Pollens of wheat and rice remain viable for 30 minutes. Pollens of some plants may remain viable for several months. Pollens can be cryopreserved in liquid Nitrogen (-196°C) in pollen banks. Pollen of carrot grass (Parthenium), Chenopodium, Amaranthus etc. may cause pollen allergy.
Megasporogensis: Process of formation of haploid megaspores from megaspore mother cells.
¶ The ovule is a small structure which is attached to the placenta by means of a stalk called funicle.
¶ The point of attachment of the body of the ovule to the funicle is known as hilum. The main body of the ovule is composed of parenchymatous cells known as nucellus.
¶ Each ovule has one or two protective integument, which encircle the ovule except at the tip having small opening called micropyle.
¶ Opposite to micropylar end is chalazal end Generally a single embryosac or female gametophyte located in nucellus.
¶ Cells of nucellus have abundant reserve food material and provide nourishment to the developing embryo.
¶ The functional megaspore develops in embryo sac.
¶ The nucleus of the functional megaspore (n) undergoes three successive mitotic cell division which results the formation of eight nucleate stage of embryo sac (free nuclear division).
¶ The cell wall formation starts at eight nuclear stages. Three cells are grouped together at micropylar end to form the egg apparatus (2 synergids + 1 egg cell).
¶ Three cells are grouped at chalazal end they are, called antipodal cells.
¶ The remaining 2 nuclei are polar nuclei move to the centre of embryo sac, called central cell.
Thus, typical agniospermic embryo sac at maturity is 8 nucleate and 7 celled.
(a) Biotic Agents- Bees, flies, butterflies, wasps, moths, ants, birds, rodents, reptiles, and some primates.
(b) Abiotic Agents-Wind and water.
Types of Flowers: In some plants like Coomelina, Oxalis and Viola have two types of flowers:
1. Chasmogamous Flower"
Flower remains open after maturity, both self pollination and cross pollination both can occur in the flower.
2. Cleistogamous Flower
Flower remain closed throughout their life, so, only self pollination (autogamy) occurs in such flowers.
Out breeding Devices: Flowering plants have developed many devices called out breeding devices to discourage self pollination and to encourage cross-pollination.
The types are:
(i) Non synchronization of pollen release and stigma receptivity
(ii) Position of anthers and stigma in such a way that pollen cannot come in contact of stigma of same flower.
(iii) Self-incompatibilty
(iv) Production of Unisexual flowers
POLLEN-PISTIL INTERACTION:
¶ The pistil has the ability to recognise the pollen grain, whether it is right type (Compatible) or of the wrong type (incompatible).
¶ If it is compatible then the pistil accepts the pollen grains.
¶ The pollen grains germinate on stigma to produce pollen tubes. The contents of the generative cell (or the two male gametes in those species whose pollen is liberated in the three celled stage), move into the pollen tube.
¶ Pollen tube grows through the tissue of stigma and style by secreting enzyme and enters the ovule, through micropyle via one of the synergid. Filiform apparatus guides the entry of pollen tube.
Double Fertilisation: The pollen tube releases two male gamete into the cytoplasm of synergid. One male gamete move towards egg cell and other male gamete towards the central cell.
¶ Syngamy: One male gamete + Egg cell → Zygote (2n)
¶ Triple Fusion: Second male gamete + 2 polar nuclei → PEN (3n) .
¶ Since two types of fusion takes place in embryo sac, hence it is called double fertilisation.
POST FERTILISATION EVENTS:
(i) Endosperm and embryo development
(ii) Maturation of ovule & ovary
FATE OF FLORAL PARTS:
Ovary (2n) →Fruit
Ovary Wall (2n) →Pericarp
Ovule (2n) →Seed
Outer Integument (2n) →Testa
Inner integument (2n) →Tegmen
Zygote (2n) →Embryo
Primary Endosperm Cell (3n) →Endosperm Fall down
Sepals (2n) → Fall down
Petals (2n) →Wither away
Stamens (2n) →Wither away
Stigma, style (2n) Nucellus →Consumed/may be present as Perisperm
Synergids (n) →Degenerate
Antipodal Cells (n) →Degenerate
Development of Endosperm: The primary endosperm cell (PEC) in embryo sac divide again and again, and form triploid endosperm. The cells of endosperm are filled with reserve food material which is used for nourishment of the embryo during its development and also for the young seedling at the time of germination.
Development of Embryo: Embryo formation start after certain amount of endosperm is formed. Following are the stages in development of a dicotyledonous embryo.
the epicotyl and the portion below the level of cotyledons is hypocotyl.
Monocot Embryo: Monocot (Rice, Maize etc.) has one cotyledon called Scutellum. The embryonal axis has the radicle and root cap enclosed by a sheath called Coleorrhiza. The upper end (epicotyle) has plumule which is covered by hollow foliar structure called the coleoptile.
Polyembryony: Occurrence of more than one embryo in a seed, is known as polyembryony e.g. Orange, lemon, onion, mango, ground nut. It may be due to presence of more than one egg cell in the embryo sac or more than one embryo sac in the ovule.
Reasons of polyembryony: It is due to fertilisation of more than one egg cell in an ovule. The condition develop when an embryo sac contains more than one egg cell or ovule contain more than one embryo sac.
Seed: After fertilisation ovule mature into seed.
eg. pea, gram, ground nut.
Albuminous Seed: Those seeds, which retain a part of the endosperm because endosperm is not completely consumed by developing embryo.
eg. maize, wheat, sunflower, castor
Seed Dispersal: Seeds are dispersed to new habitat through agent like water, wind and animals.
Apomixis: Apomixis is a form of asexual repduction that mimics sexual reproduction where seeds are formed without fertilisation.
ADVANTAGES OF APOMICTIC SEED:
¶ No segregation of characters in hybrid progeny
¶ These seeds can be used to grow crop year after year
¶ These are economical as hybrid seed are not used to grow crops year after year.
Parthenocarpic fruits: The fruits which are formed (developed) without fertilisation are known as parthenocarpic fruit. Such fruits are seedless eg. Banana. This phenomenon of development of fruit without fertilisation is known as parthenocarpy.
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