CBSE Class 12 Biology Sexual Reproduction In flowering Plants Notes

Topics:

Flowers

Pre- fertilization- structure and events

Pollination

Fertilization

Double fertilization

Post fertilization- events

Development of seed and fruits

Apomixes

Polyembryony

Types of fruits

Introduction

Sexual reproduction in flowering plants involves transformation of diploid sporophytic cell into haploid gametophytic cells by meiosis.Fusion of haploid gametes of opposite sex to form diploid zygote.The zygote then develops into an embryo which ultimately forms a diploid plant body. In flowering plants all these steps of sexual reproduction occur within specialized reproductive organs called the flowers.

Pre- fertilization: Structures and Events

The parts of a stamen are filament and anther.It is bilobed and dithecous. The anther is a four- lobed structure consisting of four micro sporangia located at the corner, two in each lobe. Developed micro sporangia are called pollen sacs.

Structure of the pollen grains or male gametophyte

1. Wall layers: the outer hard layer-exine is made up of the most resistant organic material Sporopollenin which is resistant to high temperature, pH, and enzymatic action.

2. Exine has apertures called germ pores

3. Inner wall- intine is made up of cellulose and pectin 

Structure of an ovule or megasporangium
The ovule has a stalk-funicle- Hilum is the junction between ovule and funicle- two integuments envelop the ovule except at micropyle. Opposite end is called chalazal end. It encloses mass of cells called nucellus, with an embryo sac.

Megasporogenesis:
                             Nucellus
                                   ↓
                       Megaspore mother cell
                                   ↓
                             (One cell enlarges)
                                   ↓
                                 meiosis
                                   ↓
                             4-Megaspores
           3- degenerate and one functional megaspore remains.
                                   ↓ Nucleus divides mitotically
                                   ↓
           2 - Nucleate (move to opposite poles)
                                   ↓
                          II Mitosis
                      4 -Nucleate
                                   ↓
                                  III Mitosis

2) Geitonogamy
Transfer of pollen grain from the anther to the stigma of another flower in the same plant.

3) Xenogamy
Transfer of pollen grain from the anther to the stigma of a different plants.

Outbreeding devices
• Dichogamy – pollen release and stigma receptivity are not synchronized.
• Heterostyly – anther and stigma are placed at different positions.
• Self- incompatibility
• Production of unisexual flowers.

Pollen- pistil interaction
1) The pollen grains germinate in pollen tubes and the pollen tube passes through style, reaches ovary.
2) Enters ovule through micropyle with the help of synergids
3) Two male gametes are released.
(Artificial hybridization is achieved by emasculation and bagging techniques (removal of anther and dusting stigma with anther of desired type to prevent contaminations of its stigma with unwanted pollen)

Double fertilization
.One male gametes (n) + egg → (n) Syngamy (2n) → Zygote
Another male gamete (n) + polar nuclei (2n) → PEN(3X)
Primary endosperm nucleus develops into endosperm.

Post- fertilization events
A) Development of endosperm and embryo B) maturation of ovule into seed C) ovary into fruit.

Endosperm development
PEN undergoes repeated nuclear divisions to form into free- nuclear endosperm and if the cell wall formation occurs, forms to cellular- endosperm

Embryo development (Embryogeny)
Zygote → pro- embryo → globular embryo
                                             ↓
                                     Heart shaped embryo
                                             ↓
                                     Matured embryo

Dicot embryo consist of an embryonal axis and two cotyledons.

Monocot embryo
Single cotyledon (scutellum) in one side of the embryonal axis
• radical with root cap enclosed by coleorrhizae.
• Above the attachment of scutellum is, coleoptile.( covers plumule)

Residual, persistant, nucellus is called as perisperm
Ex: black pepper and beet The embryo may enter a state of inactivity called dormancy.seed coats near micropyle facilitate entry of oxygen and water during germination.
Embryo is dormant, till favorable conditions are onset.
Fruitis (Ripened Ovary) – the wall of the ovary called Pericarp.

Fruit

False fruit: if the parts other than ovary, becomes fruit it is false fruit.
Ex: Apple,Strawberry, cashew

True fruit: Ovary become fruit
Ex: Mango

Parthenocarpic fruit: Fruit developed without fertilization
Ex: banana

Importance of seeds
Dispersal to new habitat
Nourishing young seeding
Protection of new embryo
Leads to variation

Apomixis
Seed production without fertilization

Special modes of reproduction.
1) polyembryony
2) artificial reproduction involves emasculation, bagging and tagging techniques.

Fertilisation: Process of fusion of sperm with ovum

Site of fertilisation in human female: Ampullary - isthmic junction

Secretion of acrosome helps the sperm entry into cytoplasm of ovum through zona pellucida and plasma membrane. Sperm entry induce the completion of the 2nd meiotic division of secondary Oocyte.

Placenta: An intimate connection between foetus and uterine wall of the mother to exchange materials.

Function of Placenta : Nutrition, Respiration, Excretion, as barrier, Endocrine function.

Placenta as Endocrine tissue: Placenta Produces several hormones such as - Estrogen, hcG, hPL Progesterone and relaxin (in late phase of pregnancy).

Embryonic Development: at various month of Pregnancy After-1 month = Heart, 2 months = Limbs and digits, 3 months = External genital organ. 5 months = First movement, 6 months = body covered with fine hairs, eye lid, eye lashes, 9 months = Fully developed and ready for delivery.

POINTS TO REMEMBER

Blastula: A stage of embryogenesis which comes after morula and has a hollow fluid filled space called blastocoel

Endometrium: Innermost glandular layer lining the uterine cavity.

Foetus: An advanced stage of embryo within the uterus.

Gestation Period: A period between fertilisation of ovum and the birth of baby

Hymen: A thin membrane partially covering the vaginal aperture.

Implantation: Fixing of embryo fertilized egg in uterus. It leads to pregnancy

Menarche: The beginning of first menstron in female on attaining puberty

Menopause: Permanent cessation of mensual cycle in female. It occurs between the age 45 to 50 years in human female.

Oogenesis: Formation and development of ova in ovary

Ovulation: Process of release of mature ovum (Secondary oocyte) from the ovary 

Parturition Process of delivery of the foe Child birth)

Puberty A stage at which immature reproductive system of boy or girl becomes mature

Scrotum: A muscular pouch which houses two totoo

Spermatogenes's Pinces of fromation of snem fime male germ Cell in the tentes

 FLOWER
♦ 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)

Meiosis

Microspore (n)

Mitosis

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. 

Megasporogenesis

                    Megaspore mother cell (2n)
                                  ↓               Meiosis
                       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.

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 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.

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 donot germinate on the stigma of the same flowerSelf incompatibilityartificial HybridisationHeterostyly

Types of cross-pollination performed by man for cropimprovement. 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 notneeded. Flower bud bagged & when the stigmabecomes receptive, pollination is done using desired pollen & the flower is rebagged

Pollen –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

DOUBLE FERTILIZATION :- Fusion of one male gamete with egg and the other gamete with secondary nucleus

(forming 3n endosperm nucleus)

FISSION :- Fraction of nucleus with cytoplasm.

GOOTEE :- bark of healthy and woody branch for grafting.

HELOBIAL :- The mitosis is followed by cytokinesis forming two unequal cells. Subsequent divisions are free nuclear making the endosperm cellular after cytokinesis.

INCOMPATIBILITY :- The inability of certain gametes, even from genetically similar plant species to fuse with each other. This is also called intraspecific incompatibility, self sterality.

NUCELLUS:- The nucleus undergoes repeated divisions & nuclei so produced get arranged in the periphery leaving a large central vacuole-cytokinesis begins from the periphery towards the centre making it cellular at maturity. e.g. maize, wheat, sunflower.

PARTHENOCARPY :- Development of fruit in an unfertilized flower resulting in a seedless fruit. e.g. grapes, banana, tomato.

REPRODUCTION :- The process of producing offsprings and a means of self perpetuation.

Very Short Answer Type Questions

Question. Name the common function that cotyledons and nucellus perform.
Answer : The common functions that cotyledons and nucellus perform are as follows
(i) Storage of reserve food material.
(ii) Nourishment Cotyledons nourishes embryo and nucellus nourishes embryo sac.

Question. Complete the following flow chart
Answer : 

The process of formation of microspores from a Pollen Mother Cell (PMC) through meiosis is called microsporogenesis. The microspores, as they are formed, are arranged in a cluster of four cells i e . ., the microspore tetrad.
As the anthers mature and dehydrate, the microspores dissociate from each other and develop into pollen grains (male gametophyte). When the pollens mature, it contains two cells the vegetative cell (bigger) and generative cell (smaller).

Question. Indicate the stages where meiosis and mitosis occur (1, 2 or 3) in the flow chart.
Megaspore mother cell ¾¾1® Megaspores ¾¾2® Embryo sac ¾¾3® Egg
Answer : Megaspore mother cell (2n) Meiosis ¾¾¾¾® Megaspores Mitosis ¾¾¾® Embryo sac (n) Meiosis ¾¾¾¾® Egg.
The diploid Megaspore Mother Cell (MMC) undergo meiosis and forms a linear tetrad of four haploid megaspores. Three mitotic divisions, inside the functional (one) megaspore form
the embryo sac (eight haploid nuclei), while the other three megaspores degenerate.
The embryo sac is a seven-celled and eight nucleated structure. Three micropylar, Three chalazal and one central. The three micropylar cells are collectively known as egg-apparatus, which, consists of two synergids and one egg cell. While three chalazal cell form antipodal cell. The central cell is in the form of two nucleated cell till the fertilisation occurs and called as polar nuclei.  

Question. Name the component cells of the ‘egg-apparatus’ in an embryo sac.
Answer : The component cells of the ‘egg-apparatus’ in an embryo sac include, two synergids, one egg cell and the filiform apparatus.

Question. Name the part of gynoecium that determines the compatible nature of pollen grain.
Answer : The pistil has the ability to recognise, the pollen, whether it is of the right type (compatible) or of the wrong type (incompatible). If it is of the right type, the pistil accepts the pollen and promotes post-pollination events that lead to fertilisation. If the pollen is of wrong type, the pistil rejects the pollen. The ability of the pistil to recognise the pollen is followed by its acceptance or rejection. It is the result of a continuous dialogue between pollen grain and the pistil mediated by chemical components of the pollen interacting with those of the pistils.

Question. In the diagram given below, show the path of a pollen tube from the pollen on the stigma into the embryo sac. Name the components of egg apparatus.
Answer : Following compatible pollination, the pollen grain germinates on the stigma to produce a pollen tube through one of the germ pores. The contents of the pollen grain (2 mole nuclei) move into the pollen tube. Pollen tube grows through the tissues of the stigma and reaches the ovary.
After reaching the ovary, pollen tube, enters the ovule through the micropyle and then enters the embryo sac by passing into one of the synergids through the filiform apparatus. It leads to the degeneration of that synergid.
The pollen tube breaks to release its contents (2 male nuclei). Out of the two male gametes one fuses with egg and the other fuses with central cell and fertilise. The component cells of the egg-apparatus in an embryo sac include, two synergids, one egg cell and the filiform apparatus.

Question. Can an unfertilised, apomictic embryo sac give rise to a diploid embryo? Ifyes,  then how?
Answer : Yes, if megaspore develops into embryo sac without meiotic division egg will be diploid.
Diploid egg develops into embryo by mitotic divisions.
Note Apomixis is a form of asexual reproduction to produce seeds without fertilisation.

Question. Which are the three cells found in a pollen grain when it is shed at the three celled stage?
Answer : In over 60% of angiosperms, pollen grains are shed at the two cell further stage (vegetative cell and generative cell). In the remaining species, the generative cell divides mitotically to give rise to the two male gametes before pollen grains are shed at the (three-celled stage (are vegetative cell and two male gametes).

Short Answer Type Questions

Question. Does self-incompatibility impose any restrictions on autogamy? Give reasons and suggest the method of pollination in such plants.
Answer : Self-incompatibility imposes restrictions on autogamy. The season far this may be as Majority of flowering plants produce hermaphrodite flowers and when pollens grains comes in contact with the stigma of the same flower to continue self-pollination. Such type of continued self-pollination result in inbreeding depression. That’s why flowering plants have developed many devices to discourage self-pollination and to encourage cross-pollination. One of the major way to prevent self-pollination is self-sterility.
Self-sterility in some bisexual flowers, if the pollen grains fall on the stigma of the same flower, germination does not occur. But the same pollen grains germinate when they fall on
the stigma of other flowers of the same species. It is a genetic mechanism to prevent self pollination.

Question. In the given diagram, write the names of parts shown with lines.  

Answer : Embryos of monocotyledons possess only one cotyledon. In the grass family the cotyledon is called scutellum that is situated towards one side (lateral) of the embryonal axis. At its lower end, the embryonal axis has the radical and root cap enclosed in an undifferentiated sheat called coleorrhiza. The portion of the embryonal axis above the level of attachment of scutellum is the epicotyl. Epicotyl has a shoot apex and a few leaf primordia enclosed in a hollow foliar structure, the coleoptile.

Question. Why does the zygote begin to divide only after the division of Primary Endosperm Cell (PEC)?
Answer : The primary endosperm cell divides repeatedly and forms a triploid endosperm tissue. The cells of this tissue are filled with reserve food materials and are used for nutrition of the developing embryo. Embryo develops at the micropylar end of the embryo sac where zygote is situated. Most zygotes divide only after certain amount of endosperm is formed. This is an adaptation to provide assured nutrition to the developing embryo.

Question. The generative cell of a two celled pollen divides in the pollen tube, but not in a three-celled pollen. Give reasons.
Answer : In over 60 % of angiosperms, pollen grains are shed at this 2-celled stage tube cell or vegetative cell generative cell. In the remaining species, the generative cell divides mitotically to give rise to the two male gametes before pollen grains are shed tubecell or vegetative cell two male gamete 3-celled stage.
In 3 celled stage, the pollen grains further germinate on the stigma to produce pollen tube through ane of the germ pores. The contents of the pollen grains move into the tube pollen tube grows through the tissues of the stigma and style and reaches to ovary.
Inplants, when pollen grain are shed at 2 celled stage the generative cell divides and form two male gametes during the growth of the pollen tube in the stigma.

Question. In the figure given below label the following parts-male gametes, egg cell, polar nuclei, synergid and pollen tube
Answer : The following are the parts of this figure 

Important Questions for NCERT Class 12 Biology Sexual Reproduction in Flowering Plants

Ques. In a cereal grain the single cotyledon of embryo is represented by
(a) coleoptile (b) coleorhiza
(c) scutellum (d) prophyll.

Answer: C

Ques. The embryo in sunflower has
(a) two cotyledons (b) many cotyledons
(c) no cotyledon (d) one cotyledon. 

Answer: A

Ques. Embryo sac occurs in
(a) embryo (b) axis part of embryo
(c) ovule (d) endosperm. 

Answer: C

Ques. Perisperm is
(a) remnant of endosperm
(b) persistent nucellus
(c) peripheral part of endosperm
(d) disintegrated secondary nucleus. 

Answer: B

Ques. Tegmen develops from
(a) funiculus (b) chalaza
(c) inner integument (d) outer integument.

Answer: C

Ques. Seed formation without fertilisation in flowering plants involves the process of
(a) somatic hybridisation
(b) apomixis
(c) sporulation
(d) budding. 

Answer: B

Ques. Nucellar polyembryony is reported in species of
(a) Citrus (b) Gossypium
(c) Triticum (d) Brassica. 

Answer: A

Ques. What is common between vegetative reproduction and apomixis?
(a) Both are applicable to only dicot plants.
(b) Both bypass the flowering phase.
(c) Both occur round the year.
(d) Both produce progeny identical to the parent.

Answer: D

Ques. Apomictic embryos in Citrus arise from
(a) synergids
(b) maternal sporophytic tissue in ovule
(c) antipodal cells
(d) diploid egg. 

Answer: B

Ques. In a type of apomixis known as adventive embryony, embryos develop directly from the
(a) nucellus or integuments
(b) zygote
(c) synergids or antipodals in an embryo sac
(d) accessory embryo sacs in the ovule. 

Answer: A

Ques. Adventive embryony in Citrus is due to
(a) nucellus (b) integuments
(c) zygotic embryo (d) fertilized egg. 

Answer: A

Ques. The polyembryony commonly occurs in
(a) tomato (b) potato
(c) Citrus (d) turmeric. 

Answer: C

Ques. Study of formation, growth and development of new individual from an egg is
(a) apomixis (b) embryology
(c) embryogeny (d) cytology. 

Answer: B

Ques. Nucellar embryo is
(a) amphimictic haploid (b) amphimictic diploid
(c) apomictic haploid (d) apomictic diploid.

Answer: D

Ques. Development of an organism from female gamete/ egg without involving fertilization is
(a) adventitive embryony
(b) polyembryony
(c) parthenocarpy
(d) parthenogenesis. 

Answer: D

Ques. Formation of gametophyte directly from sporophyte without meiosis is
(a) apospory (b) apogamy
(c) parthenogenesis (d) amphimixis. 

Answer: A

Ques. Prothallus (gametophyte) gives rise to fern plant (sporophyte) without fertilization. It is
(a) apospory (b) apogamy
(c) parthenocarpy (d) parthenogenesis.

Answer: B

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