REPRODUCTION IN LOWER AND HIGHER PLANTS - Notes for Class 12 Biology Chapter 1 : Maharashtra Board New Syllbaus

Notes, Imp Points, Definitions and Diagrams 

For Class 12th Biology Chapter 1 REPRODUCTION IN LOWER AND HIGHER PLANTS Based on HSC Syllabus

Introduction :-

Reproduction : - Reproduction is the production of young ones like parents.

Two categories of Reproduction :-

1) Asexual Reproduction         2) Sexual Reproduction

1.1 Asexual Reproduction

Definition : It is the process resulting in the production of genetically identical progeny from a single organism and inherits the genes of the parent.

Clones : Morphologically and genetically identical individuals.

Different Modes a Asexual Reproduction : - 

1) Fragmentation : - When multicellular organisms break into fragments, these fragments grow into new individuals.

Example - Spirogyra

2) Budding : - This mode of reproduction occurs by producing one or more outgrowths (buds) from the parent body. These buds on seperation develop into new individual.

Example - Yeast

3) Spore Formation : Reproduction that occurs by flagellated, motile zoospores which can grow independently into new individuals.

Example : Chlamydomonas .

Other methods of Asexual Reproduction 

• Binary Fission - Amoeba, Paramoecium
• Conidia Formation - Penicillium
• Gemmules Formation - Sponges

VEGETATIVE REPRODUCTION

This mode of reproduction occurs in plants through their vegetative parts.

New plants formed are genetically identical to their parents.

Artificial Methods of Reproduction in Plants :

A. Cutting : 

• The small piece of any vegetative part of a plant having one or more buds is used for propagation.
• Stem cutting - Rose, Bougainvillea , Leaf Cutting - Sansvieria , Root Cutting - Blackberry

B. Grafting :

Here parts of two plants are joined in such a way that they grow as one plant.

Process : Part of stem containing more than one bud (Scion) is joined onto a rooted plant called stock, is called Grafting.

When only one bud is joined on the stock , it is called bud grafting.

Eg: Apple, pear, rose, etc.

C. Tissue culture : 

In this method a small amount of plant tissue is carefully grown to give many plant lets.

Micropropagation method is also used now a days.

1.2 Sexual Reproduction 

It involves fusion of two compatible gametes or sex cells when the organisms reach maturity.

In Plants : End of juvenile or vegetative phase = beginning of reproductive phase.

(a. Juvenile phase – The period between birth and sexual maturity is called juvenile phase. In plants it is known as vegetative phase)

Reproductive Structure : Flower ( in Plants )

Function : Produce Haploid Gametes & Ensure fertilization takes place.

Two major events of sexual reproduction :- 

1) Meiosis 

2) Fusion of gametes

Three Stages of Sexual Reproduction :

1) Pre-fertilization 

2) Fertilization 

3) Post-Fertilization 

Androecium : Male Reproductive whorl of flower

Stamen : Individual member of Androecium

Parts of Stamen :

1) Filament 2) Connective 3) Anther

Structure of Anther :

 It is generally dithecous (having two lobes) & tetrasponrongiate (containing 4 pollen sacs)

In Monothecous anther contains two pollen sacs.

T.S of  anther-

It consist of anther wall and microsporangia ( pollen sacs).

There are four layers of the wall of mature anther.

 Epidermis : It is the outermost protective layer made of protective tabular cells.

Endothecium : Sub-Epidermal layer made of radially elongated cells with fibrous thickenings.

Middle Layer : Made of thin walled cells. It is multi-layered and surrounds microsporangium . It degenerates at maturity.

Tapetum :- It is the innermost wall layer which surrounds the sporogenous tissue of microsporangium.

Tapetum provides nutrition to developing pollen grains ( microspores ). 

Tapatem contain ubisch bodies which produce sporopollenin ( a component of exine).

B. Microsporangium ( pollen Sac ):

Microsporangium is present inner to the tapetum.

Initially it is made up of a compact mass of diploid sporogenous tissue. 

Cells of sporogenous tissue functions as microspore mother cells or pollen mother cells (PMC). 

At maturity diploid (2n) PMC undergoes meiosis to form four haploid (n) pollen grains (microspores). Process is called as microsporogenesis. 

Pollen grains are produced in the form of tetrad.

Structure of microspore: ( Pollen grain ) 

1) Typical pollen grain is a non-motile, haploid, unicellular body with single nucleus.

2) It is surrounded by a two layered wall - sporoderm.

Sporoderm consist of two layers :-

 1. Exine : Outer Layer, Thick, made of Sporopollenin.

It can be smooth or with sculptured pattern ( characteristic of species)

It is resistant to chemicals.

 2. Intine : Inner, made of Cellulose & Pectin.

Germ Pores :  At some places exine is very thin showing thin areas known as germ-pores.

Pollen viability - Viability is the functional ablity of pollen grain to germinate to develop male gametophyte) 

It depends upon environmental conditions of temperature and humidity. 

Eg. It is 30 minutes in rice and wheat.

 In some members of family Solanaceae, Rosaceae, Leguminosae, it lasts even for months

Development of Male Gametophyte

Pollen Grain = Beginning of male gametophyte

Steps of Development of male gametophyte : -

1. First Mitotic Division :- Production of bigger vegetative cell (rich in food) and small generative cell.

2. Second Mitotic Division : - Generative cell give rise to two non-motile male gametes,.

It takes place in pollen grain or pollen tube.

In Most Angiosperms, pollen grains are shed from anther at this stage.

Gynoecium : Female reproductive whorl of flower . (PISTIL)

Carpel/ Megasporophyll : Individual member of gynoecium.

Apocarpous : Many Free carpels            Syncarpous : Many carpels fused together.

Parts of Carpel : Ovary, Style & Stigma.

Structure of Anatropous Ovule

Anatropous Ovule :- Micropyle is directed downwards and Is present adjacent to the funiculus.

Ovule is attached to placenta by funiculus. Narrow Opening at apex of ovule is called micropyle.

Funiculus is attached to main body through hilum. Chalaza is Base of Ovule.

Central Parenchymatous Tissue : Nucellus

Nucellus is covered by two protective coverings (integuments) : Outer Integument & Inner Integument

Embryo Sac is present inside nucellus.

MEGASPOROGENESIS

Megasporogenesis : It is the process of formation of haploid megaspores from diploid megaspore mother cell (MMC).

Development Of Female Gametophyte :-

1) Megaspore mother cell undergoes meiosis to form linear tetrad of haploid cells i.e. megaspore. 

2) Upper three megaspores abort and lowest one towards centre of nucellus remains functional. 

3) It undergoes three successive, free nuclear mitotic divisions forming 8 nuclei, four located at each pole. 

4) One nucleus from each pole migrates towards the centre (polar nuclei).

5) Three nuclei towards micropylar end constitute egg apparatus. Large & Central - egg cell and two Supporting - synergid Cells.. Synergids contain filiform apparatus to guide pollen tube towards egg cell. 

6) Antipodal cells are group of three cells present at the chalazal end. 

7) The two haploid polar nuclei of large central cell fuse to form diploid secondary nucleus or definitive nucleus, just prior to fertilization. 

8) This seven- celled and eight nucleated structure is called an embryo sac. 

9) Female gametophyte is colourless, endosporic and is concealed in the ovule enclosed by ovary.

Monosporic Development - It is a  method of embryo sac development from a single megaspore. .

Endosporous Development :   The development of female gametophyte happens within the megaspore.(In angiosperms)

1.6 Pollination 

Pollination is the transfer of pollen grains from anther to the stigma of the flower.

TYPES OF POLLINATION :- 

Self Pollination :- Self pollination is a type of pollination which occurs in a single flower or two flowers on a single plant. (inbreeding or selfing.)

Cross Pollination :- Cross pollination is the transfer of pollen grains from the anther of

one flower to the stigma of another flower of different plants of same species.

 TYPES OF POLLINATION : - (on basis of source of pollination)

Points	Autogamy (self pollination) :	Geitonogamy :	Xenogamy (cross polination/ out breeding) :
Definition	It is a type of pollination in which bisexual flower is pollinated by its own pollen grains.	It is the transfer of pollen grain to a stigma of a different flower produced on the same plant.	It is a type of cross pollination when pollen grain of one flower is deposited on the stigma of a flower of different plant belonging to same species
Genetical Variation	Offsprings are genetically identical to their parents	Offsprings are genetically identical to their parents	Offsprings are not genetically identical to their parents .
Self/Cross pollination	A type of self pollination.	Similar to Autogamy (self pollination) and Xenogamy (cross pollination)	A type of cross pollination.
Pollinating Agents	Not involved	Involved	Involved
Example	Pea	Cucurbita maxima.	Papaya

For pollination, pollinating agents are required. They are of two types , 

Abiotic ( Wind, water, etc.)  & Biotic (Birds, Insects, Bats, Snails, etc.)

Abiotic Agents :  Non-living agents including wind and water.

1.       Pollination by Wind (Anemophily)

Found in Most important crop plants.

 

Adaptations in Anemophilous flowers :

1.       Flowers – small, inconspicuous and colourless.

2.       They lack nectar and fragrance.

3.       Pollen Grains – light, dry

4.       Large number of pollen grains are produced to increase chances of pollination.

5.       Stigma – feathery (traps pollen grains)

6.       Stamens – long filaments , Versatile anthers

7.       Stamens and Stigmas exposed to air

Eg:- Rice , Wheat, Maize, etc.

Fig :- Pollination by wind –  Maize

 

2.       Pollination by Water ( Hydrophily )

Found in 30 genera of aquatic monocots.

Adaptations in hydrophilous flowers :

1.       Flowers – Small , inconspicuous.

2.       Perianth and other floral parts unwettablev

3.       Pollen grains- long, unwettable (mucilage present)

4.       They lack nectar and fragrance.

Types of Hydrophily :-

1.       Hypohydrophily : Pollination occurs below surface of water.

Pollen grains are long, without exine, heavier than water , sink down to meet stigma of female flowers.

Eg: Zostera (sea horse)

2.       Epihydrophily :  Pollination occurs on the surface of water.

Specific gravity of pollen grains is equal to that of water, so they float on water.

Eg: In Vallisneria ,Female flowers reach water surface to ensure pollination.

 

Fig – Male and female plant Vallisneria

B. Biotic Agents : It Includes living agents like insects,birds,etc. About 80% plants require it.

 

1.       Pollination by Insects (Entomophily) :

Adaptations in entomophilous flowers :-

1)      Flowers – Large, brightly coloured.

2)      They have nectar glands and produce sweet smell (fragrance).

3)      Stigma – Rough (hair present) or Sticky (mucilaginous secretion)

4)      Pollen grains – spiny , surrounded by pollen kit

5)      Special adaptations in some plants e.g. turn-pipe mechanism in Salvia.

Eg: Rose, Jasmine, Cestrum, etc.

2.       Pollination by Birds (Ornithophily)

Few Birds specialized – like – Sun birds and humming birds . Small size , long beaks.

Adaptations in Ornithophilous Flowers :-

1.       Flowers – Large, brightly coloured.

2.       Secrete profuse, dilute nectar.

3.       Pollen grains – sticky and spiny.

4.       They don’t have fragrance .

Eg: Butea, bombax, Callistemon (Bottle Brush)

 

3.       Pollination by Bats (Chiropteryphily)

Bats can transport pollens over long distances.

Adaptations in Chiropterphilous Flowers :-

1.       Flowers – dull coloured

2.       Abundant nectar and strong fragrance.

3.       Large amount of edible pollen grains produced.

Eg: Anthocephalous (kadamb tree), Adansonia (Baobab tree), Kigelia (Sausage Tree), etc.

1.7 Outbreeding Devices (contrivances)

Outbreeding devices : Mechanisms that discourage or prevent self pollination.

I helps to increase genetic diversity which is an essential factor for evolution by natural selection.

Continued self pollination results in inbreeding depression.

Different inbreeding devices are :-

1) Unisexuality : Plant bears either male or female flowers. (dioecism).

eg: Mulberry, Papaya.

2) Dichogamy : Anthers and stigmas mature at different times in a bisexual flower.

Dichogamy is of two types : 

- 1) Protandry : Androecium matures earlier than gynoecium.

eg: disc florets of sunflower .

- 2) Protogyny : Gynoecium matures earlier than androecium.

eg : Gloriosa

3) Prepotency : Pollen grains of other flowers germinate rapidly over the stigma than the pollen grains from the same flower .

eg : Apple.

4) Heterostyly (heteromorphy) : There are two or more types of flowers in which stigmas and anthers are placed at different levels .

In heteromorphic flowers, pollen grains produced from anther pollinate stigmas produced at same level.

5) Herkogamy : Natural physical barrier between two sex organs and prevent contact of pollen with stigma of same flower . 

eg : Calotropis - Pentangular stigma is positioned above the level of anthers.

6) Self Incompatibility (self sterility) :  Genetic mechanism due to which the germination of pollen on stigma of same flower is inhibited.

eg: Tobacco, Thea.

1.8 Pollen-Pistil Interaction

All the events from the deposition of pollen grain on stigma to the entry of he pollen tube in the ovule ( synergid) are referred as pollen - pistil interaction. 

What happens when wrong type of pollen lands on a stigma of a flower ?

Answer : When the wrong type of pollen lands on the stigma it is discarded by pistil. The pistil has ability to recognise and accept the right or compatible pollen of the same species. It is done with help of special proteins . The process involves pollen recognition followed by promootion or inhibition of pollen.

 The journey of pollen-pistil interaction :

1) The compatible pollen absorbs water and minerals from the surface of the stigma, germinates and produces pollen tube.

2) The sigmatic surface provides essential prerequisites for a successful germination, which are absent in pollen grain.

3) The pollen tube is finally pushed through the ovule and reaches the embryo sac.

4) The tips of the pollen tube finally enters into one of the synergids and then ruptures to release the contents.

Sucrose :  Induces pollen germination & tube growth in vitro.

Boric Acid : Addition facilitates & accelerates pollen germination.

Artificial Hybridization : 

 Only the desired pollen grains are hand pollinated and used for fertilization. This is accomplished through emasculation and bagging procedure.

1.9 Double Fertilization

- It is a complex fertilization mechanism in which both the male gametes from a pollen grain participate.

Discovered by - Nawaschin - in liliaceous plants like Lilium & Fritillaria..

The growth of pollen tube is guided by chemicals secreted by synergids.

Entry of pollen tube into the ovule happens in 3 ways : -

1)  Porogamy :  Pollen tube enters ovule through the micropyle.

2) Chalazogamy :  Pollen tube enters ovule through the chalaza.

3) Mesogamy :  Pollen tube enters the ovule by piercing the integuments.

Note : Pollen tube penetrates embryo sac of ovule through it's micropyar end.

Siphonogamy :  When non-motile male gametes are carried through hollow pollen tube.

 Two events of sexual reproduction takes place in angiospermic plants.

1) Syngamy :  It is the fusion of haploid male gamete with haploid female gamete (egg) to produce a diploid zygote.

2) Triple fusion :  It is the fusion of haploid male gamete with diploid secondary nucleus producing primary endosperm nucleus  (PEN).

Diploid zygote develops into an embryo.

Primary Endosperm Nucleus develops into triploid endosperm.

Syngamy - Generative Fertilization

Triple Fusion :- Vegetative fertilization.

SIGNIFICANCE OF DOUBLE FERTILIZATION : - 

1) Unique feature of angiosperm . It ensures that parent plant invests a seed with a food store, only if the egg is fertilised.