REPRODUCTION IN PLANTS (12th BIOLOGY)
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Topic List
 Theory
1.
Introduction
2.
Modes of reproduction: (Asexual and sexual)
3.
Sexual Reproduction In Angiosperms
4.
Development Of Female Gametophyte
5.
Pollination : Types and agencies
6.
Fertilization
7.
Post - Fertilization Changes
 
Short Q & A
1.
Short Question Answer Page
 
Exercise
 
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3. ATP (Adenosine triphosphate)
1.The plant body in angiosperms is Sporophyte which is diploid in nature and reproduces sexually.

2.In angiosperms, flower is the highly modified and condensed shoot meant for sexual reproduction.

3. During reproduction, the flower produces pollen grains by microsporogenesis and megaspores by megasporogenesis after meiotic or reduction division.

4. The pollen grain is the first cell of male gametophyte and megaspore is the first cell of female gametophyte.

5. Then through the process of pollination male gametes and female garnets are brought close to each other.

6. Male gametophyte produce male gamete and famale gametophyte produce female gamete.

7. The male and female gametes fuse together i.e. fertilization and form a diploid zygote which is the first cell of next sporophytic generation.

8. After fertilization the ovary is converted into fruit and ovules into seed.

In angiospertnic flowers, androecium and gynoecium are the necessary or essential floral whorls as they are directly involved in sexual reproduction.

Androecium (Andros - male) :

1. It is male reproductive whorl, made up of units called stamens or microsporophylls.

2. Each stamen consist of filament, anther and connective.

                 

3. Filament is'basal, slender stalk at the apex of it sac like fertile or genesious part anther is present.

4. Anther with two lobes is called dithecoHS or bithecous anther and with one lobe is called monothecous anther.

5. Two lobes of anther are connected together by sterile narrow strip of parenchymatous tissue called as connective.

6. Each anther lobe has two chambers or sacs internally called pollensacs or microsporangia or pollen chamber which contain pollen grains.

7. Microsporogenesis i.e. formation of microspores or pollen grains takes place inside microsporangia.

8. A sterile stamen, (without pollen grains) is called staminode.

9. Function : In anther, pollen grains or microspores are produced which forms male gametes.

Types of attachment of Anther to filament:

                       

1. Basifixed or innate anther : The filament is attached to the base of anther, eg. mustard, raddish, sunflower.

2. Adnate anther : The filament is attached along entire length of anther from base to apex.

eg. Michelia, Magnolia.

3. Dorsifixed anther: The filament is attached to the back or dorsal side of anther.

eg. Chinarose, Passion flower.

4.Versatile anther : The filament is attached to the dorsal side of anther at one point in the middle so that anther can move freely in air. eg. Maize, grasses, jowar Gynoecium or Pistil (gyne - female) :

1. It is female reproductive whorl, made up of units called carpels or megasporophylls.

2. Each carpel consist of ovary, style and stigma.

3. Ovary is basal swollen portion present on thalamus having one or more chambers or loculi internally which contains one to many Ovules attached to cushion like Placenta.

4. After fertilization, ovary gives rise to fruit and ovule give rise to seed.
                       
5. Stigma is terminal, receptive part which receives and nourishes pollen grains and also provides site for germination of pollen grains.

6. A sterile carpel is known as Pistillode.

7. Function : It produces ovules which contain egg or female gamete.

Dioecious plant : Male and female flowers are present on different plants.

eg. Papaya, Mulberry, datepalm.

Monoecious plant: Male and female flowers are present on same plants.

eg. Hibiscus, Datura, Maize, jowar etc.

Polygamous plant : Plants have more than two types of flowers i.e. unisexual, bisexual, neuter.

eg. Mango.

Neuter flowers : Flower in which both essential whorls are absent. Only meant for attracting insects for pollination.

Development of Gametophytes (Preferfiiization changes) :

In sexual reproduction, the gamete producing structures are called as gametophytes which are highly reduced and dependent on sporophyte.

All the changes which leads to the formation of male or female gametophyte before fertilization are called as Pre-fertilization changes.

Stamen is a male reproductive organ of flower and consist of filament, connective and fertile or genesious part anther.

Anther:

1. A typical anther is a bithecous anther made up of two anther lobes connected by connective.

2. Each lobe is further divided into two pollen sacs or pollen chambers or microsporangia.

3. Thus, there are totally four pollen sacs or microsporangia in an anther, hence also called as tetrasporangiate or tetralocular anther.

4. Anther is a fertile part which produces haploid pollen grains or microspores.

T.S. Of Anther :

1. In T.S., Mature anther shows following parts :

I) Anther wall,

II) Pollen sacs or Microsporangia.

I. Anther wall:

i) It is the outermost covering of anther,

ii) Anther wall consist of - outermost epidermis, Hndothecium, 2-3 middle layers and innermost tapetum.

                     

1. Epidermis:

1) It is single layered, outermost covering of anther.

2) It is protective in nature and made up of flattened parenchymatous cells.

2. Endothecium:

i) It is present just below epidermis and made up of parenchymatous cells.

ii) Normally, it is single layered but in some plants; it is multilayered.

iii) It is also called as fibrous layer as the cells'develops fibrous thickenings of callose.

iv) It is hygroscopic in nature and helps in the dehiscence of anther.

v) The endothecial cells situated in the shallow groove between two pollen sacs remain thin walled and represent the line of dehiscence called as stomium.

3. Middle layers:

i) Inner to endothecium, 2-3 layers of parenchyma cells are present called as middle layers.

ii) It surrounds each pollen sac.

iii) The cells of this layer degenerates at maturity i.e. after the formation of microspore.

iv) Due to this the two pollen sacs merge to form one pollen chamber in each lobe.

4. Tapetum :

i) It is the innermost, single layered which covers the pollen sacs.

ii) It is made up of large elongated cells with prominent nuclei and dense cytoplasm

iii) It is nutritive layer and provides food material to developing pollengrains.

iv) It may be multinucleate with one or more diploid nuclei or have large polyploid nucleus,

v) It also produces ubisch granules which forms sporopollenin i.e. component of exine of pollen grain.

II. Pollen sacs :

i) In dithecous (typical) anther, there are four pollen sacs or microsporangia.

ii) Young pollen sac contains diploid cells called as pollen mother cells (PMC) or Microspore mother cells (MMC).

iii) Each pollen mother cell divides by meiosis or reductional division to form haploid pollengrains or microspores.

iv) The two anther lobes are connected by connective which is formed by parenchyma. The connective contains a vascular bundle which conducts water and food to anther.

Microsporogenesis in Angiosperms :


The process of formation of microspores (Pollen grains) from pollen mother cells by meiosis is called microsporogensis,

1. The process of microsporogenesis occurs in pollen sacs.

2. Each functional pollen mother cell by reduction division produces four microspores or pollen grains.

3. Initially, these microspores are in tetrads i.e. in group of four pollen grains enclosed in a common wall called tetrahedral tetrad. Later on they separate from each other.

Structure of pollen grain :

1. Pollen grains or microspores are unicellular, uninucleated and haploid,

                                    

2. They are small in size (0.025 to 0.125 mm in diameter) and vary in shape, generally spherical, oval, pyramidal, polyhedral, lobed, triangular etc.

3. The wall of pollen grain is called sporoderm which is made up of two layers, outer is exine and inner is intine.

a) Exine:

i) It is outermost, thick, hard and protective convering.

ii) Exine may be spiny (in insect pollinated plants) or smooth (in wind pollinated plants),

iii) There are few thin, circular areas (opening) in the exine called as germpore. When the thin areas are elongated slit like called as germ furrows. In regions of germ pore, exine is either absent or very thin.

iv) The pollen grains in monocots are mostly uniporate with single germfurrow and with three germpore i.e. triporate in dicots.

iv) Exine is made up of a complex substance called Sporopollenin which is resistant to chemical, physical and biological decomposition.

b) Intine:

i) Intine is inner, thin, delicate and smooth layer.

ii) Intine is made up of cellulose and pectin.

iii) It forms pollen tube during germination of pollen grain,

iv) Intine of pollen grain encloses dense cytoplasm and prominent haploid nucleus in the centre.

Functions:

1. Pollen grains germinate to form male gametophyte containing two non-motile male gametes.

2. Exine and intine both are protective in function.

3. Intine comes out through germ pore forming pollen tube during development of male gametophyte.

Development of male gametophyte (Microgametogensis) :

1. The male gametophyte or microgametophyte develops from haploid pollen grain/microspore.

2. Pollen grain is the 1st cell of male gametophyte. Also called as partially developed male gametophyte.

3. The process of development of mature male gametophyte from pollen grain is called microgametogenesis.

4. The development of male gametophyte starts in pollen sacs of anther before pollination and completes on the stigma of gynoecium after pollination.

5. All the stages of development of male gametophyte before pollination are called Pre-pollination stages and all the stages after pollination are Post -pollination stages.

                            

Stages in the development of male gametophyte:

A. Pre-pollination stages (Before pollination in pollen sac):


i) Development of male gametophyte is 'in situ' i.e. it begins within pollen sac. It is also called as Precocious it means further development of any structure occurs in a place where it is formed.

ii) The unicellular, uninucleate, haploid pollen grain undergoes mitosis to form two Unequal cells.

iii) The larger cell is called as vegetative or tube cell and smaller cell is called as generative cell.

iv) Generative cell has large nucleus, thin cytoplasm. Vacuole and reserved food is absent in it.

v) The vegetative cell has large vacuole, cytoplasm, large nucleus and reserve food material.

vi) Initially generative cell is attached to wall of pollengrain. Soon, the generative cell loses contact with the microspore wall to separate from it and may lie freely in the cytoplasm of tube cell.

vii) At this 2-celled stage of pollen grains, anthers dehisce and pollen grains are released from anther and transferred to receptive stigma during Pollination.

viii) Such two celled pollen grain is also called as young or partially developed male gametophyte.

B. Post-pollination stages (After pollination on stigma) :

i) The pollen grains falls on right stigma after pollination, then stigma secretes water and other sugary substances which stimulates the germination of pollen grains.

ii) Pollen grains absorbs the sugary liquid secreted by stigma through its germ pore.

iii) As a result the volume of cytoplasm increases which exerts pressure on intine of pollen grain.

iv) Intine comes out through the germ pore in the form of short tube called pollen tube or germ tube.

v) Pollen tube grows through stigma and enters the stylar canal towards ovules in the ovary.

vi) In the style, the pollen tube may pass through stylar canal. If it is solid, it secretes pectinase and other hydrolytic enzymes for making passage.

vii) The pollen tube absorbs nourishment from the cell of style for its growth.

viii) The nucleus and cytoplasm of tube cell and generative cell migrates towards pollen tube.

ix) The generative cell divides by mitosis to form two Male gametes or sperms which are Unicellular, non-motile with single haploid nucleus.

x) The structure i.e. pollen tube along with it's cytoplasm, tube nucleus and two non-motile haploid male gametes is called Male gametophyte.

xi) In certain cases, the tube nucleus degenerates even before the formation of pollen tube or before fertilization rather said that the growth of pollen tube is controlled by tube nucleus.

xii) In angiosperms, the male gametophyte is highly reduced, microscopic only three celled - (one tube cell and two male gametes), endosporic (develops within the spore) and totally dependent on sporophyte. It is formed by one meiosis and two mitosis.

xiii) In some plants, prior to pollination, the generative cell, when pollen is still within the anther lobe divides and produces two male gametes. The pollen grains of such plants are shed at three celled stage.
 
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