Sexual Reproduction in Flowering Plants, Chapter Notes, Class 12, Biology, Part -1 PDF Download

REPRODUCTION IN FLOWERING PLANTS

INTRODUCTION

Angiosperm originated in Mesozoic era.

Angiosperm originated either in the begining of Cretaceous period or in ending of Jurassic period of Mesozoic era. It means they are originated between Cretaceous and Jurassic period on the earth.

Angiosperm dominated over the earth in Coenozoic era. So this era is known as "Golden Period of Angiosperms".

First of all N. Grew realized the fact, that Stamens are male sex organ of flower (Anatomy of plants)

Sexuality in plant first of all reported by Jacob Camerarius.

He reported Anthers are the male sex organ and Ovary with style and stigma are female sex organ and for the formation of seed, interaction is essential in between both the sex organs.

Significance of pollination and role of insects in pollination was recognized by Josheph Kolreuter.

C.F. Wolf – Father of plant Embryology.

Prof. P. Maheshwari – Father of Indian plant Embryology. He wrote a book – 'An Introduction to Embryology of Angiosperms'.

Classification

Class         -            Dicotyledonae

Subclass    -            Polypetalae

Series        -            Thalamiflorae

Order        -            Parietals

Family       -            Cruciferae or Brassicaceae

Genus       -            Capsella

Species      -            Bursa pastoris [Common name "Shepherd's purse"]

Most of the important Angiospermic characters are found in Capsella so that for the study of Angiosperms, it is considered as a "Typical Angiosperm".

It is an annual plant and grows as weed during the winter season in the field.

The main plant of the Capsella is a sporophyte. Which is diploid and it is differentiated into root, stem and leaves.

Capsella is a heterosporous plant it means there are two different types of spores are formed in the life cycle which is classified into two categories in which male spores are called Microspores and female spores are called Megaspores.

The process of reproduction takes place in this plant through a special structure, called flower.

Calyx, Corolla, Androecium and Gynoecium are present in the typical or complete flower.

The calyx and corolla are termed accessory whorls of the flower. Because these structures do not participate in the process of reproduction, only helps.

The androecium and gynoecium are known as essential whorls, because they are directly related with the reproduction.

Monocarpic Plants :

The plants in which flowering and fruiting takes place only once in the whole life cycle are called monocarpic e.g. Annual & Biennial plants.

Polycarpic Plants :

The plants in which flowering and fruiting takes place many times in the entire life cycle are known as polycarpic e.g. Perennial plant.

Bamboo, Palms, Banana, Centuray plant (Agave Americana) are perennial plants but they are the example of monocarpic plants.

Reproduction in Flowering Plants

"Reproduction is one of the important processes by which every living organism make a copy of itself. It is the means of multiplication and perpetuation of species because the older individual of each species undergo senescence and die"

All the reproductive methods of plants are broadly categorized into two types -

Sexual Reproduction

Asexual Reproduction

Sexual Reproduction

In Angiosperms male and female gametes are formed in male and female sex organs by the process of meiosis. Both the gametes fuse together to form a diploid zygote which gives rise embryo. It means the process in which embryo is formed by meiosis and fertilization is called Amphimixis.

Male reproductive organ - Androecium

Male reproductive organ is called androecium and their unit is called stamen. Stamen is also known as microsporophyll. There are 6 stamens are present in Capsella.

A typical stamen is differentiates into three parts –a long, thin structure is called filament which joins the stamen to the thalamus. The free end of the filament, a swollen spore bearing structure is called anther.

Anther and filament are attached together with help of small region, called connective. Connective contains vascular tissues. The main parts of the stamen is the anther.

Each anther generally bilobed structure i.e. anther has two anther lobes .

Each lobe of anther has two chambers (Dithecous) which are called pollen sacs or microsporangia or pollen chambers.

Therefore, a typical anther has four pollen sacs is called tetrasporangiate.

Pollen grains are formed inside the pollen sac through the meiotic division of pollen mother cells.

Pollen grains are formed inside the pollen sac through the meiotic division of pollen mother cells.

At the maturity of the pollen grains, sterile tissue degenerate which are present in between the pollen sacs. Both the pollen sac fused together. Because of this reason, only one chamber appears in each anther lobe at maturity. So two chambers are seen in the mature anther at the time of dehiscence.

In Capsella, which is member of the cruciferae or Brassicaceae, anther are dithecous and tetrasporangiate type.

But in Malvaceae, the anther of stamen has only one theca in each anther lobe. This is called monothecous and it contains only two pollen sacs called Bisporangiate.

STRUCTURE OF ANTHER :

The development of anther in origin is Eusporangiate type i.e. It is developed from more then one archesporial cells.

In the transverse section of anther, it is seen almost spherical.

The following structures are present in the anther :-

Epidermis :- It is the outermost layer of anther. It is single celled thick and continuous layer but not archesporial in origin. It forms the outermost protective layer.

In Arceuthobium (Smallest Parastic Angiosperm) fibrous thickening present in epidermis so it is called exothecium.

Endothecium :- This layer is present below the epidermis. It is single celled thick layer. During the maturation of anther, various changes takes place in different walls of cells of endothecium. The outer wall of these cells remains thin walled, but inner walls and radial walls become thick due to thickening of a-Cellulose fibers. Callose bands are also present along the radial walls. At some places callose bands and fibrous thickening are absent. These places are called stomium. The dehiscence of anther takes palce only from these places. Endothecium becomes hygroscopic nature due to presence of fibrous thickening. So it helps in dehiscence of anther.

Middle layer :- Middle layer is consist of parenchymatous cells. This layer is one to three celled thick structure. Food is stored by parenchymatous cells of this layer. Middle layer is ephemeral in nature and absent in a mature anther.

In Holoptelia plant 3 to 4 called thick middle layer is present.

In Najadaceae & Lemnaceae families middle layer is absent.

In wolffia middle layer is absent.

Tapetum :- It is the inner most layer which acts as nutritive layer. It is known as tapetum. Pollen sacs surrounded by tapetum. This is also single celled thick layer. The cells of the tapetum initially diploid but they become polyploidy due to endomitosis (Chromosome duplication without nuclear division). It means these cells contain many chromosomes and sometime binucleate
(As karyokinesis not followed by cytokinesis)

Tapetum absorbs food from the middle layer and provide nutrition to the microspore mother cells or microspores. The cells of tapetum secrete hormones and enzymes. The tapetum layer disappears in the mature anther.

Note : In Nicodia and Costum plants, tapetum is multilayered.

Pollen sacs : Four Pollen sacs are present in the anther. Pollen sacs are also known as microsporangia. Inside the pollen sacs, microspores are formed by the meiotic divison of microspore mother cells.

TAPETUM IS OF TWO TYPES :-

Amoeboid Tapetum/Invasive Tapetum/Periplasmodial Tapetum :

It is found in primitive Angiosperm. Such type of tapetum absorb all foods from the middle layer. So middle layer immediately degenerates. In the beginning, all food materials stored by tapetum. Tapetal cells convert the absorbed food into special food granules called protoplast bodies. The innermost layer of tapetum dissolve and release its protoplast into the cavity of the microsporangium. Now inside the pollen sacs protoplast bodies are known as periplasmodium. Microspore mother cells are surrounded by periplasmodium and provides nourishment to the developing microspores. This type of tapetum provide nutrition to the microspores after degeneration.

Example : Typha, Alisma and Tradescantia.

Glandular or Secretory Tapetum :

It is developed type of tapetum. It is not degenerates quickly. It absorbs nutrients from the middle layer and secreted into the cavity of the microsporangia (Pollen sacs) e.g. Usually it is found in most of Flowering plants (Capsella).

Before degeneration of cells of tapetum, they form special granules called Proubisch bodies in cytoplasm. Proubisch bodies transfer between cell wall and cell membrane of tapetal cells. Here they are surrounded by Sporopollenin. Now they are called Ubisch bodies or orbicules. At last tapetum degenerates and ubisch bodies released into pollen sacs.

Generally, sporopollenin participates in the formation of outer covering (Exine) of Pollen grains.

Tapetum helps in transfer of food, storage of food, formation of sporopollenin and pollenkitt materials.

MICROSPOROGENESIS :

The anther appears as outgrowth like structure in the initial stage which shows spherical or oval shaped structure.

At this stage, it is a mass of undifferentiated and homologous meristematic cells which is surrounded by a single cell thick outer layer. This layer is known as epidermis. First of all vascular tissue are formed in middle region Simultaneously four cells located just below the epidermis in vertical rows in the region of hypodermis at the four corners are become large has visible nucleus with dense cytoplasm. Due to this reason they are different from the rest of the cells. These cells are called archesporial cells.

These cells divide periclinally to form primary parietal cells below the epidermis and primary sporogenous cells towards the centre. Both of the cells usually undergo further divisions to form complete structure of anther except epidermis.

Primary parietal cells undergo further periclinal and anticlinal division to form a series of 3-5 layers making the walls of the anther.

Out of them outer most layer of anther is formed just below the epidermis by primary parietal cells is called Endothecium or fibrous layer. The endothecium is followed by 1-3 celled thick layer is termed middle layer. The innermost layer of the anther which surrounds pollen sacs, is called tapetum. Later the tapetal cells play a significant role during the meiotic cell division in microsporogenous cells and in pollen development.

The primary sporogenous cells divide twice or more than two times by mitotic division to form sporogenous cells and later sporogenous cells differentiated into microspore mother cells (MMC / PMC) during the formation of wall of pollen sac.

Each microspore mother cell divide to form four haploid microspore or pollen grain by meiotic division or reduction division.

During this period spherical bodies are formed inside the tapetal cells before their disintegration. These spherical bodies are known as Ubisch-body. Ubisch body is made up of a complex substance called sporopollenin. It is biopolymer (Heteropolysaccharide)..    

After the formation of ubisch body, the tapetum layer degenerates. Ubisch bodies participate in the formation of exine of the microspores inside the pollen sacs. Now thick walled microspores are called pollen grains.

At the initial state all four microspores are attached together with the help of callose layer. This group of microspores is called tetrad. After some time, this callose layer dissolve by callase enzyme. Which is secreted by tapetum.   

Normally each microspore mother cell can form tetrad by meiotic division. But in some plant like Zostera, some microspore mother cells become sterile and provide nutrition to rest of microspore mother cells.

Similarly, tapetum is not well developed in Gentianaceae family so some cells of sporogenous tissue become sterile and provide nutrition to remaining sporogenous cells.  

Types of tetrads : The arrangement of the microspores in tetrad condition as follows :-

Tetrahedral tetrad : Four haploid microspores arranged in tetrahedral form.         

Example : Dicotyledons – (Capsella)

Isobilateral tetrad : This condition is found in monocotyledons. Microspores are arranged at the lateral side of each other.

Decussate tetrad : In this two microspores lies at the right angle of other microspores

Example : Magnolia

T-Shaped tetrad : Two microspores lie longitudinally and two microspores lie at transversly in this type of tetrad.

Example : Aristolochia & Butomopsis.

Linear tetrad : In this tetrad all four pollens arranged in linear order. e.g., Halophylla, Halophia.

All the above type of tetrads are found in Aristolochia elegans.

Most common type of tetrads is Tetrahedral.

Structure of Microspore or Pollen Grain :

Pollen grain is the first cell of a male gametophyte.

Pollen grain is termed as immature male gametophyte. Usually, they are in round shape. Pollen grain surrounded by two distinct layers. The outer layer (wall) is thick, rigid and ornamented, called exine. This layer is formed by cutin and sporopollenin. Sporopollenin is Highly resistant material (Resistant to temperature, ph, Enzyme, Electric shock etc.)   It is nonbiodegradable.

The internal layer is thin, soft and elastic in nature. It is called intine. It is made up of pectin and Cellulose or pecto-cellulose.

The number of germ pore, structure and ornamentation of exine is a significant feature of taxonomy.

A detail study of pollen grains is called Palynology.

Three colpus type (slit type) of germpore are present in pollen grain of Capsella. This type of pollen grains are called tricolpate. Only one germ pore is present in monocots and pollen grain is called monocolpate.

The plants in which pollination takes place by insects, their pollen grains having oily layer around the pollen grain. It is called pollen kitt. It is composed of lipids and carotenoids.  

Function of pollen kitt :

This oily layer protects the pollen grain from the harmful ultraviolet rays.

Its sticky surface helps to attach with the insects.

Its yellow colour attracts the insects. Pollen kitt is present on the pollens of Capsella.

Type of Germpore :  

Dehiscence of Anther :          

During the maturation of anther, various changes are takes place in walls of anther. 

In the begining, middle layer degenerates due to absorption of food by tapetum.      

 When the micropores are formed inside the pollen sacs, at the same time ubisch bodies are formed in cells of tapetum, then after it degenerates. Ubisch bodies participate in the formation of exine of pollens.

In this way, in a mature anther only two layers epidermis and endothecium are present in the form of outer covering.         

The sterile tissues are present between both the pollen sacs of each anther lobe degenerate. So both pollen sacs of the each anther lobe fuse together to form single pollen sac.  

Therefore, in the T.S. of mature anther only two pollen sacs are present.                

Dehiscence of anther takes place during the dry season. Due to the hygroscopic nature of endothecium loss of water takes place from the cell of endothecium.         

Walls of endothecial cells try to contract due to the loss of water but inner and radial walls do not contract due to presence of fibrous thickening whereas outer thin walled cells of endothecium contract and become concave or incurved. 

Incurving of outer walls creates pulling force or tension over the entire surface of anther. Due to tension, thin walled stomial cells breaks off and dehiscence of anther takes place and pollen grains are present in pollen sacs released into the atmosphere.       

Dehiscence of anther in Angiosperms either longitudinal or Apical pore or Transverse or Valvular type. Dehiscence of anther of Capsella is longitudinal.      

Micro-Gametogenesis or Development of Male Gametophyte :      

In flowering plants, pollen grain or microspore considered as first cell of male gametophyte. Germination or development of pollen grain takes place before dehiscence of anther is called precocious development. Development of pollen also takes place at mother place [inside pollen sac of anther] is called In-situ development.        

Pre Pollination development -           

In the beginning of the process, only nucleus of pollen grain divided by unequal mitotic division, resulting two unequal size of nucleus are formed. Small nucleus present near the walls is called generative nucleus and large nucleus present inside the cytoplasm is called tube or vegetative nucleus.

Both the nucleus surrounded by cytoplasm and it becomes dense, then again followed by unequal cytokinesis, resulting two unequal size of the cells are formed.

Larger cell in which large nucleus is present known as Vegetative cell and smaller cell in which small nucleus is present, called generative cell.

Now pollen grains come in bicelled and binucleated stage. In 60% Angiosperms pollination of pollen grains take place in bicelled and binucleated stage. While in 40% in 3 celled, 3 nucleate stage. The development of gametophyte take place inside the pollen grain is also known as endosporic development.

This stage of pollen grain is called immature or partially developed male gametophyte.

Generative cell detached from the wall and changed into vermiform or spindle shaped structure and enter inside the vegetative cell.

Post Pollination development -          

Rest of the further development of pollen grain [Immature male gametophyte] takes place on the stigma of carpel after pollination.

 Pollens absorb moisture and sugar content from the stigma. Due to this absorption volume of internal contents of cytoplasm increased. It exerts pressure on the both layers. Because of this pressure intine comes out through any one germopore in the form of tube like structure called pollen tube.

Facts about types of Pollen grains :

In Asclepiadaceae (Calotropis) and Orchidaceae family, all the pollen grains joined together to form "Pollinium".

More than four pollen grains are found in tetrad called "Polyspory" e.g. Cuscuta.

In some plants, four pollens of tetrad join together permanently are called "Compound pollens" e.g. Drosera, Typha, Drimys and Elodea.

Pollinium of Calotropis is called "Translator apparatus".

Compound pollens of Mimosaceae (Mimosa) family contains 6-8 or 64 pollen grains stick together to form a small unit that is called "Massullae" e.g. Mimosa and Neottia plants.

Due to the presence of sporopollenin, fossils of pollen grain are always found in good condition. The presence of fossils of pollen grains can forecast the presence of natural resources like petroleum, coals etc. in the earth.

Pollen grains of some plants present in air cause allergy are called "aero allergens" e.g. chenopodium, Parthenium, Sorghum and Amaranthus. ["Hay fever" is caused by pollens of Ambrosia].

In Cyperaceae family only one pollen grain is formed from pollen mother cell e.g., Cyperus.

Largest pollen – Mirabilis.

Smallest pollen – Myosotis.

Longest pollen – Zostera (Filiform pollen)

Eight nucleated embryo sac type of pollen is found in Hyacinthus orientalis the pollen nucleus divide to become 8 nucleate [This type of pollen grain discovered by Nemec. So it is called Nemec phenomenon]

Pollen grains are rich in nutrients. It has become a fashion in recent years to use pollen tablets as food supplements in western countries. Large number of pollen products in form of tablets and syrups are available in market. Pollen consumption has been claimed to increase the performance of athletes and race horse.

Pollen grains of many plant species cause severe allergies and bronchial disorders in some people leading to chronic respiratory disorders like - asthma, bronchitis. For ex. Parthenium (carrot / congress grass) – in India cause pollen allergy and asthma.

Viability of pollen grains is depands on temperature and humidity. In some cereals like rice and wheat – pollen grains loss viability within 30 minutes of their release. In members of Rosaceae, Leguminoseae and Solanaceae pollen grains are viable for months.

Pollen grains can be stored like semen of humen being for years by cryopreservation in liquid nitrogen at –196º temperature.

Such pollens can be used as pollen banks similar to seed banks in crop breeding programmes.

Female Reproductive Organ - Gynoecium

Gynoecium is the female reproductive organ. The free unit of gynoecium is called pistil or carpel.

Carpel is also known as megasporophyll.

The carpel is differentiate into three distinct region -

(i) Stigma                                                                                                     

(ii)  Style                   

(iii)  Ovary

The free end of the carpel which receives pollen grains is called stigma. A long, narrow tubular structure is present in between the stigma and ovary called style. The basal swollen part of the carpel is called ovary. The ovule is also known as megasporangia which are borne on a cushion-like tissue called placenta in the ovary. One or more than one ovules are present inside the ovary.

The gynoecium of the Capsella is bicarpellary, syncarpous, unilocular and superior. It becomes bilocular due to the formation of false septum or replum at maturity.

Structure of Ovule or Megasporangium :    

A ridge or stalk like out growth is formed from the placenta of the ovary on which body of ovules are present. Each ovule attached to the placenta by means of a thin stalk called funicle or funiculus/Funiculum

Number of ovule in a ovary may be one (in wheat, Rice, Mango) or many (in papaya, water melon orchids).

The point of attachment of the funicle with the ovule is called hilum.

The main region of the ovule is composed of mass of parenchymatous cells called nucellus. Nucellus is the main part of ovule. The nucellus is covered by one or two coats called integuments.

In most of the ovule, funicle is attached to the main body of ovule for some distance (at lateral side) to form a ridge like structure known as Raphe.

Vascular tissues are present inside the funiculus which supply food material from the placenta to the body of ovule.

Special Integuments :

ARIL – It is the type of third integuments which develops from funicle at the base of the ovule e.g. Myristica, Asphodelus and Litchi.

ARILLODE – It develops from the tips of the outer integument and grows downwards and surrounds the entire ovule e.g. Pitchecolobium (Inga dulce)

SARCOTESTA – When outer integument becomes fleshy then it is called sarcotesta e.g. Mangoliaceae (Magnolia).

OPERCULUM – It is a stopper or break like structure which is formed on the micropyle. It is formed due to the elongation of inner integuments or endostome/exostome/Nucellus projections e.g. Lemnaceae family (Lemna).

CARUNCLE OR STROPHIOLE – It is formed due to the proliferation (out growth) of outer integuments over the micropyle. e.g. Ricinus communis (Castor). It is formed by sugary contents so helps in absorption of water during germination of seeds and dispersal of seeds by ants called myrmecochory.

COMA – In some of the plants unicellular filaments like structures are present on the seed which is formed by cells of outer surface of outer integument. Such seeds are known as "Comose seeds". e.g. Calotropis and Gossypium.

A place from where funicle and integuments arise is called Chalaza.

Integument is absent just opposite to the chalaza, so that a narrow passage (pore) is formed which is called micropyle.

Micropyle in bitegmic ovule has two parts – outer region which is surrounded by outer integument is called exostome.

The inner part of micropyle which is surrounded by inner integument called endostome.

In most of the Angiosperm entire part of the nucellus is utilized by developing embryo sac but in some of the Angiosperm some part of the nucellus remain inside the ovules that part of the nucellus present inside the seed in the form of a thin layer known as perisperm. Perisperm is commonly found in Piperaceae (Piper nigrum) and Zingiberaceae Families (Turmeric, Ginger) and Beet plant. 

Some filaments are attached with funicle [some times placenta] are known as "Obturators".

The function of obturators is to guide the passage of pollen tube towards the micropyle inside the ovary.

Types of Ovule on The Basis of Integuments :

A single integumented ovule is called unitegmic ovule – example – members of Gamopetalae and Gymnosperm.

Two integumented ovule is called bitegmic ovule. Example – In most of Angiosperm [Polypetalae – Capsella and Monocots].

The ovule in which integuments are absent is called Ategmic ovule e.g. Olax, Liriosma, Loranthus & Santalum.

Types of Ovules on The Basis of Nucellus :

Tenuinucellate – The nucellus is either less developed or present in the form of single layer.

Example : Gamopetalae group.

Crassinucellate – The nucellus is massive type i.e., it is made up of many layers.  

Example : Polypetalae group and Monocots

The nucellus degenerates in plants of Compositae family and integuments becomes active to form a nucellus like tissue. This is called endothelium or integumentary tapetum. It is multinucleate. structure.     

The nucellus dissolves in the members of Podostemaceae family to form a nutritional cavity. This is termed pseudoembryo-sac.    

TYPES OF OVULES :    

There are six different types of ovules are found in Angiosperms on the basis of relationship of the micropyle, chalaza, and hilum with body of the ovule and orientation on the funiculus : 

Atropous or Orthotropous :

The body of ovule is upright in position. The micropyle, chalaza and hilum lie in one straight line, so that this ovule is called straight or upright ovule. Example : Betel, Piper, Polygonus and in Gymnosperms. It is the most primitive and most simple type of ovule of Angiosperms. Raphe is absent.

 Hemitropous or Hemi-Anatropous ovule :

In this ovule, the body of the ovule bent on funcile at 90º angle, i.e., body of ovule present at right angle to the funiculus. This is intermediate type between ortho and anatropous ovules. This ovule is also called horizontal ovule because body of ovule present in horizontal position on the funiculus. Micropyle and chalaza are present in the same line but micropyle is situated away from hilum. Example: Ranunculus, Primula, Golphimia.  

Campylotropous Ovule :

In this ovule, the body of ovule curved (Curvature is not effective) in this way so micropyle and chalaza do not present in straight line. The embryo sac and nucellus both are present in curved position. Micropyle comes close to the hilum. Example – Leguminosae, Capparidaceae, Cruciferae family [Capsella] 

Anatropous Ovule :

In this type, the body of the ovule completely turned at 180º angle, due to unilateral growth of funiculus, so it is also called inverted ovule. The chalaza and micropyle lie in straight line. The hilum and micropyle lie side by side very close to each other. This type of ovule is found in 80% families of Angiosperms but not in Capsella. In this ovule micropyle facing downward condition. This is the most common type of ovule so that it is considered as a "typical ovule" of Angiosperms. eg. Members of Malvaceae, Cucurbitaceae, Solanaceae, Compositae family. It is also called resupinate ovule. 

Amphitropous Ovule :

In this type of ovule, curvature is more pronounced or effective in the nucellus and due to this effect of nucellus, embryo sac becomes horse shoe shaped. Micropyle comes close to the hilum. It is also called as transverse ovule. e.g. Mirabilis, Lemna and Poppy, Alisma, Butomaceae family.    

Circinotropous Ovule :

This type of ovule, first of all body of ovule inverted once and again turned into straight position due to the growth of funiculus so that body of ovule present on funicle at 360º. The entire body of ovule is surrounded by funiculus. It is also known as coiled ovule. Micropyle is situated away from hilum e.g. Cactaceae family-Opuntia.

MEGASPOROGENESIS :

During the development of ovule, in the begining of this process, nucellus develops form the placenta in the form of a small rounded out growth like structure. At this stage, all the cells of nucellus are undifferentiated and homologous and meristematic. This mass of cells surrounded by single celled thick layer of epidermis.

Any one hypodermal cell of nucellus is differentiated and increase in size. It becomes different from rest of the cells due to presence of distinct nucleus. It is called archesporial cell. Archesporium divides periclinally to form an outer primary parietal cell and inner Primary Sporogenous cell. Activity of primary Parietal cell depends on type of plants. If plant belongs to gamopetalae then it forms tenuinucellate type ovule and if plant belongs to polypetalae then it forms crassinucellate type of ovule is formed. The primary sporogenous cell directly act as a megaspore mother cell (MMC). It divides meiotically to form, four haploid megaspores.  

The four haploid megaspores generally arranged in linear tetrad. Generally the lower most or chalazal megaspore remains functional out of tetrad of megaspores and the other three lie towards the micropyle degenerate. This functional megaspore produces female gametophyte. In most of Angiospersm (Capsella), Chalazal megaspore remains functional.         

DEVELOPMENT OF EMBRYO SAC OR FEMALE GAMETOPHYTE :

Megagametogenesis : Megaspore is the first cell of the female gametophyte. This megaspore grows in size and obtains nutrition from the nucellus. The nucleus of megaspore divides mitotically to form a two nuclei. Each nucleus moves towards the opposite pole and reached at their respective poles. Both the nuclei lie at poles divide twice mitotically. Resulting, four-four nuclei are formed at each poles [Total 8-nuclei].      

Out of the four, one-one nucleus migrates from the both poles [one nucleus from chalazal side and one nucleus from micropylar side] towards the centre. They are known as polar nuclei. Both polar nuclei are present in the centre.   

Remaining three-three muclei at each pole surrounded by the cytoplasm to form cells as a result of cytokinesis. Three cells are formed towards the micropyle in which one cell is large and more distinct out of three cells. This is called egg cell and remaining two smaller cells are known as synergids. These three micropylar cells collectively known as egg-apparatus. [1 Egg cell + 2 Synergids]   

The three cells are formed toward the Chalaza are called antipodal cells. Both the polar nuclei present in the large central cell. But just before the process of fertilization they unite or fuse together in the centre to form secondary nucleus. It is diploid in nature [2n] and one in number.

 Therefore, seven cells and eight nucleated structure is formed. This eight nucleated and seven celled structure is called female gametophyte or embryo sac of Angiosperms. This type of embryo sac is known as polygonum

type because it is discovered by Strassburger in Polygonum plant. Polygonum type embryo sac is most common type in Angiosperms [Capsella]. Polygonum type of embryo sac develops from single megaspore so it is also known as monosporic embryo sac.

Fingers like processes are produced from the outer wall of the synergids are known as filiform apparatus. With the help of these structures, synergids absorb food from the nucellus and transfer to the embryo sac. Filiform apparatus is less developed in antipodal cells. Filiform also secrete some chemicals which attracts the pollen tube.

In some plants, barrier are present either above or below the female gametophyte. These barrier are made up of thick walled cells of nucellus. They prevent the movement of embryo sac towards the chalaza or micropyle. The barrier which is present towards the chalaza is called hypostase e.g. Umbelliferae family, Zostera and Crozophora plants.        

The barrier which is present toward the micropyle is called epistase e.g. Costalia and Costum.

TYPE OF EMBRYO SACS :      

Monosporic Embryo sac – It is of two types –         

Polygonum type – It is eight nucleated and seven celled embryo sac.    

Oenothera type – Exceptionally it is four nucleated in which only one nucleus in a central cell and three nucleus in egg apparatus. Antipodal cells are absent. [Micropylar megaspore become functional]

Bisporic Embryo sac – It is formed by two megaspores. It means it develop from two nucleus of megaspores. It is of two types –         

Allium type – Eight nucleated and seven celled [Chalazal megaspores]

Endymion type – Eight nucleated and seven celled [Micropylar megaspores]  

Tetrasporic Embryosac – It is formed by all four megaspore nuclei because meiosis is not accompanied by cytokinesis, so that four nuclei of megaspores are formed.  

All four nuclei are collectively known as "Coenomegaspore".         

POLLINATION

"Pollination is defined as the process of transfer of pollen grains from anther to the stigma of the same flower or of different flower of the same species."

Pollination is of two types :    

SELF POLLINATION OR AUTOGAMY :       

If the pollen grain are transferred from an anther to the stigma of the same flower is called self pollination or autogamy.

CROSS POLLINATION OR ALLOGAMY :    

When the pollen grains are transferred to the stigma of other flower of the same species is called Cross pollination or Allogamy. It takes place in between two different flowers.     

Heterostyly  There is difference in between the length of the filaments of stamens and length of style in different flowers of some plants. Some of the plants having long stamen and short style, and in some of the plants bears long style and short filament. Due to this reason, self pollination can not possible in these plants e.g. Primrose (Primula), Lathyrum, Oxalis.   

Primrose (Primula vulgaries) is a commonly cited example of this phenomenon. It has two types of flowers (distyly) :

Pin-eyed or long-styled, with long style, long stigmatic papillae, short stamens and small pollen grains.

Thurum-eyed or short styled, with short style, small stigmatic papillae, long stamens and large pollen grains.

The stigma in the thrum-eyed flowers is at the level of anthers in the pin-eyed flowers and vice-versa. As a rule, pollen from thrum-eyed flowers can bring about effective pollination only in pin-eyed flowers and, similarly, pollen from pin-eyed flowers can effect legitimate pollination in thrum-eyed flowers, but not in their own type.

Some flowers, such as Lythrum and Oxalis, show tristyly. They have three types of flowers with respect to the length of the style and stamens. Pollen from flowers of one type can effectively pollinate stigma of only the other two types of flowers and not of its own type.

Self sterility or self incompatibility or intraspecific incompatibility : In this condition the pollen grains of the flower can not germinates on the stigma of the same flower. This condition is called self sterility. This is a parental [Genetical] characteristic feature which is controlled by genes. Such as in Pitunia, Malva, Thea, Passiflora, Vitis, Apple (Malus).

Incompatibility involves many complex mechanisms associated with interactions of pollen and stigmatic tissues. It leads to prevention of pollen germination, retardation of pollen tube growth, deorientation of pollen tube or even failure of nuclear fusion. It is controlled by genes with multiple alleles (s-allele).

Self incompatibility or intraspecific incompatibility :

Plants rejects the self pollens grains i.e. pollens from same flower or different flower of same plant to prevent genetic self pollination.

Recognition of "self" pollen in based on genes for self incompatibility called 'S'-gene. In the gene pool of a plant population, there can be dozons of alleles of an S-gene.

If a pollen grain has an allele that matches an allele of the stigma on which it land, the pollen tube fails to grow.

Depending on species, self recognition blocks pollen tube growth by one of the two molecular mechanism.

Gametophytic selfincompatibility (GSI)

Sporophytic self incompatibily (SSI)

Gametophytic self incompatibility – In GSI, the S-allele in the pollen genome regulates the blocking of fertilization.

For ex. An S₁ pollen grain from an S₁S₂ parental sporophyte will fail to fertilize egg of an S₁S₂ Flower but will fertilize an S₂S₃ flower.

An S₂ pollen grain would not fertilize either flower.

Self recognition of this type involves the enzymetic destruction of RNA within a rudimentary pollen tube RNA hydrolyzing enzymes are secreted by style of carpel which destroy pollen tube.

Ex. Liliaceae, Poaceae, Solanaceae, Trifolium.

Sporophytic self in compatibility (SSI) – In this system pollens of a plant behave similarly, irrespective of S-allele they carry.

For ex. A plant carrying S₁S₂ alleles, the pollen carrying S₁ or S₂ allele, would behave as S₁ if S₁ is dominant or as S₂ if S₂ is dominant and S₁ + S₂ if no dominance. So the presence of even one of the alleles of the stylar tissue in sporophytic tissue of male parent make the all  pollen of that plant non-functional with respect to that particular style.

So this S₁S₂ plant produce pollens S₁ or S₂ are completely incompatible to plant carrying S₁S₂, S₁S₄, S₁S₅ or S₂S₃, S₂S₄, S₂S₅ but 100 percent compatible with a plant carrying S₃S₄ or S₃S₅

In SSI system rejection occurs on the stigma the pollen tube is destroyed as can not penetrate cuticle of stigma.

Ex. Astaraceae, Brassicaceae

Growth of its pollen tube of self pollinated pollen grain is very slow and growth of the pollen tube of cross pollinated pollen grain in very fast so pollen tube of cross pollinated pollen grains reach earlier inside the ovule. This is termed prepotency.