Megasporogenesis

Sexual Reproduction In Flowering Plants

Megasporogenesis:-

The process of formation of megaspores from megaspore mother cell is called as Megasporogenesis. First of all a dome shaped outgrowth of undifferentiated, homologous and meristematic mass of cells is formed known as Nucellus. A cell just below the upper layer i.e., the epidermis, becomes visibly different from other cells and gets differentiated and increase in size. Due to the presence of a distinct nucleus, it looks different from the other cell. Now it is called as Archesporial cell. This Archisporial cell divides periclinally to form two different cells, the outer Primary Parietal cell and inner Primary Sporogenous cell. The Primary Sporogenous cell differentiate into Megaspore Mother cell (MMC). The Megaspore Mother cell further divide meiotically to form four haploid cells, the megaspores. The four haploid megaspore gets arranged in a linear fashion from chalazal to micropyler end. Out of the four haploid cell three present near the micropyle gets degenerate and only one which is near the chalaza remain functional. The chalazal megaspore which is functional, forms the female gametophyte.

Types Of Ovule

Sexual Reproduction In Flowering Plants

Types Of Ovule:

On the basis of Integuments Ovule is of following three types:-

1. Unitegmic Ovule - When only one Integument is present is called Unitegmic Ovule.

2. Bitegmic Ovule - When two Integument is present, then this type of Ovule is called Bitegmic Ovule.

3. Ategmic Ovule - When Integument is absent, then this type of Ovule is termed as Ategmic Ovule.

On the Basis of Nucellus Ovule is of mainly two types:-

1. Tenuinucellate Ovule - Nucellus is in the form of single layer present around the embryo sac or it is less developed type, this is called Tenuinucellate Ovule.

2. Crassinucellate Ovule - Nucellus is multi layered or it is massive type, then such type of Ovule is called Crassinucellate Ovule.

On the basis of relationship among Micropyle, Chalaza and Hilum with the body of Ovule and orientation of these on the Funiculus, Ovule is of six types:-

1. Atropous or Orthotropous - This is a straight or upright Ovule, because the Micropyle, Chalaza, and Hilum lie on the same line i.e., in one straight line.

2. Hemitropous or Hemi-anatropous Ovule - This Ovule is a horizontal type of Ovule, In this the body of Ovule makes  90 degree angle with the funicle and is horizontally situated. The micropyle and Chalaza lie on the same line and Micropyle away from the Hilum.

3. Campylotropous Ovule - The body of Ovule is curved and Micropyle and Chalaza is not present in the same line. The nucellus and Embryo sac is also distorted i.e., curved. The micropyle comes little close to the Hilum.

4. Anatropous Ovule - This is an inverted type of Ovule. In this type of Ovule body of Ovule is turned completely 180 degree to the Funiculus. Micropyle and Chalaza lie on the straight line. Micropyle gets more close to the Hilum. This is a typical type of Ovule found in Angiosperms and it is also the most common type of Ovule.

5. Amphitropous Ovule - This Ovule is known as transverse Ovule. In this type of Ovule curvature is very effective in nucellus, due to this effect Embryo sac becomes horse shoe shaped. The Micropyle and Hilum gets very close.

6. Circinotropous Ovule - This type of Ovule is called as a coiled Ovule. In this type of Ovule funicle coils in such a way that the body of Ovule present on the funicle at 360 degree. Micropyle is far away from the Hilum.

The Structure of Gynoecium - Class 12, Chap 2, Lec 8

Sexual Reproduction In Flowering Plants

The Structure Of Gynoecium:

The female reproductive organ of a plant is Gynoecium. The unit of Gynoecium is Pistil or Carpel. Pistil or Carpel is also known as megasporophyll. 

The pistil or carpel is divided into three parts- 

1. Stigma - Stigma is the upper part of pistil or Carpel on which pollen grain lands. Stigma provides moisture, nutrients and other factors to the germinating pollen grain.

2. Style - The middle part of pistil which is thin, long and narrow tube like structure is called style.

3. Ovary - The basal swollen part of pistil or carpel is called ovary. 

The Structure of Ovule - Class 12, Chap 2, Lecture 9

Sexual Reproduction In Flowering Plant

The Structure Of Ovule:

The Ovule develops from the base of the ovary. Ovule is also known as megasporangium. First of all undifferentiated, homologous mass of meristematic cell grow at the base of ovary, this is known as Placenta. A stalk like out growth comes out of the Placenta to which the body of Ovule is attached, this structure is known as Funicle or Funiculus or Funiculum. The point of attachment between Funicle and the head of ovule is called as Hilum. At some distance in the lateral side of Funicle the body of ovary is attached with Funicle this is known as Hyphae. The place from where Funicle and the Integuments arise is called Chalaza. The major portion of Ovary is made of parenchymatous cell and act as nutritive layer, this is known as Nucellus. One or two coat around the Nucellus is present and called as Integument. Just opposite to the Chalaza, Integuments are absent and form a narrow passage or pore this is called as Micropyle. Few filaments are attached with funicle or placenta and these filaments guide the growing pollen tube towards the Micropyle is called as Obturators. In the middle of the Ovule a distinct large cell is present, this form the Embryosac.

Objective Type Questions :-

Dehiscence - Class 12, Chap2, Lecture 7

Sexual Reproduction In Flowering Plants

Dehiscence:

Dehiscence is the process of rupturing of anther wall to release pollen grains. During the maturation of anther a lot of changes takes place in the walls of anther. The middle layer after giving its nutrients to the tapetum degenerate. The tapetum transfer these nutrients to the pollen sac and also convert them in special material called Ubisch body. when the microspore mother cell divide to form pollen grain at the same time tapetum release Ubisch body in microsporangia i.e., pollen sac and degenerate. The sterile cell which is present between two pollen sac also degenerate. Only the epidermis and endothecium remain intact . In the cells of endothecium alpha-cellulose deposition and callose band is present, but at some places these materials are absent which is known as stomium and these place become weak. Being hygroscopic in nature, when water dry up the wall of endothecium contract and become concave and incurved . Now due to this incurving process the weak stomium gets cracked and finally break-off causing the release of pollen grains.   

Structure of pollen grain - Class 12, Chap2, Lec-6

Sexual Reproduction In Flowering Plants

Structure Of Pollen grain:

The formation of pollen grain take place from microspore mother cell. pollen is a haploid cell and called as first cell of male gametophyte. Pollen grains are round in shape. Pollen grains are surrounded by two layers, 1st layer is outer side called exine and the 2nd layer is inner side called intine. The exine is a very tough, rigid and ornamented structure.  The exine is formed by cutin and sporopollenin. Sporopollenin is the hardest and non-biodegradable material. The intine of pollen grain is soft, thin and elastic. The intine is formed of pectin and cellulose or pecto-cellulose. The exine is very thin or absent at few places of the outer surface of pollen grain, these places are called as germ pore. The germ pore gives a passage to the intine during the development of pollen tube in a germinating pollen grain. In the pollen grain of insect pollinating plants have oily layer around the pollen grain, it is called pollen kit. Pollen kit is made up of lipids and carotinoids. The bright colour of pollen kit attracts insects and being sticky gets attached to the body surface of insects. The pollen kit material also protects pollen grain from the harmful effects of Ultraviolet rays (UV-rays).

                                        

* Due to the presence of sporopollenin, fossilized pollen grains are found in good condition and indicates the presence of natural resources such as petroleums, coals etc..

* The pollen grains of chenopodium, Parthenium, Sorgum, Amaranths etc. cause allergy ( chronic respiratory disorders like- asthma and bronchitis) and called aero allergens.

* Pollen grains are very nutritious and used as tablets and syrup. 

* Smallest pollen grain - Myosotis.

   Largest pollen grain - Mirabilis.

   Longest pollen grain - Zostera ( Filliform pollen ).

* In India pollen allergy and asthma is caused by Parthenium ( Carrot/Congress grass ).

Types of Tetrad (arrangement of pollen grain) - Class 12, Chap 2, Lecture 5

Sexual Reproduction in flowering Plants

Types Of Tetrad:

All the four microspores formed from the microspore/pollen mother cell ( MMC/PMC ), initially remain attached together with the help of callose layer, so the group of microspores formed is called Tetrad. This arrangement of microspores is different for different plant types, which is given below:-

1. Tetrahedral tetrad - Four microspores are arranged in a tetrahedral form. e.g., - Dicotyledon.

2. Isobilateral tetrad - Four pollen grain/microspores are arranged laterally . e.g., - Monocots.

3. Decussate tetrad - Out of four microspores two microspores are arranged at the right angle of the other microspores. e.g., - Dicotyledon.

4. T-Shaped tetrad - Out of four microspore two microspore lies longitudinally and other two lies transversally . e.g., - Dicotyledon.

5. Linear tetrad - When all pollen grains are arranged in a linear order, then the formed tetrad is linear tetrad. e.g., Monocots.

* Most common type of tetrad is Tetrahedral Tetrad.

Microsporogenesis - Class 12 , Chapter 2 , lecture -4

Sexual Reproduction in Flowering plants

Microsporogenesis-

 

Microsporogenesis is the process of formation of microspores. Microsporogenesis takes place in pollen sac of Anther. Initially anther is a mass of homologous, undifferentiated, meristematic cells. Initially in the center of anther connective tissue forms the connective. Later on each each cell from the four corners of the anther, just below the epidermis becomes distinct from the rest of the cell and gets differentiated into the special cell known as Archesporial cell. Now the Archesporial cell divide mitotically to form two different type of cell i.e., Primary Parietal Cell and Primary Sporogenous Cell. The primary parietal cell divide mitotically to form the three layers of anther such as, Endothecium, Middle layer, Tapetum. Now the primary sporogenous cell divide mitotically to form sporogenous cell. Sporogenous cell again divide to form microspore mother cell (MMC/PMC). The microspore mother cell divide meiotically to form four haploid microspores. These four microspore reman intact, and covered by callose, this structure is called as tetrad. On maturity of microspore, the tapetal cell   release hormones and enzyme callase. Callase dissolve the callose and the pollen grain or microspore becomes free. 

Objective type Question-

1. Microsporophyll is called-

a). Androecium         b). Gynoecium

b). Filament               c). Ovary

2. Archesporial cells do not form-

a). Tapetum               b). Middle layer 

c). Endothecium        d). Epidermis

3). Microsporophyll contains which among these-

a). Ovule                    b). Pollen grain

c). Both                      d). None

4. Meiosis is observed in-

a). Primary parietal cell  b). Tapetal cell

c).Sporogenous cell        d). Microspore mother cell

5. The plant obtained from pollen grain culture is-

a). Haploid plant             b). Diploid plant

c). Heterosis                     d).Polyploid plant

6. Which is surrounded by  callose -

a). Middle layer 

b). Pollen grain 

c). Microspore mother cell

d). Egg

7. Pollen grain is formed by the meiotic cell division of-

a). Primary sporogenous cell

b). Primary parietal cell

c). Sporogenous cell

d). Microspore mother cell

8. Tetrad is a condition observed as-

a). Four microspore covered by callose

b). Four microspore free in pollensac

c). Microspore mother cell covered by callose

d). Four layers of the tapetum

https://youtu.be/Mun84hiCnZY

Types of Tapetum - CLASS 12 , Chap-2 , Lec-3

                                            Sexual Reproduction in Flowering Plants

                                

 Types of Tapetum

Tapetum is of following type:

1.Amoeboid or Periplasmodial or Invasive Tapetum

Such type of tapetum is mostly found in primitive type of angiospermic plants. This tapetum initially absorb all nutrients from the middle layer and middle layer degenerates immediately. Now the tapetum store all food material and later on it convert all these food material into special material which is called as protoplast bodies.

                       Now , the tapetum finally degenerates and release the protoplast bodies in the pollen sac, here it is known as periplasmodium and supply nutrition to the developing microspore.

2.Secretory Tapetum or, Glandular tapetum  

This type of tapetum is well developed and stable . This type of tapetum do not absorb all nutrients from middle layer immediately. This tapetum absorb nutrients  and transfer it to the pollen sec. Tapetum convert the absorbed nutrients in a special material called it proubisch body , making it permeable across the cell membrane. Just before the maturity of microspore tapetum integrate sporopollenin and proubisch body now this material is called Ubisch body and tapetum degenerates to release ubisch body in microsporangia. Now in microsporangia sporopollenin form the exine of pollen grain.

Objective type question-

1. which of the following is responsible for formation of Ubisch bodies?

a). Endothecium     b). Embryo sac

d). Tapetum             e). Pollen grain

2. The developed type of tapetum is-

a). Periplasmodial tapetum   b). Glandular tapetum

c). Both                                  d). None

3. Sporopollenin is integrated with -

a). Protoplast                  b). Ubisch body

c). Pro-ubisch body         d). None

Structure of Anther - CLASS 12 , Chapter 2, Lecture - 2

Sexual Reproduction in Flowering Plants

The Structure of Anther:

The male reproductive organ of a plant is the Androecium. Androecium is constitute of one or many stamens. The upper swallon part of stamen is called anther.  The development of anther takes place by more than one Archesporial cell , so it is Eusporangiate in origin.

                                                                                                    Anther is generally spherical and have following structures-    

1. Epidermis - Epidermis is the uppermost layer . It is single celled thick continuous layer . Epidermis is protective in nature . Epidermis is not Archesporial in origin .

2.Endothecium - Endothecium is present just below the epidermis. Endothecium is also single celled thick , continuous layer. The cells of Endothecium has deposition of alpha-cellulose in its inner wall and callose band in radial wall but the outer remain thin . Endothecium is Archesporial in origin. In some cells of endothecium the inner wall and radial wall deposition of alpha-cellulose and callose band is not present , this place is called stomium. At the point of stomium dehiscence takes place.

3. Middle layer - Middle layer is next to the Endothecium and 1 to 3 celled thick . Middle layer is Archesporial in origin . Middle layer have parenchymatous cells. Tapetum absorb nutrition from middle layer after which middle layer degenerates. so it shows ephemeral nature and degenerates in mature anther.

4. Tapetum - Tapetum is present next to the Middle layer . Tapetum is single celled and surrounds the pollen sac. Tapetum initially absorb nutients from the middle layer and convert it into special material depending upon the type of tapetum then after it pours it into the pollensac. Tapetum also secretes hormone and enzymes. The tapetum  layer is absent in mature anther. Tapetum layer is the nutritive layer.

5. Pollen sac - In a young or immature anther four pollen sacs are present. Pollen sacs are special chamber in which special cell called microspore mother cells are present. These cells meiotically divide to give rise microspores. Pollen sac is also called as microsporangia. 

 

Objective type questions ----- ( All these questions are asked in previous exams-)

1. Microsporophyll contains which among these -

a). Nucellus            b). Pollen grain

c). Pollen tube        d). All the above

2. Flowering in a biennial plant take place-

a). Many times       b). Ones

c). Twice                d). None

3. The nutritive layer is-

a). Exothecium      b). Endothecium

c). Tapetum            d).Sporogenous tissue

4. Archesporium gives rise to which structure in a flowering plant -

a). Wall and the tapetum

b). Only wall of the sporangium 

c).Only tapetum and sporogenous cells

d). Both wall and sporogenous cells

5. The function of tapetum is -

a). Support       b). Division

c). Nutrition     d). None

6. Meiotic cell division is present in-

a). Microspores  b). Megaspores

c). Tapetal cell    d). Spore mother cell

7. Which is correct for a anther wall

a). In between endothecium and middle layer tapetum is present

b). Just below the endothecium tapetum is present

c). Between endothecium and tapetum middle layer is present

d). Just below endothecium middle layer is present

8. Which layer secretes hormones and enzyes -

a). Epidermis       b). Tapetum

c). Endothecium   d). Middle layer

9. Which is not Archesporial in origin -

a). Endothecium   b). Middle layer

c).Tapetum           d). Epidermis

10. In which layer of the tapetum stomium is present -

a). Middle layer   b). Endothecium

c). Both                d). None