Sexual reproduction and Asexual reproduction in Plants

• 

 

 XII CH-1.

Reproduction in lower and higher plants.

Reproduction-

Reproduction is an essential process as it leads to the continuation of species as well as to maintain the continuity of life. Hence, reproduction is one of the most important characteristics of all living organisms.

Types of reproduction:- 

Two types of reproduction:-  

 1)Asexual  reproduction 

2)Sexual   reproduction   

1)Sexual reproduction-   

1. It requires two parents except for bisexual plants.

2. Formation and Fusion of gametes occurs

3. Offspring produced are not similar to parents.

4. It is common in most plants and animals.

5It is a less rapid method of multiplication

6. Meiosis occurs in reproductive cells.    

7. It produces genetically variable offs  

2)Asexual reproduction-                        

1. It requires only one parent.   

2. Do not occur Formation and Fusion of gametes.  

 3. The offspring are similar to their patents. 

4. It is present in lower animals and plants.  

5 It is a rapid method of multiplication.

6. Meiosis does not take place.                           

7. It produces genetically identical offspring.

1) Asexual Reproduction-

The reproduction takes place without the formation and the fusion of gametes is called asexual reproduction. 

By asexual reproduction, the genetically identical progeny is produced from a single organism.

Such morphologically and genetically identical individuals are called clones.                               

Modes of Asexual reproduction-   

i) Fragmentation 

ii) Budding 

iii)  Spore formation /Sporulation:                  

Modes of Asexual reproduction-

1)Fragmentation- Multicellular organisms can break into fragments due to one or different reasons. These fragments grow into new individuals. e.g. Spirogyra .

 2)Budding- It is the most common method of asexual reproduction in unicellular yeast.One or more small outgrowths are produced during favorable conditions, from a parental cell is called as bud. These buds on separation develop into new individuals.

3)Spore formation-

Asexual Reproduction occurs in Chlamydomonas, flagellated,  motile zoospores are grown to form new individuals.

Binaryfission-inAmoeba,Paramoecium;Conidiaformation =  in  Penicillium Gemmules  formation =  in Sponges .

4)Vegetative propagation - 

a)Natural vegetative propagation-

1) Root= Sweet potato, Asparagus, Dahlia   

 2)stem = PotatoCynodon  rhizome (turmeric), tubers (potato), bulbs (onion)   

(3)Leaf=Bryophyllum, Kalanchoe, Begonia.

b)Artificial vegetative propagation - 

(1)cutting  (2) Grafting and  budding  (3) Tissue culture 

Vegetative  Reproduction-  Pants reproduce asexually through their vegetative parts like root, stem leaf.  Hence, the new Plants formed are genetically identical to their parents.           

There are two methods- 

1)Natural methods of vegetative propagation. 

2)Artificial methods of vegetative propagation                                                             

Artificial methods vegetative  propagation                      

a)Cutting= The small piece of any vegetative part of plant having one or more buds is used for propagation viz. 

  1)Root cutting e.g. Apple, Tamarind, Blackberry, Lemon, Sweet potato, Guava et .

 2)Stem cutting e.g. Sugarcane, Rose, China rose, Coleus, Duranta, Bougainvillea, etc.;  

3)Leaf-cutting  - e.g. Bryophyllum, Sansevieria, and Peperomia 

(b) Grafting and budding-   The parts of two plants are joined in such a way that they grow as one plant.  

In this method,  parts of the stem containing more than one bud  (Scion) are joined onto a rooted plant called stock. which is called Grafting. 

Where budding is called bud Grafting in which only one bud is joined on the stock, e.g.  Apple, Pear, Rose. 

 c)Tissue culture - It is a  method by which a small amount of plant tissue is carefully grown to give many plantlets. The micropropagation method is also used nowadays. 

Significance of vegetative reproduction -  

1)It is an easy and cheaper method.  

2)It produces genetically identical plants.

3)the grafting and budding are helpful to increase the yield. 

4)Micropropagation is the rapid method of propagation.

Reproduction = 

It is the production of young ones like parents. 

Reproduction is an essential process as it leads to the continuation of species as well as to maintain the continuity of life. 

Hence, reproduction is one of the most important characteristics of all living organisms.

Types of reproduction:-   Two types of reproduction:- 

 1)Asexual  reproduction

 2)Sexual   reproduction

 2) Sexual Reproduction   

The type of reproduction takes place by formation and fusion of two compatible gametes or sex, cells are called sexual reproduction. All organisms must reach maturity in their life before they can reproduce sexually. 

1. In plants, the end of the juvenile or vegetative phase marks the beginning of the reproductive phase and can be seen easily in the higher plants at the time of flowering. 

 2. The flower is a specialized reproductive structure of a plant in which sexual reproduction takes place.

3. The function of the flower is to produce haploid gametes and to ensure that fertilization will take place

 4. Typical flower consists of four different whorls viz. calyx, corolla, androecium, and gynoecium.

5. Sexual reproduction involves two major events viz. meiosis and fusion of gametes to form a diploid zygote and the production of genetically dissimilar offsprings, Sexual reproduction create variations.     6.Variations are useful for survival and the evolution of species, over time.

Sexual Reproduction-

Sexual reproduction is characterized by fusion of the male and female gametes (fertilization), the formation of  zygote and embryogenesis Sequential events of sexual reproduction are grouped into three distinct stages

  1)Pre-fertilization  2)Fertilization  3)Post-fertilization 

Androecium or Male reproductive part=

 The male reproductive whorl of the flower is called  Androecium. An individual member of  Androecium is called the stamen.  The stamen consists of the anther, connective, and filament. Pollen grains are produced inside the anther.

Structure of Anther=

1. An immature stage of the anther is represented by a group of parenchymatous tissue surrounded by a single-layered epidermis2.Anther is of two types- Monothecous anther and Dithecous anther

3.Each monophagous anther contains two pollen sacs.

4.Each dithecous anther having two lobes and four pollen sac is Tetrasporogiate.                                                                                             

5. Some hypodermal cells get transformed into archesporial cells.

6. Archesporial cell divides into sporogenous cells and primary parietal cells.

7.Each  sporogenous cell forms a microspore tetrad and Parietal cell to form another wall layer.

T. S. of Anther =

 A)Anther wall= A mature anther wall consist of four layers as follows

1. Epidermis:- It is the outermost common wall of anther made up of flattened cells. It is protective in function.

2. Endothecium:- It is inner to epidermis made up of a single layer of cells.

Endothecium cells show characteristic fibrous thickenings of callose.

It shows thin-walled cells which help in dehiscence of anther at maturity called as stomium. Fibrous thickenings and hygroscopic nature help in the dehiscence of anther at maturity.

3. Middle layers:- Inside the endothecium, 1 to 3 layers of parenchyma cells are present called middle layers. Middle layers surround the microsporangium. They degenerate at maturity.

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

 B. Microsporangium ( pollen Sac ):- Each microspore    mother    a cell    divides Meiotically  to  form tetrad of haploid microspores (pollen grain ). 

Structure of microspore –

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

2)It is surrounded by a two-layered wall Sporoderm. The Sporoderm consist of two layers:- 

 i. Exine - The outer layer of Sporoderm is exine which is thick and made up of a complex, non- biodegradable, substance called sporopollenin.  

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

 These are meant for the growth of emerging pollen tubes during the germination of a pollen grain.

ii. Intine - The intine is inner, thin, smooth, and made up of cellulose and pectin.Development of male gametophyte- 

1.Pollen grain marks the beginning of male gametophyte.   

 2. It  undergoes  first  mitotic division  to  produce  bigger, naked vegetative or tube cell and a small thin-walled generative cell.   

 3.The vegetative cell is rich in food and having an irregularly shaped nucleus.

4.The  second mitotic division is concerned with generative cells only and gives rise to two non-motile male gametes.   

 5.The mitotic division of the generative cell takes place either in pollen grain or in the pollen tube.  

 6.The pollen grain is shed from the anther at this two-celled stage in most of the angiosperms

Development of male gametophyte

 1.Pollen grain marks  the  beginning of male gametophyte. 

 2. It  undergoes  first  mitotic division  to  produce  bigger, naked vegetative or tube cell and a small thin-walled generative cell.   

3.The vegetative cell is rich in food and having  irregularly shaped nucleus.  

4.The generative cell floats in the cytoplasm of vegetative cells.

5.The  second  mitotic division  is  concerned with   generative cell only and gives rise to two non-motile male gametes. 

6.The mitotic division of the generative cell takes place either in pollen grain or in the pollen tube.  7.The pollen grain are shed from the anther at this two-  celled stage in most of the angioperms.               This pollen tube with two male gametes, thin cytoplasm, and a tube the nucleus is called a male gametophyte.

Gynoecium (Pistil).  

Female reproductive structure I. Female a reproductive whorl of a flower is called gynoecium (Pistil).

II. The individual member of the gynoecium is called carpel (megasporophyll).

 III. Typical carpel has three parts viz, stigma, style, and ovary.   A flower with free carpels is called apocarpous (e.g. Michelia).

IV. A flower with many carpels fused together is called syncarpous   (e.g. Brinjal).

V.The number of ovules in the ovary varies e.g. paddy, wheat, and mango are uniovulate whereas tomato and lady's finger are multilobulated          

 Structure of anatropous ovule - 

In angiosperms anatropous ovule is the most common type of ovule    Formation of megaspore (egg) I .e.megasporogenesis and development of female gametophyte takes place inside the ovule  In Angiosperms embryo the sac is monosporic, endospores, 7 celled, and 8 nucleate. 

V. S. of mature ovule : 

It consists of two parts  

 1. Stalk-

Stalk of the ovule is called a funicle, lt attaches the ovule with the placenta. The part of the funicle which remains attached with the ovule and persists in seed also, called as raphe.

The point of attachment of funicle with the body of the ovule is called as hilum or hilus Body.

2. Body Stalk:- 

Nucellus:- It forms the central mass  of the ovule also called megasporangium proper

Chalaza:- The base of the ovule is called chalaza. Integuments:- These are two protective coverings of nucellus i.e. outer and inner so-called as bitegmic.

Micropyle :- The integuments leave a narrow opening at the tip called the micropyle. During fertilization, the pollen tube enters the ovule through the micropyle.

Embryo sac:- In mature ovule inside the ovule oval-shaped, haploid embryo sac (female gametophyte ) is present.

Egg apparatus -At the micropylar end, there is 3, central egg and two lateral synergids.

Secondary the nucleus or polar nuclei. -A center cell having two nuclei present.

Antipodal cells -at the chalazal end three cells present.

Megasporogenesis:

The process of formation of haploid megaspores from diploid megaspore mother cell (MMC) is called megasporogenesis. Megaspore mother cell is any cell of the nucellus, more or less in the center but towards the micropylar end of the ovule.

Development of female gametophyte

Development of female gametophyte- 

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

2) Upper three megaspores get degenerates and the lowest one towards the center of the nucellus remains as a functional megaspore.

3)Functional megaspore acts as the first cell of the female gametophyte.

4)It undergoes three successive free nuclear mitotic divisions to form a total of eight nuclei are formed, four of which are located at each pole.5)One of the nuclei from each pole migrates towards the center and is called polar nuclei.

6)Three nuclei towards micropylar end constitute egg apparatus. It consists of a large central,  egg cell, and two synergid cells.

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

8)The two haploid polar nuclei of large central cell fuse to form a diploid secondary to the nucleus or definitive nucleus.

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

10) In angiosperms, the development of female gametophyte is endosporous i.e. within the megaspore.

11) The female gametophyte is colorless, endospores  and  is concealed in the ovule enclosed by the ovary.

Click here PDF PPT open