Reproduction in lower and higher plants

Points we cover in chapter 

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1.      Asexual Reproduction

2.      Sexual Reproduction 

3.      Microsporogenesis 

4.      Structure of Anatropous ovule 

5.      Megasporogenesis 

6.      Pollination :

7.      Outbreeding devices (contrivances)

8.      Pollen - Pistil Interaction

9.      Double Fertilization 

10.   Development of Endosperm 

11.   Development of Embryo 

12.   Seed and Fruit Development 

13.   Apomixis 

14.   Parthenocarpy 

15.   Polyembryony 

Introduction

Reproduction is production of young  ones like parents.

It essential process as it leads to continuation of species as well as to maintain the continuity of life.

Each organism has its own particular method of reproduction.

·         These methods generally fall into two categories:.

• In this type of reproduction no requirement of two compatible gametes or sex cells.
• It is the process resulting in the production of geneticallyy identical progeny from a single organism and inherits the genes of the parent.
• Such morphologically and genetically identical individualss are called clones.
• Organisms choosee to reproduce asexually by different 

 Modes or ways: 

1. Fragmentation 
2. Budding 
3. Spore formation 

I. Fragmentation :

• organisms can break into fragments. 
• These fragments grow into new individuals.
• Eg Spirogyra 

ii. Budding : 

• It is the most common method in unicellular Yeast. 
• producing one or more outgrowths (buds). 
• it takes place during favourable conditions 
• These buds on seperation develop into new individual.

iii. Spore formation : 

•  In Chlamydomonas asexual reproduction occurs by flagellated, motile zoospores
•  which can grow independently into new individuals.

Vegetative Reproduction : 

• Plants reproduce asexually through their vegetativee parts.
• Hence, the new plants formed Are genetically identical to their parents.
•  in Agriculture And horticulture exploit vegetative reproduction In order to multiply fresh stocks of plants. 
• Artificial methods are used to propagate desired Varieties according to human requirements.

The various methods are as follows : 

1. Cutting
2. Grafting
3. Tissue culture 

a. Cutting :

 The small piece of any vegetative part of a plant having one or more buds is used for propagation 

 1. Stem cutting - e.g. Rose, 

 2. Bougainvillea; leaf cutting - e.g. Sansvieria;

3. root cutting e.g. Blackberry.

b. Grafting : 

•  Here parts of two plants are joined in such a way that they grow as one plant.
• In this method, part of the stem containing more than one bud (Scion) is joined onto a rooted plant called stock, is called grafting. 
• Whereas buddingg is also called bud grafting in which only one bud is joined on the stock, e.g. Apple, Pear, Rose, etc. 

c. Tissue culture :

• It is a method by which a small amount of plant tissue is carefully grown to give many plant lets.
•  Micropropagation method is also used now a days.

• Involves fusion of two compatible gametes or sex cells.
• Organisms become mature  before they can reproduce Sexually.
• End of juvenile or vegetative phase marks the begining of the reproductive phase
• Seen easily in the higher plants at the time of flowering.
• Flower is specialized reproductive structure
• Produce haploid gametes and to ensure that fertilization will take place.

Flower consists of four different whorls 

1. Calyx
2. Corolla
3. Androeciumm
4. Gynoeciumm .

Sexual reproduction events

• It involves two events  viz. meiosis and fusion of gametes(fertilization)
• After fusion form diploid zygote and the production of  genetically dissimilar offsprings.
• Variations are useful from the point of view of the survival and the evolution of species, over the time.
• Sexual reproduction is characterised by fertilization, the formation of zygote and embryogenesiss.
• Sequential events  are grouped into three distinct stages
• viz, 

1. Pre-fertilization,
2.  Fertilization
3. Post-fertilization.

Flower whorls

A. Essential Whorls

Reproductive parts of flowers

1.  Androecium ( works as Male)  it contain Anther , filament, and connective

2.      Gynoecium ( Works as Female) it contain Stigma , Style, and Ovary

B. Non essential WhorlsSupporting whorls

1.      calyx

2.      corolla

Male reproductive whorl

Structure of Anther

• At immature stage represented by group of parenchymatous tissue surrounded by single layered epidermis.
• Anther is generally dithecous (having two lobes) and tetrasporongiate.
• Each monothecous anther contains two pollen sacs.
• In dithecous anther four pollen sacs are present.  Therefore, it is tetrasporongiate( having four pollen sacs)
• The heterogenesity  (differenciation) arises when some hypodermal cells get transformed into archesporial cells.

 T.S of Anther

• Archesporiall cell divide to Form   inner sporogenous cell and outer primary parietal  cell.
• Sporogenous cell forms sporogenous tissue. Sporogenouss tissue is capable of giving rise to a microspore tetrad.
• Parietal cell undergoes divisions to form anther wall layer.

  The wall of Anther consist of Four layer

• Epidermis is outer layer made up of Tubular cells
• Endothecium is Sub epidermal layer made up of radial elongated cell
• Middle layer made up of thin walled cells
• Tapteum is Innermost layer. Nutritive layer of anther wall.

Development of male gametophyte

• Pollen grain initiates the formation of male gametophyte.
• It undergoes first mitotic division to produce bigger, naked vegetative cell and small, thin walled generative cell.
• The vegetative cell is rich in food and having irregular shaped nucleus.
• The generative cell floats in the cytoplasm of vegetative cell.
• The second mitotic division occur in  generative cell only and produce two non-motile male gametes.
• Second division of  generative cell takes place either in pollen grain or in the pollen tube.

Female reproductive whorl

• Gynoecium  (Pistil) is female reproductive Whorl.
• Individual member of gynoecium is called carpel (megasporophyll).
• Typical carpel has three partss viz, ovary, style and stigma.
• A flower with many, free carpels is called apocarpous (e.g. Michelia).
• A syncarpous flower is one that has many carpels fused together (e.g. Brinjal).
• The number of ovules in the ovary varies e.g. paddy, wheat and mango are uniovulate
• whereas tomato and lady’s finger are multiovulate.

Structure of Anatropous ovule:

• Ovule develops inside the ovary and is attached to the placenta by a small stalk called funiculus.
• The place of attachment of funicular with the main body of ovule, is called hilum.
• The ovule consists of central parenchymatous tissue, the nucellus  which is surrounded usually by two protective coverings called integuments viz. Outer and an inner integument.
• A narrow opening at the apex of the ovule is called micropyle.
•  Chalaza is the base of ovule directly opposite to micropyle.
• Embryo sac (female gametophyte) is oval multicellular structure embedded in the nucellus.

Megasporogenesis

• It is the process of formation of haploid megaspores from diploid megaspore mother cell (MMC).
• Megaspore mother cell becomes distinguished in the nucellus, more or less in the centre but towards micropylar end of ovule

Development of female gametophyte :

• Megaspore mother cell undergoes meiosis to form linear tetrad of haploid cells i.e. megaspore.
• Upper three megaspores abort and lowest one towards centre of nucellus remains functional.
• It acts as the first cell of female gametophyte. (is functional megaspore).
• It undergoes three successive, free nuclear mitotic divisions.
• Thus total eight nuclei are formed, four of which are located at each pole. One nucleus from each pole migrates towards the centre and is called polar nuclei.
• Three nuclei towards micropylar end constitute egg apparatus
• It consists of large central, haploid egg cell and two supporting haploid synergid cells.
• Synergid shows hair like projections called filiform apparatus, which guide the pollen tube towards the egg.

Antipodal cells

• Antipodal cells are group of three cells present at the chalazal end.
• The two haploid polar nuclei of large central cell fuse to form diploid secondary nucleus or definitive nucleus, just prior to fertilization.
• This seven-celled and eight nucleated structure is called an embryo sac.
• This method of embryo sac development from a single megaspore is described as monosporic development.
• In angiosperms, the development of female gametophyte is endosporous i.e. within the megaspore.
• Female gametophyte is colourless, endosporic and is concealed in the ovule enclosed by ovary.

• Pollination is the transfer of pollen grains from  anther to the stigma of the flower .
• Gametes  are produced at two different sites.
• Pollen grains are non-motile and they are usually carried from flower to flower by means of external agents.
• The external agents are abiotic agents (wind, water) and biotic agents (birds, insects, snails ) for pollination.
• Flowers, feeds the visitors and exploiting their mobility for pollination.
• After pollination fertilization was teaks place.

Type of pollination

1. Self pollination

Pollination occurs in single flower or two flowers on a single plant. It results in inbreeding or selfing.

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.

Pollination can be further divided into three types on the basis of source of pollination.

Types on the basis of source of pollination.

A.    Autogamy(self pollination) :

• Bisexual flower is pollinated by its own pollen grains.
• Off springs are genetically identical to their parents e.g. pea.

B.     Geitonogamy:

• It is the transfer of pollen grain to a stigma of a different flower produced on the same plant.
• It  involves pollinating agents, but it cannot bring about genetic variations. e.g. Cucurbita maxima
• 
  

 C. Xenogamy (cross polination/ out breeding) :

• Pollen grain of one flower is deposited on the stigma of aflower of different plant belonging to same species.
• It involves pollinating agents
• It generates genetically varied off springs. 

The agents responsible for pollination

A.    Abiotic Agents  (non-living agents )

1. Anemophily (Pollination by wind)  :

• Most of the important crop plants are wind pollinated.
• Ex. These includes wheat, rice, corn, rye, barley, oats and Palms.

Adaptations in anemophilous flowers :

• Flowers are small, inconspicuous, colourless, without  nectar and fragrance (odour).
• Pollen grains are light in weight, dry and produced in large numbers to increase chances of pollination considering wastage of pollen grains .
• Stigma is feathery to trap pollens carried by wind currents.
• Stamens are exserted with long filaments and versatile anthers.
• Stamens and stigmas are exposed to air currents. 

       2. Hydrophily (Pollination by water ) 

Found only in some 30 genera of aquatic monocots. E.g. Vallisneria, Zostera, Ceratophyllum etc.

Adaptations in hydrophilous flowers :

Flowers are small and inconspicuous.

Perianth and other floral parts are unwettable.

Pollen grains are long and unwettable due to presence of mucilage.

Nectar and fragrance are lacking in flowers.

Hydrophily is of two types -

a. Hypohydrophily :

• Pollination occurs below the surface of water.
• Here the pollen grains are heavier than water, sink down and caught by stigmas of female flowers,
• e.g. In Zostera (sea grass) the pollen grains are long, ribbon like and without exine.

b. Epihydrophily 

• The pollen grains float on the water surface and reach the stigma of female flower.
• e.g. Vallisneria is a submerged dioecious, fresh water aquatic plant in which female flowers reach the water surface temporarily to ensure pollination and male flowers float on the surface of water. 

B.  Biotic Agents : These are  living agents.

About 80% of plants require the help of other living, moving creatures such as insects, birds, bats, snails to transfer their pollens from one flower to another.

These also sustain our ecosystems and produce natural resources by helping plants to reproduce

A. Entomophily (Pollination by insects ) :

• It occurs in Rose, Jasmine, Cestrum, etc.

Adaptations in entomophilous flowers :

• They are large, showy and often brightly coloured.
• The flowers produce sweet odour (smell) and have nectar glands.
• The stigma is rough due to presence of hair or is sticky due to mucilaginous secretion.
• The pollen grains are spiny and surrounded by a yellow sticky substance called pollen kit.
• Some plants have special adaptations for the insect visitor to help in cross pollination,
• e.g. lever mechanism or turn pipe mechanism in Salvia.

B.Ornithophily (Pollination by birds ) :

• Only a few types of birds are specialised for pollination.
• They usually have small size and long beaks e.g. Sun birds and humming birds.
• Some ornithophilous plants are Bombax, Callistemon (Bottle Brush), Butea, etc

Adaptations in ornithophilous flowers :

• Flowers are usually brightly coloured, large and showy.
• They secrete profuse, dilute nectar.
• Pollen grains are sticky and spiny.
• Flowers are generally without fragrance, as birds have poor sense of smell.

C. Chiropteryphily (Pollination by Bats ) :

• Bats can transport pollens over long distance, some times several kilometers.
• Adaptations in Chiropterphilous flowers :
• Flowers are dull coloured with strong fragrance.
• They secrete abundant nectar.
• Flowers produce large amount of edible pollen grains,
• e.g. Anthocephalous (kadamb tree), Adansonia (Baobab tree), Kigelia (Sausage tree).

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