CROSS POLLINATION and METHODS of Cross pollination

 Cross pollination

It involves the transfer of pollen grains from the flower of one plant to the stigma of another plant. It is called cross pollination, xenogamy or allogamy.

Methods of cross pollination

The methods of cross pollination are categorised in two groups -

 a) Abiotic agents i.e wind current, gravity, water etc.

b) Biotic agents - such as animals, birds, insects, bats etc.

Abiotic agents or methods

i) Anemophily (anemos means wind and philein-to love)

It is the mode of pollination or transfer of pollen grains from anther to stigma through agency of wind. The flower which are wind pollinated are called anemophilous.

The characteristics of anemophilous flowers are - 

i) Flowers are small colourless, inconspicuous, odourless and nectarless.

ii)Calyx and corolla are either reduced or absent. Anthers are usually versatile.

iii) When flowers are unisexual, male flowers are more abundant than female flowers. In bisexual flowers, the stamens are generally numerous.

 iv) pollen grains are small, light, dry, dusty and sometimes winged so that they are easily blown away to long distance.

v)The flowers are well exposed in the air.

vi) Pollen grains of anemophilous flowers are produced in huge quantity.

vii) The sigmas are large, well-exposed, hairy, feathery or branched to catch the air -borne pollen grains.

viii) In some plants, the anthers burst suddenly to throw the pollen grains into the air (gun powder mechanism).

Pollination by wind

Example of wind pollinated flower are- 

Grasses, Sugarcane, bamboo, coconut,palm, date palm, cannabis, maize etc.

ii) Hydrophily(water pollination)

It is a mode of pollination or transfer of pollen grains from the anthers to the stigma through the agency of water.

The characteristics of the Hydrophilous plants are as follows - 

i) Flowers are small, colourless, inconspicuous, odourless and nectarless.

ii) Calyx, corrola and other floral parts are unwettable.

iii) Pollen grains and stigma are generally unwettable.

iv) The stigma are long and sticky.

Example of Hydrophilous plants are -Vallisneria, Zostera,Ceratophyllum, Hydrilla etc.

 In Vallisneria, a submerged fresh water hydrophyte, is a dioecious plant i.e male flower borne on male plants and female flower borne on female  plants. Mature male flower are abscised from the spadix and float on the surface of water. The mature female flowers also float on the surface of water,but remain attached to the female plant with the help of long stalks. The floating female flower have large sticky stigmas , so when anthers burst to release the pollen grains,it get attached to the female stigmas for the purpose of pollination on the surface of water. After fertilization, the female flowers are pulled down inside the water by the coiling of the flower stalk.

Pollination in VALLISNERIA

iii) Ornithophily (pollination by Birds)

It is the mode of pollination performed by birds. The most common bird pollinators are Sun bird, Humming bird,Crow, Bulbul, Parrot,Mynah etc. 

The characteristics of the ornithophilous flowers are as follows -

i) the flowers are usually large in size. They have tubular or funnel shaped corollas.

ii) The flowers are bright coloured which attract the birds from the long distances.

iii) The flowers produce abundant watery nectar.                               

iv) They are usually scentless.

                                                                Pollination by birds

Example of ornithophilous plants are - Bombax( Red Silk Cotton), Bottle brush, Bignonia, Agave etc.

iv)  Chiropterophily(Bat Pollination)

It is the mode of pollination performed by bats. The bats are nocturnal which move swiftly and transport pollen grains to long distance.

 Characteristics of the chiropterophilous 

plants are- 

i) The flowers are large, dull coloured and have a strong scent.

ii) These flowers produce abundant pollen grains and secrete more nectar than ornithophilous flowers.

Example of chiropterophilous plants are - kigelia pinnata, Adansonia,Bauhinia megalandra, Anthocephalus.

v) Entomophily ( Insect pollination)

It is the mode of pollination or transfer of anthers to the stigmas  by the insects.

 

The entomophilous flowers are characterized by the following adaptations - 

 

i) The flowers are usually large, conspicuous, brightly coloured and showy to attract insects.

ii) In this, petals of flowers are large and attractive. When they are small and inconspicuous, other parts of the plants are enlarge and become attractive.

iii) Small flowers bloom in bunches to attract the insects. Sometimes, they are condensed together to form a head.(e.g sunflower).

iv) Different colour of flowers attract different insects.

v) Entomophilous flowers usually blossom at specific time when particular insects or pollinators are available.

vi)These types of flowers usually produce specific odour to attract specific insects.

vii) pollen grains of certain flowers are edible. Many insects visit these flowers to eat their pollen grains or to carry them for their brood of larvae.

viii) Nectar secrete from their nectaries, is an important source of food for certain insects pollinators.

ix) The outer surface of pollen grains may be rough, spiny or sticky.  It possess yellow sticky substance called pollenkit which acts as insect attractants and as an adherent to the insect body.

A detailed example of entomophily pollination in SALVIA

 

The salvia genus belongs to family Labiate in which the gamopetalous corolla is two -lipped . The lower lip provides platform for the visiting insects and upper lip is just like a hood which protects the floral organs. The flowers are protandrous. Each flower has two epipetalous stamens located anterio-lateral in position. Each stamen has a short filament and an elongated curved connectives. The anther has two parts - one half is sterile and another half is fertile. Both parts of anther are separated apart due to elongation of connective. The elongated connective has two unequal arms. The upper arm is long and curved. It bears the fertile lobe of anther. The lower arm of connective is short and bears sterile lobe of anther. The two sterile lobes jointly form a sterile plate of tissue which is placed at the mouth of corolla tube and partly blocks the path of the visiting insects.As a bee visit the young flower and moves inward in search of nectar,it's head pushes sterile plate which brings down the fertile lobes to strike against its back. The pollen grains are deposited upon the back of the bee. When the bee visit older flowers, its back rubs against the mature stigma bringing about the pollination. 

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Pollination and types of pollination

 Pollination

   The transferral of pollen grains from the opened anthers to the receptive stigma of the carpel is called pollination. It is of two types-    SELF POLLINATION and CROSS POLLINATION.

SELF POLLINATION

     Self pollination involves the transfer of pollen grains from the anthers of a flower to a stigma of the same flower or genetically similar flower. It occurs by two types s- 

a) Autogamy- ( autos=self, gamos=marriage)

        It is a kind of pollination in which the pollen  from the anthers of a flower are transferred to the stigma of the same flower. It occurs by three methods -

i) Cleistogamy (kleisto=closed,gamos= marriage) 

 Some plants never open to ensure complete self pollination. This condition is called Cleistogamy, e.g, oxalis, viola etc. These cleistogamous flowers are bisexual small, inconspicuous, colourless and do not secrete nectar.

ii) Homogamy 

 Anthers and stigma of bisexual flowers of some plants mature at the same time. They are bought close to each other by growth, bending or folding to ensure self pollination. This condition is called Homogamy.

E.g Mirabilis( Four 'o, clock), potato, sunflower plants etc.

iii) Bud pollination 

 Anthers and stigma of the bisexual flowers of some plants mature before the opening of buds to ensure self pollination, e.g , Wheat, Rice, peas etc.

B) Geitonogamy ( geiton- neighbour, gamos- marriage)

It is a kind of pollination in which pollen from anthers of one flower are transferred to the stigma of another flower borne on same plant. It usually occurs in plants which shows monoecious conditions ( unisexual, male and female flowers are borne on the same plants).

ADVANTAGES of SELF pollination -

1. Chances of pollination are more.

2.self pollination maintains purity of the race and avoids mixing.

3.It needs not to produce a large number of pollen grains.

4. Flowers need not  to possess devices such as large and showy petals , presence of scent and nectar,etc to attract pollinators.

DISADVANTAGES of Self pollination -

 1 . Progeny continuously gets weaker after every generation.

2. Less chances of the production of new species and varieties.

CROSS POLLINATION

It involves the transfer of pollen grains from the flower of one plant to the flower of another plants. It is called xenogamy or allogamy . 

The main floral characteristics which facilitate cross pollination are- 

i) Herkogamy - Flowers possess some mechanical barriers on their stigmatic surface to avoid self pollination e.g, presence of gynostegium and pollinia in calotropis.

ii)Dichogamy - pollen and stigma of the flower mature at different times to avoid self pollination. It is of two types - protogyny and protandry.

iii) Self incompatibility - In some plants, the mature pollen fall on the receptive stigma of the same flower but fails to bring about self pollination. It is called self incompatibility.

iv) Male sterility- the pollen grains of some plants are not functional.Such plants set seeds only after cross pollination.

v) Dioecism- Cross pollination always occurs when plants are unisexual and dioecious i.e , male and female flower occurs on separate plants e.g, papaya, cucurbits etc

vi) Heterostyly - the flower of some plants have different lengths of stamens and styles so that self pollination is not possible e.g, primula, Linum etc.

Agents for cross pollination

 a) Abiotic agents - such as wind current, gravity, water, etc.

b) Biotic agents -  such as animal pollinators , insects, birds.

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Important terms used in genetics

 1.Gene. -  

A gene may be defined as the unit of inheritance which is carried from the parent by a gamete in a chromosome and expression of a character in the young one in cooperation with its allele, other gene and environment. Chemically, a gene is a linear segment of DNA. 

2.Allelomorphs( Allele) -

 A pair of genes that control the two alternative expressions of the same character and have the same loci(sites) in the homologous chromosomes are called allelomorphs or Allele. The term allele used for any two genes of a single character.

Example - The gene T and t for tallness and dwarfness for height of pea plant. Now TT, Tt, tt are considered as allele. 

3.Gene locus -

 A specific region of a chromosome representing a single gene or allele is known as gene locus. The allelic genes occupy the corresponding loci in a pair of homologous chromosomes.

4.Dominant and Recessive Allele -

When two alleles representing the alternative of a character come together in an individual, usually one allele is expressed fully and the other is not at all. The overpowering of one allele by another is called law of Dominance. 

  The allele which always expresses itself even in the presence of its contrasting allele is known as the dominant Allele.

   On the another hand,  the allele which fails to express itself in the presence of its contrasting dominant Allele is called recessive allele.  

  Example - a guinea pig with allele Bb for coat colour is black. The B allele for black colour coat being dominant express itself and the b for white coat colour being recessive, does not express itself.

Wild and Mutant alleles-

An original allele, dominant in expression and wild spread in the population, is called wild allele.

   An allele formed by a mutation in the wild allele, recessive in expression and less common in the population is termed as mutant alleles.

Homozygous organism -

An organism in which both the alleles of a character at the corresponding loci in homologous chromosomes are identical is said to be homozygous or genetically pure  organism. Homozygous individuals breed true for the specific trait i.e produce offspring with similar genotype and phenotype on inbreeding. They are called pure individual.

Example - The pea plant with TT and tt allele pair are homozygous for height. The combination Of TT allele is homozygous dominant; and the combination of tt allele is homozygous recessive.

 5.Heterozygous organism - 

An organism in which the two allele of a character at the corresponding loci in the homologous chromosomes are unlike is said to be heterozygous for their character. Heterozygous individual do not breed true and are said to be hybrid for their character.

Example - The pea plant with Tt allele pair is heterozygous for height ia heterozygote, one allele codes for dominant trait and the other is recessive trait, and usually the dominant allele is expressed and the recessive allele remains latent.

  So, the homozygous and heterozygous individual are called homozygote and heterozygote respectively.

6.F1 generation - 

It is the first filial generation of hybrid offspring produced in a genetic cross between two genetically different but pure parents.

Example - 

F1 generation produced by crossing TT and tt parent Pea Plants consists of Tt individuals.

7.F2 generation - 

It is the second filial generation of young ones produced in a genetic cross between two F1 organisms.

8.Pure Lines - 

A race of genetically pure,true breeding organisms resulting from continued selfing or inbreeding is called a pure Lines. In nature, pure Lines do not produce vigorous offspring because defective alleles may come to exist in homozygous state.

9.Genotype - 

 It is the sum total of genes inherited from both parents irrespective of whether they are expressed or not.

Example - TT, Tt and tt are genotype for the size of stem in pure tall, dwarf and hybrid tall garden pea.

10.Phenotype -

It refers to the expressed or observable structural and functional trait produced by the interaction of genes and environment.

Example - Tallness in pea plant, colour blindness in man are phenotype.both are noticed by eyes.

         Individual of the same genotype have the same phenotype and breed alike. Individuals of different genotype may also have the same phenotype e.g the genotype TT and Tt give rise to tall pea plant.

11.Genome -

A complete set of gene (DNA instructions) contained in the haploid dose of chromosomes and inherited as a unit from one parent is known as genome. A haploid cell contains two genomes, one parental, other maternal. A cell may have more than two genomes also. Such cells are called polyploids.

12.Gene pool- 

 All the genotype of all the individuals in an inbreeding population make up a gene pool.

  

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