POLLINATION - SELF POLLINATION & CROSS POLLINATION

STD-12      UNIT-6     CHA-2

REPRODUCTION IN FLOWERING PLANT

POLLINATION - SELF POLLINATION 

POLLINATION

• The transfer of pollen grains from the anther to the stigma is called pollination . 
• Pollen grains are immobile . 
• They cannot reach the stigma by themselves . 
• An external agent is required for this . It can be wind , water , animal , gravity or growth contact
• Pollination is of two types self pollination and cross pollination

Self Pollination 

• It is the transfer of pollen grains from the anther of a flower to the stigma of either the same or genetically similar flower . 
• Accordingly , self pollination is of two types ,autogamy and geitonogamy 

CROSS POLLINATION 

Cross Pollination ( Xenogamy , Allogamy )

• Cross pollination is the transfer of pollen grains from the anther of one flower to the stigma of a genetically different flower . It is also called xenogamy ( Gk . xenos- strange , gamos , marriage )

• The term allogamy ( Gk , allos - other , gamos , marriage ) includes both geitonogamy and xenogamy . 
• Cross pollination is performed with the help of an external agency . 
• The latter may be abiotic ( e.g. , wind , water ) or biotic ( e.g. , insects , birds , bats , snails ) . 
• Cross pollination is named after the agency that assists it , viz ; anemophily ( wind pollination ) , hydrophily ( water pollination ) , entomophily ( insect pollination ) , ornithophily ( bird pollination ) , chiropterophily ( bat pollination ) and malacophily ( snail pollination ) .

Self Pollination - TYPES 

1. Autogamy ( Gk . autos self , gamos , marriage ) .

• It is a type of self pollination in which an intersexual or perfect flower is pollinated by its own pollen . 
• Autogamy is possible only when anther and stigma are close together and there is synchrony in pollen release and stigma receptivity . 
• Autogamy occurs by two methods . 

• ( a ) Homogamy . 
• The anthers and stigmas of chasmogamous or open flowers are brought together by growth , bending or folding . 
• In Catharanthus ( = Vinca ) , the growth of style brings the stigma in contact of ripe anthers present on the mouth of corolla tube. 
• In Mirabilis ( Four O'Clock ) , the bending of filaments brings ripe anthers in contact with stigma.
• Potato flowers show curling of style for carrying stigma to ripe anthers.
• When cross pollination fails in Sunflower , the bifid stigma curls back so as to pick the pollen sticking to the surface of style. 
• It is fail safe mechanism of self pollination

• ( b ) Cleistogamy
• The flowers are intersexual . 
• They remain closed causing self pollination .
• Cleistogamy occurs late in the flowering season in some plants , e.g .. Commelina bengalensis , Balsam . Oxalis , Viola . These plants , therefore , possess both chasmogamous and cleistogamous flowers . 
• In cleistogamous flowers , the anthers dehisce inside closed flowers . 
• Growth of style brings the pollen grains in contact with stigma . Pollination and seed set are assured . 
• Pollinators are not required .
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2. Geitonogamy ( Gk . geiton- neighbour , gamos - mariage )  . 

• It is a type of pollination in which pollen grains of one flower are transferred to the stigma of another flower belonging to either the same plant or genetically similar plant . 
• In geitonogamy , the flowers often show modifications similar to ones found in xenogamy or cross pollination .

Advantages of Self Pollination 

1. It maintains the parental characters or purity of the race indefinitely . 
2. Self pollination is used to maintain pure lines for hybridisation experiments . 
3. The plant does not need to produce large number of pollen grains . 
4. Flowers do not develop devices for attracting insect pollinators . 
5. It ensures seed production . Rather it is used as fail safe device for cross - pollinated Nowers .

Disadvantages of Self Pollination 

1. New useful characters are seldom introduced . 
2. Vigour and vitality of the race decreases with prolonged self pollination . 
3. Immunity to diseases decreases . 
4. Variability and hence adaptability to changed environment are reduced .

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