Sexual reproduction in flowering plants (New syllabus chapter 1 / old syllabus Chapter 2)
Pollination|reproduction in flowering plants|short notes| NCERT| standard 12| Biology
Note 8
>> Definition:
->Transfer of pollen grain (shed from anther) to the stigma of a pistil is termed as Pollination.
>> Objective of Pollination:
-> Both gametes i.e. male gamete (produced by pollen grain) and female gamete (produced by embryo sac) are non-motile.
-> For fertilization, both gametes need to come in contact with each other. Hence Pollination is needed to help male gamete reach to the female gamete.
> Plants have various adaption for pollination.
> Plants make use different types of external agents for pollination.
> There are 3 major type of Pollination:
1) Autogamy
2) Geitonogamy
3) Xenogamy
>> Autogamy:
> Auto- self and gamy- fusion of gametes
> Pollination is achieved within the same flower.
> pollen grains are transferred from anther to the stigma of the same flower.
> Such flowers can be called as self-pollinating flowers
> This type of pollination does not require any type of pollinating agent.
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| Self-pollinating flower |
> 2 requirements for Autogamy-
--> (i) Synchrony :
-------> Pollen release (anther dehiscence) and stigma receptivity should occur simultaneously
-------> if pollen are released, but stigma is not at the maturation for receiving pollen - autogamy cannot occur successfully.
--> (ii) Closeness :
--------> anther and stigma should remain close to each other
> There are 2 types of flowers:
--> (i) Chasmogamous -
--------> these are the normal type of flowers we see in our surroundings
--------> they open at the maturation.
--------> Anther and stigma remain exposed to environment
--------> that means pollen grains can go and land on stigma of different flower and similarly pollen grain from other flower can land on the stigma.
--------> Hence, complete autogamy is not possible.
--> (ii) Cleistogamous -
--------> These flower remain close - does not open at all.
--------> anther and stigma are situated close to each other
--------> dehiscence of anther occur inside flower bud only
--------> pollen grain have no other options but to land on the stigma - and pollination occurs
--------> hence, Cleistogamous flowers are compulsorily autogamous, because there is no chance of cross-pollen coming in contact with the stigma
--------> Cleistogamous flower produce assured seed-set even in the absence of pollinators - no requirement of pollinating agents
--> Commenlia, Oxalis, Viola - produce both type of flowers (Chasmogamous and Cleistogamous)
>> Geitonogamy:
> Transfer of pollen grains from anther to stigma of another flower on the same plant.
> This involves pollinating agents
> Functionally, it is cross-pollination
> Genetically, it is similar to autogamy - because pollen grains come from same plant.
> So male and female gametes of same plants are mating each other - and the product seed-set would be similar to that of autogamy
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| Cross-pollinating flower |
>> Xenogamy:
> Transfer of pollen grains from anther to stigma of flower of different plant
> Pollinating agents are required
> It is cross-pollination
> This is the only type of pollination where genetically different type of pollen grains land upon stigma
Thank you for reading!
Happy learning!
Manish Mevada
Urvi Bhanushali
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