Q. 26

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Answer :

(a) The types of flowers produced by Viola (Pansy) are: chasmogamous and cleistogamous flowers.

They differ from each other in the following way:


Chasmogamous flowers have exposed anthers and stigma whereas cleistogamous flowers do not open at all.


Cleistogamous flowers can only be self-pollinated while chasmogamous flowers can be either self-pollinated or cross-pollinated.


Cleistogamous flowers does not need an external pollinating agent while chasmogamous flowers do need an external pollinating agent.


(b) The kind of pollination in one of the types of flowers that ensures seed-set production is termed as cleistogamy. Cleistogamous flowers do not open at all and in them, the anthers and stigma are close to one another. There is no means by which cross-pollination between two flowers can take place here, so this makes them autogamous (the process of self-fertilization in plants.) So, assured seed-set is produced even when pollinators aren't present.


(c) Vallisneria is a water pollinated plant which grows in freshwater. In Vallisneria, the female flower reaches the water surface by the long stalk that is present whereas the male flowers or pollen grains are released on to the surface of the water. Then these pollen grains are carried passively by water currents. Some of these pollen grains would eventually reach the stigma of the female flower and in this way, pollination is carried out by water as the pollinating agent.


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This diagram shows the female and male flowers in Vallisneria and a brief on how the pollination is taking place by water acting as the external agent.


OR
(a) The technique that can help a healthy married woman who is unable to produce viable ova but wants to bear a child is Gamete intra fallopian transfer (GIFT) where an ovum collected from a donor is transferred to fallopian tube of the healthy married woman who cannot produce ova, but would be able to provide suitable conditions for fertilization and for the further foetus development till parturition (process of delivery of the foetus).


(b) Ovum is a single cell released from either of the ovaries of the female reproductive system. When it is fertilized, it is capable of developing into a new individual.


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(c) Polyspermy is when an egg has been fertilized by more than one sperm.


Polyspermy is prevented in humans as during fertilization,binding of one sperm to the egg or when it comes in contact with the zone pellucida layer of the ovum, it induces depolarization of the egg plasma membrane which eventually blocks the entry of further sperms thus does not let any other sperms to fertilize the egg.



The diagram shows how after one sperm has come in contact with the zona pellucida, no other sperm is allowed entry because of depolarization of the plasma membrane.


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