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  • (a) Describe any two devices in a flowering plant which prevent both autogamy and geitonogamy.  (b) Explain the events upto double fertilisation after the pollen tube enters one of the synergids in an ovule of an angiosperm. 

(a) Describe any two devices in a flowering plant which prevent both autogamy and geitonogamy. 

(b) Explain the events upto double fertilisation after the pollen tube enters one of the synergids in an ovule of an angiosperm. 
 

 

 

 

 
 
 
 
 

Answers (1)

 a) Two devices which prevent both autogamy and geitonogamy- 
 
1.  In several species such as papaya, male and female flowers are present on different plants, that is each plant is either male or female (dioecy). This condition prevents both autogamy and geitonogamy.  

2. Another device is self-incompatibility. This is a genetic mechanism and prevents self-pollen (from the same flower or other flowers of the same plant) from fertilising the ovules by inhibiting pollen germination or pollen tube growth in the pistil. 
 
b)  

  1. After entering one of the synergids, the pollen tube releases the two male gametes into the cytoplasm of the synergid.
  2.  One of the male gametes moves towards the egg cell and fuses with its nucleus thus completing the syngamy. This results in the formation of a diploid cell, the zygote.
  3.   The other male gamete moves towards the two polar nuclei located in the central cell and fuses with them to produce a triploid primary endosperm nucleus (PEN) .
  4. As this involves the fusion of three haploid nuclei it is termed triple fusion.
  5.  Since two types of fusions, syngamy and triple fusion take place in an embryo sac the phenomenon is termed double fertilisation, an event unique to flowering plants.
  6.  The central cell after triple fusion becomes the primary endosperm cell (PEC) and develops into the endosperm while the zygote develops into an embryo. 

  

 

Posted by

Priyanka Kumari

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