why in most of the  flowering plant only one megaspore is functional and other three are degenerated while in microspore all the four cells are functional

In flowering plants the megaspore mother cell undergoes meiotic division to form four megaspores. out of these 3 degenerates and one if left function which further develops into embryo sac. 

Possible reasons behind the degeneracy of 3 megaspores are- 

  • The position of megaspore is fixed,it would not be possible to accommodate 4 embryo sac at one point. 
  • If the remaining 3  are functional then there is a division of nutrients among them which can cause nutrient shortage.
  •  Its the pollen grain which need to travel to megaspores so there are chances of them to get affected or lost while travelling, hence they are more in numbers. 
  • If all 4 megaspore would be functional it need 4 pollen to fuse with them resulting in the formation of 4 pollen tubes which becomes very complex for the plant.

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