Question

What is the difference between 2-celled stage and 3-celled stage of pollen grain

Answer 1

Hello !
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2 - celled :  when cell contains vegetative and generative cells 
3 - celled : when generative cells divides by mitotic division to form exactly two same cells (males ) with vegetative cell 
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Answer 2

In 2- celled staged of pollen grains, each pollen grain contains vegetative cell and generative cell whereas in 3- celled stage, the generative cell undergoes meiotic division to form two sperm cells.

In 2 celled- stage, generative cell divides in the pollen tube but not in a 3-celled stage.