Explanation

In $Pinus,$ the microspore nucleus divides by a periclinal wall and forms a very small prothallial cell and large central cell. The central cell cuts off a second prothallial cell and antheridial cell. The nucleus of the antheridial cell divides to form generative cell and tube cell. Thus, the pollen grain of $Pinus$ is sheded at four-celled stage when it consists of two vegetative prothallial cells, a generative cell and a tube cell.