In flowering plants, pollen grains are formed within the anther.
As the anther develops, four patches of tissue grow and become four chambers or pollen sacs. These chambers are where the pollen will develop.
This image of a young Lilly anther clearly shows the four large pollen sacs developing within the anther. Within these pollen sacs will be cells. These are the “pollen-grains-in-training” and are at this stage known as microsporocytes.
From here, the pollen sacs and the cells within them undergo some major changes. Through cell division, the microsporocytes change to form clusters or groups of four called quartets within the four chambers .
As the anther matures, the pollen sacs walls between adjacent pairs breakdown, creating two sacs instead of four.
The microspores within the chambers are almost ready to become pollen grains but first they must undergo three more changes which occur almost simultaneously.
1. Members of the quartet separate from one another.
2. Each microspore then undergoes nucleus division. The two nucleus that are developed are the generative nucleus and the vegetative nucleus which are involved in how the pollen grain achieves fertillization.
3. A two layered wall develops around each microspore.
When these events are completed the microspores have become pollen grains!