The process of formation of microspore or pollen grain within the pollen sac of anther is known as microsporogenesis.
Stamen is the male fertilizing organ of flower. It typically consists of anther and filament. Collectively the stamens form the androecium.
Anther is the fertile portion of stamen. In angiosperm anther is typically dithecous and tetrasporangiate. In family malvaceae anther is kidney shaped and bisporangiate.
HOW DOES MICROGENESIS TYPICALLY OCCUR IN ANGIOSPERM?
1. Young anther consist of undifferentiated mass of meristematic tissue which later forms four lobed structure.
2. Few cells of hypodermis become distinct with prominent nucleus and dense cytoplasm forming archiesporial cells.
3. These archiesporial cells divide periclinally forming two layers of cell.
4. The outer layer is known primary parietal layer. It later divides periclinally and anticlinally to form the concentric layer of anther wall which consist of epidermis, endothecium, middle layer and tapetum.
5. The inner layer is known as primary sporogenous layer. The cells of this layer divide mitotically to form spore mother cell or directly behave as spore mother cell.
6. Spore mother cell undergoes meiosis to form 4 haploid daughter cells.
7. Thus formed microspore is present in the form of spore tetrad. Spore tetrad is aggregation of 4 microspore formed by the meiotic division of a spore mother cell.
8. The spore tetrad later separate forming pollen grains.