Why is Ericaceae pollen often found in groups of four grains?

In the family Ericaceae, pollen is frequently found in groups of four grains known as tetrads. This occurs because, during the development of pollen within the anther, diploid microsporocytes undergo meiosis to form a group of four joined haploid microspores. While in many other angiosperms these grains eventually separate, in many members of the Ericaceae, they remain united as tetrads.12

These tetrads are often held together in clumps by viscin threads—fine, sticky structures that allow the pollen to adhere to pollinators as they travel between flowers.1