The Pollen grains aggregates as tetrads, diads, polyads and many more.
These are refered as compound Pollen grains.
Most of the Angiosperm families exhibit this type of Pollens.
They germination is mostly difficult and occurs when ever
the aggregate losses the cementing (exine chemical) substances around them.
The pollen aggregate (unweils) seperates from each other while drying or due to seperation this favours their germination into pollen tubes.......
Go through the below image for How pollen gets attached with exine secretions.
