They are associated with low density.<span />
One of the ways is through the wind. The pollen then needs to be very light, and have a shape that supports being carried by the wind. It is then caught by the sticky surface of the stigma.
Another way is through animals, such as bees. The bees feed on the flowers and the pollen sticks to their legs. When they fly off to another flower, they will leave the trace of this pollen there. In fact many flowers attract bees for this reason, with their colour or smell.
