Marilyn sez, "Until 375,000 years ago, plants had be by physically close to each other in order to reproduce. Pollen changed all that. From the article by Rob Dunn in the Dec. issue of National Geographic:"
Update: not sure about Marylin's source for the 375k year stat above, but it looks like pollen is at least as old as the late Devonian
In the 300,000 pollen-bearing plant species on Earth, there are 300,000 different forms of pollen. The great variety in colors, shapes, and textures of the grains has evolved in accordance with each plant's biological particulars. Beetle-pollinated plants tend to have smooth, sticky pollen, the better to adhere to the lumbering beetles' backs. Plants pollinated by fast-moving bees or flies may have spiny pollen that lodges easily between the insects' hairs. Plants pollinated by bigger animals, such as bats, sometimes have bigger pollen, though not always -- perhaps not even most of the time. In the details of pollen's variety, more remains to be explained than is understood.
A friend with allergies once compared living through high-pollen-count days as "being the involuntary star in a vegetage-kingdom bukkake movie." I haven't been able to think of pollen the same way since.