Microscope picture of a typical pollen sample. The two clearly visible pollen grains are Pine (looks like a Mickey Mouse hat) and Tupelo.

The study of organic walled microfossils like pollen and spores is called palynology. Paleoecologists use palynology as a tool for deciphering past environments. Because temperature and precipitation determine the range of many plants, changes in pollen assemblages in sediment cores can serve as a proxy record of climate change. Wind pollinated plants like pine, oak, grass, and ragweed shed vast quantities of pollen into the air. Although poorly preserved when it settles on dry soil surfaces, pollen that ends up in water bodies can last for thousands of years in anoxic bottom sediments. To concentrate the pollen in sediments, palynologists use chemicals to destroy the unwanted material. 
One of my favorite pollen grains is this one from a prickley pear cactus.
For more information on Palynology
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University of South Alabama
Last modified: 2/29/00